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lY. 


GEOLOGY  OF  THE  BOSTON  BASIN 


WILLIAM  0.  CROSBY. 
YOL.  L 

PART  I.— NANTASKET  AND  COHASSET. 


BOSTON: 

BOSTON    SOCIETY   OF   NATURAL   HISTOEY. 
■1893. 


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Occas.  papers  Bost.  Soc.  Nat.  Hist.  IV. 


Plate  3. 


<'^y''frmT> 


GEOLOGY 


OF    THK 


BOSTON    BASIN. 


WILLIAM   0.    CROSBY. 


JN    TWO   VOLUMES. 


VOL.  L 


PART  I.— NANTASKET  AND  C0HAS8ET. 


BOSTON : 

BOSTON   SOCIETY   OF   NATURAL   HISTORY 

1893. 


The  expense  of  ijublisking  this  part  is  defrayed 
chief y  hy  Mr.  Thomas  A.  Watso7i. 


QOSRWCOUfQE 


FEB  05 


NANTASKET  AND  COHASSET. 

INTRODUCTION. 


It  is  obvious  upon  the  most  casual  observation  that  the  town 
of  Hull,  Mass.,  which  is  virtually  an  island,  being  joined  to  the 
main  land  of  Cohasset  only  by  the  narrow  barrier  beach  con- 
nectino;  the  Green  Hill  drumlin  with  the  OTanite  ledg-es  alono" 
the  Jerusalem  Road,  is  sharply  divided  into  two  districts  which 
are  very  strongly  contrasted  in  tlicir  topographic  and  geologic 
features.  In  the  order  of  geologic  age  and  interest  these  are  : 
(1)  The  highly  irregular,  broken  and  rocky  tract,  commonly 
known  as  Nantasket,  which  forms  the  southern  extremity  of  the 
town,  adjoining  the  mainland,  and  is  almost  completely  isolated 
by  Strait's  Pond,  Weir  River  Bay,  Nantasket  Harbor  and  the 
Atlantic.  (2)  The  narrow,  and,  at  some  points,  extremely « 
slender  peninsula  stretching  from  Atlantic  Hill  north-northwest 
for  more  than  three  miles  to  Point  Allerton,  whence,  turning  at 
a  right  angle,  it  extends  to  the  westward  two  miles  further  to 
Pemberton  and  Windmill  Point.  This  peninsula  thus  embraces 
Nantasket  Beach  and  the  Village  of  Hull ;  and,  with  the  excep- 
tion of  the  single  ledo-e  of  slate  on  the  south  side  of  Thornbush 
Hill,  near  the  Village,  it  is  composed  entirely  of  rounded  drift 
hills  or  drumlins  and  the  connecting  beaches  of  sand  and  shingle. 
The  dividing  line  between  these  two  districts  is  where  the  sands 
of  the  beach  rest  against  the  rocky  northern  base  of  Atlantic 
Hill. 

Although  the  main  purpose  of  this  paper  is  to  set  forth  the 
results  of  a  detailed  study  of  the  intricate  geologic  structure 
exhibited  in  the  magnificent  rock  exposures  of  the  southern  area 

OCCAS.  PAPERS  B.  S.  N.  H.        rv.         1 


(Nantasket),  the  concludiDg  pages,  especially,  relating  to  the 
glacial  and  more  recent  phenomena  and  the  non-lithified  de- 
posits of  the  district,  will  apply  to  the  northern  or  beach  area  as 
well.  Hence  the  scope  of  the  paper  really  embraces  the  entire 
area  of  Hull,  or  Nantasket  in  the  broad,  original  application  of 
the  name.^  The  mouth  of  Weir  Kiver,  or  Weir  River  Bay, 
forms,  in  the  topographic  sense,  the  natural  western  boundary  of 
the  Xantasket  area  ;  yet,  geologically,  it  is  impossible  to  exclude 
the  promontory  of  Rocky  Neck,  in  Hingham.  This  rocky  head- 
land is  lithologically  and  structurally  identical  with  the  district 
east  of  Weir  River  Bay,  while  it  is  completely  separated  from 
the  other  and  dissimilar  sedimentary  areas  of  Hingham  by  the 
ledo'es  of  o^ranite  on  the  south  and  southwest  and  the  drumlins 
of  Planter's  Hill  and  World's  End  on  the  west  and  northwest. 
The  natural  boundary  line  between  the  geologic  areas  of  Nantas- 
ket and  northern  Hingham  appears,  therefore,  to  be,  not  Weir 
River  Bay  but  the  eastern  shore  of  Hingham  Harbor.  The 
southern  boundary  or  limit  of  Nantasket  is  clearl}^  marked  by 
the  continuous  depression  formed  by  Strait's  Pond,  Lyford's 
Liking  and  Weir  River  Bay,  which  is  continued,  as  a  narrow 
strip  of  marsh,  south  of  Rocky  Neck  to  the  bay  which  sharply 
indents  the  eastern  shore  of  Hingham  Harbor.  This  topographic 
trough  defines  with  approximate  accuracy  the  present  border  of 
the  Boston  Basin,  separating  the  sedimentary  and  volcanic  rocks 
froni  tlie  somewhat  more  elevated  granitic  area  on  the  south. 
Although  the  line  of  depression  just  traced  is  a  natural  and 
important  geologic  boundary,  it  has  appeared  best  to  give  this 
paper  greater  breadth  and  completeness  by  including  in  its  scope 
not  only  the  sedimentary  and  volcanic  rocks  of  Nantasket,  and 
the  granite  floor  upon  which  they  rest,  but  also  a  sufficient 
breadth  of  the  granitic  border  of  the  basin  to  nmke  apparent 
the  marked  contrast  in    geologic  structure   on  opposite   sides  of 


'Tlic  oilier  isliiiiils  of  liosfoii  Iliiibor  (tlie  Browstcrs,  (!:ilf  U.,  Green  Id.,  etc.)  belong 
)  tlie  town  of  Hull.  But  ilic  geoloyy  of  tlie.se  formed  the  subject  of  a  previous  paper 
l^roc.  B.  S.  N.  II.,  ,\xiii.,  450-457);  and  will  be  more  fully  described  in  a  later  paper 
f  this  series.  ' 


to 
(1 

of  this  series. 


the  boandaiy.  The  geology,  so  far  as  the  hard  I'ocks  are  con- 
cerned, is  as  simple  south  of  the  line  as  it  is  complex  north  of  it. 
Hence,  although,  as  the  map  shows,  the  addition  of  the  whole 
of  Cohasset  and  a  portion  of  Scituate  much  more  than  doubles 
the  area  to  be  described,  it  extends  but  slightly  the  limits  of  this 
paper,  which  is  still,  notwithstanding  its  title  and  areal  scope, 
chiefly  a  detailed  account  of  the  ledges  of  Nantasket. 

The  numerous  islands  of  Boston  Hai'bor,  as  any  one  may  ob- 
serve in  going  from  Boston  to  Nantasket  by  water,  are,  with 
unimportant  exceptions,  composed,  like  the  Nantasket  Beach 
area,  wholly  or  almost  wholly  of  rounded  drift  hills  and  con- 
necting beaches  of  sand  and  gravel  ;  and  the  fine  sections  of 
some  of  the  drumlins  due  to  marine  erosion  show  that  in  most 
cases  they  are  pure  drift  accumulations,  at  least  above  sea  level. 
The  Boston  Basin  does  not  afford  a  stronger  topographic  contrast 
than  is  presented  when  the  Nantasket  steamer,  having  rounded 
Bumkin  Island,  the  graceful  drumlin  outline  of  which  is  abi"uptly 
terminated  on  the  west  by  marine  erosion,  and  followed  the 
winding  channel  past  the  beautiful  drumlins  of  the  World's  End 
and  Planter's  Hill,  sweeps  by  the  bold  and  massive  ledges  of 
Rocky  Neck  and  Nantasket.  We  pass  in  an  instant  from  an 
area  which  is  deeply  covered  by  drift  and  presents  only  the  most 
typical  drift-contours,  to  an  essentially  driftless  area,  where  the 
ice-sheet  left  hardly  sufficient  detritus  to  fill  the  narrow  gorges 
and  chasms  dividing  the  rocky  hills. 

The  problem  involved  in  this  unequal  distribution  of  the  drift, 
in  this  abrupt  passage  from  an  area  in  which  the  ice-sheet  acted, 
at  least  during  its  later  stages,  as  an  important  agent  of  depo- 
sition, to  an  area  in  which  its  action  was  purely  erosive,  lies 
beyond  the  scope  of  this  paper  ;  although,  if  the  drift  had  not 
been  swept  from  the  Nantasket  area,  there  would  have  been  little 
occasion  for  a  special  study  of  its  geologic  features.  We  may 
now  simply  accept  the  almost  perfect  exposures  of  these  rock 
masses  as  one  of  the  best  gifts  of  the  ice-sheet  to  the  student  of 
our  local  geology  ;  for  there  is  probably  no  equally  limited  area 
in  the  Boston  Basin  more  worthy  of  detailed  and  thorough  in- 
vesti2:ation  than  this  southeast  corner. 


The  structure  of  other  districts  is,  no  doubt,  equally  varied 
and  intricate  ;  but  Nautasket  excels  in  the  very  favorable  op- 
])Oi-tunities  Avhich  its  almost  continuous  rock  surfaces  present 
for  (leci[)hering  its  structure  lines.  It  is  especially  interesting, 
in  this  connection,  to  obsei-ve  that,  while  the  rocky  area  occur- 
ring- next  to  the  north  of  Nantasket,  in  the  middle  line  or  axis 
of  the  Boston  Basin — the  outer  islands  of  Boston  Harbor — 
affords  the  finest  and  most  typical  exposures  of  intrusive  sheets 
or  interbedded  dikes  to  be  found  in  this  region  ;  the  characters 
of  the  contemporaneous  beds  or  ancient  lava  flows  of  the  Boston 
Basin  and  their  relations  to  tlie  interstratified  conglomerate  can 
be  studied  to  the  best  advantage  at  Nantasket,  on  the  border  of 
the  basin. 

At  no  other  point  is  the  evidence  equally  clear  and  conclusive 
that  the  melaphyrs  and  porpliyrites  of  the  Boston  Basin  are,  in 
the  main,  true  contemporaneous  lavas  which  were  poured  out 
on  the  sea-floor  at  different  periods  during  the  deposition  of  the 
beds  of  conglomerate  and  sandstone.  The  Nantasket  area  also 
shows  more  <^learly  than  any  other  that  in  some  parts,  at  least, 
of  the  Boston  Basin  the  conglomerate  beds,  as  well  as  the  as- 
sociated melaph3U's  rest  directly  upon  the  fundamental  granite, 
witliout  the  possible  interposition  of  the  slate  series. 

It  may  be  pointed  out  in  this  connection  that  the  main  pur- 
pose of  the  present  investigation  is  simply  to  elucidate  tlie 
structure  and  explain  the  origin  and  sequence  of  the  rocks 
of  this  somewhat  complicated  area.  In  other  words,  this  is  a 
structural  rather  tlian  a  litliological  study  ;  and  I  wish  to  ac- 
knowlwlge  my  great  obligation  to  Mr.  Geo.  P.  Merrill  of  the 
U.  S.  National  Museum  for  undertaking  a  microsco[)ic  examina- 
tion of  t!ie  newer  eruptive  rocks  of  Nantasket.  His  determina- 
tions ha\e  ])revented  serious  errors  in  my  Avork  and  enabled  me 
{()  (haw  certain  conclusions  with  a  degree  of  confidence  which  it 
woiihl  otherwise  have  been  impossible  to  feel. 


TOPOGRAPHY. 

The  rocky  areas  or  ledges  of  Cohasset  and  Nantaskct  arc 
characterized,  considering  their  diversified  geologic  structure, 
by  a  remarkable  uniformity  of  elevation,  indicating  tliat  in.prc- 
glacial  times  this  region  was  worn  down  nearly  if  not  quite  to 
its  base-level ;  and  tlie  deeply  incised  and  generally  abru[)t 
character  of  the  valleys  now  dividing  this  old  peneplain  shows 
that  it  must  have  shared  in  the  genei'al  and  marked  elevation  of 
the  land  at  the  dawn  of  the  great  ice  age  ;  while  the  flict  that 
the  deeper  valleys  are  now  to  a  large  extent  occupied  by  the  sea 
or  its  deposits,  i.  e.,  are  or  have  been  true  fiords,  is  sufficient  evi- 
dence of  a  subsequent  subsidence  ;  and,  finally,  the  extensive 
beach  and  marsh  formations  prove  that  the  present  level  of 
the  land  has  been  maintained  for  a  very  long  time.  The  mod- 
ern base-level  is  strongly  accentuated,  not  alone  through  tlie 
constructive  action  of  the  sea,  but  marine  erosion  has  made  ex- 
tensive inroads  upon  the  drumlins  and  other  drift  deposits, 
especially  of  the  Nantasket  peninsula  and  adjacent  islands,  de- 
veloping the  prominent  sea-cliffs  of  Telegraph  Hill,  Point  Allei-- 
ton.  Strawberry  and  Green  Hills,  etc., as  well  as  broad  submarine 
platforms  or  shoals,  the  outlines  of  which  can  be  traced  on  the 
Coast  Survey  chart. 

Since  even  the  most  typical  peneplain  must  slope  gently  sea- 
ward, the  average  elevation  of  the  rock-surface  of  eastern  Mass- 
achusetts increases  gradually  as  it  i-ecedes  from  the  coast ;  and 
Nantasket  and  Cohasset  are,  therefore,  one  of  the  lower  portions 
of  it.  In  the  valleys,  the  rock-contours  are,  of  course,  cai'ried 
down  to  and  below  sea-level ;  but  the  ledges  and  rocky  hills 
separating  the  depressions,  the  real  remnants  of  the  ancient 
peneplain,  are,  even  in  the  Nantasket  area  and  along  the  Co- 
hasset shore,  rarely  below  50,  and  usually  from  75  to  100  feet, 
in  height;  and  the  elevation,  increasing  very  slightly  inland, 
attains  its  maximum  of  125  to,  possibly,  150  feet  in  the  broad 


6 

area  of  rocky  woodland  along  the  bouudarv  between  Cohasset 
and  Hinghani.  The  moderate  disparity  of  these  figures  does 
not  contradict  the  previous  statement  concerning  the  general 
uniformity  of  tlie  rocky  elevations  ;  for  the  main  point  is  that 
while  there  are  hundreds  of  rocky  hills  and  ridges  or  ledges  rising 
10  to  50  feet  or  more  above  the  ground  between  them,  there  are 
none  rising  to  commanding  heights,  that  is,  there  are  no  rock 
elevations  decidedly  overlooking  the  surrounding  country,  as 
do  the  drift  hills  or  drumlins.  It  is  of  course  this  absence  of 
true  rock  hills  that  proves  the  ancient  peneplain  ;  for  the  present 
interstream  surfaces,  where  not  encumbered  by  drift,  are  essen- 
tially ledgy  plateaus.  The  absence  of  crags  and  pinacles  of  rock 
is  easily  accounted  for  by  the  severe  glaciation  which  this  region 
has  suffered  ;  but  we  cannot  thus  exj)lain  the  fact  that  there  are 
no  dominant  rock  hills  of  rounded  or  glaciated  outline,  like 
the  Blue  Hills.  That  glacial  erosion  was  not  equal  to  the  com- 
plete obliteration  of  such  reliefs  is  proved  by  the  survival,  even 
when  quite  isolated,  of  the  numerous  hills  and  ledges  which  do 
not  rise  above  the  surface  of  the  dissected  peneplain.  Briefly 
stated,  then,  the  rock  hills  of  this  district  have  all  been  carved  out 
of  the  prcglacial  peneplain  ;  and  the  general  equality  of  relief  thus 
determined  must  continue  until  an  increased  elevation  of  the 
land,  giving  renewed  energy  to  the  ordinary  agents  of  erosion, 
permits  a  more  general  and  imequal  effacement  of  the  original 
interstream  surfaces. 

Tliat  the  depressions  or  valleys  which  now^  interrupt  the  pene- 
[)lain  are  not  due  chiefly  to  glacial,  but  to  prcglacial  aqueous, 
erosion  is  sufficiently  obvious,  in  many  cases,  from  tlieir  direc- 
tions ;  and  it  is  also  seen  in  the  fact  that  the  rock  hills  and 
ledges  of  Nantasket  and  Cohasset,  through  the  influence  of  the 
faults,  dikes  and  joints  by  which  they  are  bounded,  are,  in  the 
main,  approximately  rectangular  in  plan,  with,  frequently,  very 
sfoci)  •^''  even  precipitous  slopes,  and  showing  only  secondarily, 
mid  not  as  their  primary  form,  the  roche  niontonce  outline  due 
to  glaciation.  Hence,  cliffs,  sometimes  rising  abnn)tly  from 
the  water  or  the  salt   marshes,   and  straight,  narrow  defiles  are 


characteristic  topogra[)liic  features  of  tlie  district,  although  the 
partial  submergence  of  the  valleys  and  the  general  plateau  form 
of  the  hills  causes  the  elevations  to  aj)pcar  incommensurate  with 
the  otherwise  rugged  character  of  the  topography.  North  of 
Atlantic  Hill  we  descend  into  the  great  central  valley  of  the  Bos- 
ton Basin,  now  occupied  by  Boston  Ilarbor ;  and  the  pene- 
plain is  lost.  This  is  an  area  of  much  softer,  sedimentary 
rocks,  which  was  deeply  and  broadly  eroded,  when  the  reo"ion 
was  more  elevated,  by  the  united  Charles  and  Neponset  Rivers 
and  their  affluents. 

The  lower  portions,  especially,  of  the  old  peneplain,  in  Co- 
hasset,  are  to  a  considerable  extent  emphasized  by  level 
expanses  of  modified  drift.  The  principal  sand  plain,  through 
which  project  both  the  drumlins  and  the  higher  ledges,  rano-es 
from  40  to  60  feet  in  height  and  extends  interruptedly  over  a 
large  part  of  the  town,  mantling  but  not  greatly  masking  the 
contours  of  the  underlying  rocks.  Although  distinctly  recog- 
nizable at  many  points,  it  has  its  finest  development  in  and 
about  Cohasset  Village,  between  Little  Harbor  and  the  railroad, 
rising  very  abruptly  45  feet  from  the  level  marshes  of  Little 
Harbor  to  the  kettle-dimpled  plain  on  which  the  village  stands. 
North  of  Strait's  Pond  and  Weir  River  Bay,  in  Nantasket  and 
northern  Hull,  the  modified  drift  is  very  scantily  developed  and 
of  no  topographic  importance.  But  in  the  terraces  or  elevated 
shore-lines  traceable  at  corresponding  heights  on  some  of  the 
drumlins  in  Hull  and  also  in  Cohasset  and  Scituate,  and  accu- 
rately marking  the  varying  levels  of  the  sea  at  the  time  when 
the  sand  plains  were  formed,  we  have  a  related  feature  of  con- 
siderable interest  which  will,  in  its  proper  sequence,  be  described 
in  detail. 

While  the  modified  drift  is,  at  the  best,  only  a  minor  factor 
in  the  topography  of  this  district,  the  unmodified  drift  or  till,  on 
the  other  hand,  occurring  almost  wholly  in  the  form  of  drum- 
lins, adds  greatly  to  the  topographic  relief  and  diversity.  The 
drumlins  constitute,  virtually,  the  only  elevations  in  Hull 
north  of  Atlantic  Hill ;  in  fact  it  is  doubtful  if  this  part  of  the 
peninsula  would  have  any  existence  as  dry  land  if  it  were   not 


for  these  solid  nuclei  of  till  about  which  the  beach  deposits  have 
gathered.  As  already  noted,  and  as  the  map  shows,  the  drum- 
lins  are  not  evenly  distributed.  In  Hull,  in  the  direct  line  of 
the  beach,  like  a  string  of  beads,  are  Point  i\.llerton  and  Straw- 
berry Hills,  White  Head,  Sagamore  Head,  Hampton  Hill, 
Rockland  Hill  (a  lenticular  slope  of  till)  and  Green  Hill. 
Parallel  with  this  line,  on  the  west,  is  a  second,  including  the 
druuilins  of  Hull  Village  —  Thornbush  and  Telegraph  Hills, 
Little  Hog  Island  and  Bumkin  Island,  this  line  being  continued 
to  the  west  of  Nantasket  in  the  double  drumlin  of  the  Worlds' 
End,  Planters'  Hill  and  Pine  Hill.  From  either  of  these  lines 
we  pass  to  an  area,  including  the  Nantasket  ledges  and  all 
that  part  of  Cohasset  and  Hingham  north  of  the  railroad  and 
cast  of  Hingham  Harbor,  in  which  drumlins,  or  any  noteworthy 
accumulations  of  till,  are  almost  wholly  wanting ;  the  hard 
granite  surface  of  the  peneplain  standing  forth  clear  and  naked. 
But  south  of  the  railroad,  again,  in  the  broad  depressed  or  valley 
portion  of  Cohasset,  the  drumlins  are  thickly  planted,  greatly 
obscuring  this  portion  of  the  peneplain.  The  series  begins  on 
the  northwest  with  Turkey  Hill  (181  feet)  and  Scituate  Hill 
(177  feet),  two  of  the  most  typical  drumlins  in  the  Boston 
Basin  and  the  culminating  points  of  the  area  to  which  this  pa- 
per relates.  Southeastward,  as  the  map  shows,  the  belt  broad- 
ens, but  the  drumlins,  although  more  numerous,  are  also 
smaller  and  very  much  flatter  inform,  being,  where  they  reach 
the  level  of  the  principal  sand  plain,  as  in  Hoop  Pole  and  Mann 
Hills,  in  Scituate,  not  readily  distinguished  from  it.  But 
farther  south  in  Scituate,  we  find  again,  in  Booth  Hill,  a  drum- 
lin of  respectable  height  and  great  horizontal  extent.  West 
of  King  Street,  Scituate  Pond  and  the  head  waters  of  Bound 
Brook,  and  south  of  Turkey  and  Scituate  Hills,  no  drumlins 
liave  been  observed  in  Cohasset,  the  peneplain  being  here,  as 
iioith  of  the  railroad,  but  slightly  encumbered  by  drift  dc- 
])()sits. 

The  drainage  of  this  district  is  a  simple  story.      No  part  of 
Tltill  is  more  than  one-fourth  of  a  mile  from  the  salt  water,  and 


9 

there  iire  no  streams.  The  plateau  of  North  Cohassct  drains 
directly  by  several  short  brooks,  of  which  the  most  important 
is  Turkey  Hill  Run,  rising  in  the  swamp  between  Turkey  and 
Scituate  Hills,  into  the  Strait's  Pond  trough,  the  Atlantic  and 
Little  Harbor.  The  remainder  of  Cohasset  and  northern 
Scituate  are  tributary  either  to  Cohasset  Harbor  and  its  ex- 
tension, the  Gulf,  or  to  Bound  Brook,  which  is  the  principal 
affluent  of  the  Gulf.  Considerable  attention  has  been  given  to 
tracing  out  not  only  the  actual  drainage  lines  or  streams,  but 
also  the  areas  of  obstructed  drainage,  the  ponds,  swamps  and 
marshes  ;  and  it  is  believed  that  these  features  are  represented 
more  accurately  on  the  maps  accompanying  this  paper  than  on 
any  earlier  published  maps. 

Besides  Strait's  Pond,  which  is  due  to  an  artificial  barrier  at 
its  western  end  and  is  mainly  salt  water,  the  only  important 
pond  in  this  entire  district  of  Hull,  Cohasset  and  northern 
Scituate  is  Scituate  Pond  (sometimes  called  Lily  Pond)  in 
the  southern  central  part  of  Cohasset.  It  is  tributary  to  Bound 
Brook  ;  and  it  is  also  connected  on  the  northeast  with  a  swampy 
tract  from  which  a  much  smaller  brook  flows  directly  into  the 
Gulf.  Scituate  Pond  is  thus  very  nearly  an  example  of  a  basin 
having  two  distinct  outlets  at  the  same  level  or  in  a  state  of  equi- 
librium. It  is  probably  due  primarily  to  the  accumulation  of 
modified  drift  in  the  valley  of  Bound  Brook  ;  and  the  general 
absence  of  ponds  in  this  district  may  be  attributed  to  the  lack  of 
continuity  of  the  sand  plains.  Even  the  small  kettle  ponds 
of  other  districts,  with  a  few  exceptions,  as  in  the  vicinity 
of  Little  Harbor  where  the  principal  plain  has  its  best  devel- 
opment, are  wanting  here.  Neither  is  the  form  of  the  ground 
favorable  to  the  view  that  any  important  ponds  have  been  obliter- 
ated by  the  cutting  down  of  barriers  or  the  growth  of  silt  and  bog 
deposits.  If  the  valley  of  Bound  Brook  were  cleared  out,  the 
Gulf  would  doubtless  be  extended  inland  to  Scituate  Pond,  if 
not  considerably  beyond  ;  and  this  main  drainage  channel  and 
its  affluents  are  now  so  nearly  on  a  base-level  basis  that  they 
?ire  bordered  throughout  by  broad  swamps  and  meadows. 


10 

Besides  Bound  Brook  and  the  Gulf,  the  principal  submerged 
or  drowned  rock  valleys  of  this  district  are  Weir  River  Bay 
and  Strait's  Pond,  Little  Harbor,  and  Cohasset  Plarbor.  These 
fiords,  as  remarked,  are  all  far  advanced  in  the  process  of -silting 
up  to  the  present  level  of  the  sea.  Even  Cohasset  Harbor  is  a 
harbor  in  little  more  than  name,  presenting  with  the  ebb  of  the 
tide  a  broad  level  expanse  of  mud  and  sand  of  great  but  unde- 
termined thickness  ;  while  the  widely  extended  marshes  on  the 
east  and  south  show  how  greatly  the  area  as  well  as  the  depth 
of  the  harbor  has  been  reduced.  The  barrier  beach  bordering 
and  limiting  the  salt  marshes  on  the  north  has  nearly  separated 
the  outer  harbor  from  the  Cove ;  and  the  soundings  on  the 
chart  indicate  that  a  similar  bar  is  now  forming  across  the  mouth 
of  the  outer  harbor,  from  the  Grlades  westward.  In  more  ex- 
posed situations,  the  sea  has  ah-eady  closed  by  barrier  beaches 
two  eastern  entrances  to  Cohasset  Harbor,  the  main  entrance  to 
Little  Harbor,  the  eastern  end  of  Strait's  Pond,  and  connected 
all  the  drumlins  of  the  Nantasket  peninsula  by  beaches  fi-om 
fifty  feet  to  nearly  half  a  mile  in  width.  The  numerous  rocks 
and  islets  fringing  this  part  of  the  coast  are  not  the  result  of 
marine  erosion  ;  but  these,  and  also  the  ledges,  sometimes  of 
great  extent,  now  isolated  by  the  salt  marshes  of  Nantasket 
and  Cohasset,  testify  equally  with  the  submerged  valleys  to  the 
subsidence  of  the  land.  Continued  subsidence  would  sub- 
merge these  ledges  and  isolate  others,  leaving  the  general 
aspect  of  the  coast  unchanged. 


MAPS. 

This  paper  is  accompanied  by  two  maps  (Plates  I  and  II) .  The 
first,  on  a  scale  of  2400  feet  to  the  inch,  embraces  the  entire 
peninsula  of  Hull,  the  whole  of  Cohasset,  a  portion  of  Scituate 
and  all  but  the  southern  end  of  Hingham,  or,  in  other  words, 
the  entire  area  to  be  described  not  only  in  this  paj)er  but  also  in 
the  following  paper  on  the  geology  of  Hingham.  To])ogra]>hi- 
cally,  it    is    based  primarily    upon    the   Coast  Survey  chart  of 


n 

Boston  Harbor  and  the  atlases  of  Plymouth  and  Norfolk  Coun- 
ties ;  l)ut  certain  featui-es,  especially  the  marshes,  drainage  lines, 
(Irumlins  and  other  drift  reliefs  have  been  greatly  modified  in  ac- 
cordance Avith  original  observations.  The  aim  has  been  to  com- 
pile the  best  obtainable  data,  rather  than  to  preserve  a  uniform 
degree  of  accuracy,  and  undoubtedly  some  parts  of  the  map  arc 
much  more  reliable  than  others.  Even  in  the  absence  of  con- 
tour lines,  it  exhibits  some  of  the  more  important  relief  features, 
the  kames  and  drumlins  being  clearly  distinguished  ;  w^hile  the 
hydography,  including  the  shore-lines,  ponds,  streams,  swamps 
and  mai'shes,  is  represented  with  unusual  completeness  and  ac- 
curacy ;  and  it  shows  in  addition  the  general  structure  of  the 
hard  rocks,  and  especially  the  relations  of  the  granitic  masses  to 
the  sedimentary  and  volcanic  deposits.  The  Nantasket  area,  the 
elucidation  of  which  is  the  special  object  in  view,  forms  the  central 
part  of  this  first  or  general  map,  which  thus  illustrates  its  topo- 
graphic and  geologic  relations  to  contiguous  districts  ;  while  the 
second  or  special  map,  on  a  scale  of  BOO  feet  to  the  inch,  permits 
of  much  greater  detail. 

No  good  or  even  approximately  accurate  topographic  map  of 
all  that  large  part  of  the  Nantasket  district  west  of  the  County 
Road  has  heretofore  been  published,  and  it  has  been  necessary  to 
devote  considerable  time  and  labor  to  the  preparation  of  a  suita- 
ble basis  for  the  representation  of  the  geologic  features.  I 
am  particularly  indebted  to  Mr.  Wm.  M.  Beaman  for  the  sub- 
stantial accuracy  of  the  western  area,  between  the  railroad  and 
Weir  River  Bav,  and  for  a  general  triano'ulation  of  the  entire 
district  from  Planter's  Hill  to  Green  Hill  and  Black  Rock  ;  and 
to  Mr.  Gordon  H.  Taylor  for  the  contour  lines  upon  nearly  the 
entire  map  excepting  the  western  ar.ea.^ 

The  map  of  Rocky  Neck  is  based  upon  the  plan  of  a  land 
survey  kindly  furnished  by  Mr.  John  R.  Brewer ;  and  the 
coastal   area,    from   the  steamboat  wharf  to  Green  Hill  is   re- 

^  Messrs.  Beaman  and  Taylor  have  made  a  careful  study  of  the  topography  of  this 
section  as  students  in  the  Department  of  Civil  Engineering  in  the  Massachusetts 
Institute  of  Technology. 


12 

tliiccd,  with  some  modifications,  from  tlie  ma[)  ot"  Nantasket  in 
the  atlas  of  Plymouth  County,  1879  ;  while  I  am  personally  re- 
sponsible for  the  outlines  of  the  central  area,  between  the  County 
Road  and  railroad,  and  for  the  entire  south  shore  of  Weir  River 
Bay,  Lyford's  Liking  and  Strait's  Pond. 

The  general  absence  of  roads  or  other  artificial  land-marks 
west  of  Hull  Street  and  the  County  Road,  and  especially  west 
of  the  railroad,  and  the  fact  that  the  hills  and  other  natural 
features  in  this  section  were  still  unnamed,  caused  it  to  appear 
desirable,  in  the  interests  of  concise  and  intelligible  description, 
to  assign  names  to  the  more  important  rock  masses,  marshes, 
etc.  Of  course  it  is  not  expected  that  many  if  any  of  the  geo- 
logic and  descriptive  designations  which  aj^tpear  upon  the  map 
and  in  the  following  pages  will  gain  general  acceptance  :  and 
this  is  in  no  wise  essential  to  their  present  usefulness.  The 
actual  exposures  of  the  hard  rocks  are  so  nearly  continuous 
over  the  areas  where  they  are  indicated  by  colors  on  this  map 
that  it  has  appeared  unnecessary  to  represent  the  individual 
ledges  or  outcrops  ;  although  this  is  virtually  done  for  all  the 
ledges  in  the  marshes  and  below  the  high-tide  line.  And,  ex- 
cept in  the  case  of  some  of  the  fault-lines,  the  map  may  be 
fairly  regarded  as  a  plain  i-ecord  of  actually  observed  facts,  de- 
void of  theory. 

THE  GRANITIC  ROCKS  OF  COHASSET. 

This  heading,  in  its  broadest  application,  covers  all  the  geo- 
logical formations  of  Cohasset,  except  the  dikes  of  diabase  and 
porphyrite  and  the  drift  deposits.  The  dikes,  being  essentially 
similar  in  character  and  age  to  those  of  Nantasket  will  be  most 
conveniently  described  in  that  connection  ;  and  since  the  super- 
ficial geology  forms  naturally  one  continuous  chapter  for  the  en- 
tire area  to  which  this  paper  relates,  the  drumlins,  sand  plains, 
etc.,  of  Cohasset  will  not  be  taken  up  separately.  But  it  appears 
best  to  introduce  a  general  account,  lithological  and  structural, 
of  the  granitic  rocks  of  Cohasset  at  this  point,  because  they  form, 


18 

not  only  tlie  border,  but  the  floor  of  tliis  part  of"  the  Boston  Basin. 
They  are  alike  the  foundation  upon  which  the  newer  sediments 
and  lavas  of  Nantasket  are  piled,  and  the  principal  source  from 
which  the  material  for  building  the  Nantasket  strata  was  de- 
rived. 

Among  the  granitic  rocks  of  Cohasset  are  included  chiefly 
the  diorite  and  the  granite  proper.  The  diorite  is  in  eveiy  in- 
stance clearly  the  older,  as  well  as  the  less  abundant  and  less 
important,  rock  ;  the  relations  of  the  two  rocks  being  essentially 
the  same  here  as  elsewhere  about  the  Boston  Basin.  To  a  large 
extent  they  are  quite  intimately  associated,  the  diorite  occurring 
very  generally  in  the  form  of  irregular  fragments  or  masses,  of 
all  sizes,  enclosed  in  the  granite  ;  while  in  other  cases  the  gran- 
ite forms  irregular,  branching  dikes  in  the  diorite.  In  fact,  the 
granite,  however  massive  it  may  be,  is  rarely  entirely  free  from 
inclusions  of  diorite  ;  and  the  diorite,  even  when  farthest  from  a 
main  body  of  granite ,  almost  invariably  exhibits  a  net-work  of 
granite  intrusions.  In  other  words,  the  diorite  has  been  very 
generally  fissured,  and  in  large  part  completely  shattered,  and 
then  injected  by  the  granite.  Hence,  although  the  two  rocks 
are  always  perfectly  distinct  in  their  chronological  relations,  ob- 
serving an  invariable  sequence,  it  is,  for  considerable  areas,  a 
hopeless  task  to  trace  or  define  the  distribution  of  either  sepa- 
rately from  the  other  ;  and  it  is  for  this  reason  alone  that  they 
are  not  distinguished  on  the  map.  Any  boundaries  that  might 
be  drawn  upon  the  map  would  have  but  very  little  significance, 
since  in  no  case  could  they  be  either  exclusive  or  inclusive.  The 
granite  very  largely  predominates  ;  and  the  diorite,  except  as 
isolated  inclusions  in  the  granite,  is  quite  restricted  in  its  distri- 
bution. It  has  been  observed  chiefly  in  the  ledges  south  of  Co- 
hasset Harbor,  north  and  west  of  Little  Harbor  and  along  the 
shore  between  Little  Harbor  and  Nantasket.  The  large  ledge 
south  of  the  Cove  and  west  of  the  Gulf — Kent  Rocks  — 
is  mainly  granite,  but  encloses  considerable  diorite,  while  the 
pointed,  rocky  hill  immediately  east  of  the  Gulf  —  Government 


14 

Rocks  —  is  all  granite,  coarse  and  pinkish.  In  the  large  ledgy 
tract  nearly  half  a  square  mile  in  extent  between  the  Gulf  and 
the  Scituate  shore,  diorite  appears  to  be  the  prevailing  rock  ; 
but  the  granite  is  always  close  at  hand  and  forms  some  large 
masses.  The  ledges  along  the  Scituate  shore,  east  of  Cohasset 
Harbor  and  the  Glades,  and  advancing  from  the  south,  are, 
except  for  an  occasional  inclusion  of  diorite,  wholly  composed 
of  a  beautiful,  coarsely  but  uniformly  crystalline  and  massive 
pinkish  granite,  which  also  forms  the  adjacent  islands,  from 
Barr's  Kocks  to  the  Osher  Rocks.  On  reaching  the  prominent 
point  a  short  distance  north  of  the  Osher  Rocks,  however,  we 
pass  abruptly  from  the  granite  to  diorite  with  only  occasional 
irreofular  dikes  of  ffranite  breakingf  throuoh  it.  The  diorite  forms 
the  north  shore  of  this  point  ;  but,  crossing  a  short  shingle 
beach,  wc  find  that  the  east  shore  of  Strawberry  Point  is  chiefly 
coarse  granite  ;  while  across  the  north  side  there  is  much  diorite 
alternating  with  the  granite.  It  is  partly  in  solid,  unbroken 
masses,  more  commonly  veined  with  granite,  and  to  a  consid- 
erable extent  completely  morcellated,  yielding  a  very  coarse 
breccia  in  which  the  diorite  forms  the  fragments  and  the  granite 
the  cement.  Gull  Island  presents  at  low  tide  a  broad  flat  sur- 
face of  diorite  irregularly  veined  with  granite  ;  but  Shc]i])ard's 
Ledofc  and  the  ledo-es  southwest  of  Gull  Island  are  ofi'anite 
with  little  or  no  diorite.  These  characteristic  relations  of 
the  two  rocks  are  frequently  repeated  on  the  many  other  ledges 
and  islets  between  Cohasset  Harbor  and  Minot's  Light,  although 
the  granite  usually  predominates.  Along  the  north  side  of  Co- 
hasset Harbor,  between  the  railroad  and  White  Head,  the  ledges, 
so  far  as  observed,  are  nearly  all  granite  ;  and  following  the 
shore  northward  from  AYhite  Head,  around  Sandy  Conc,  to  the 
mouth  of  Little  Harbor,  the  rock  is  all  the  most  typical,  coarsely 
crystalline,  light  gray  graiititc,  weathering  pink  or  reddish,  very 
massive  in  structure  and  with  only  very  rarely  a  small  inclusion 
of  diorite.  North  of  the  narrow  mouth  of  Little  Harbor,  on 
I>each  Island,  the  granite  continues,  unchanged,  to  a  point  be- 
wecn    200   and  300  feet    south   of  the   artificial    hailjor  on  the 


15 

headland  opposite  Brush  Island.  Here  we  pass  very  ahru[)tly 
from  the  coarse,  massive  granite  to  the  mixed  granite  and  diorite, 
tor  the  first  hundred  feet  or  so  a  very  confused  mixture,  in  which 
the  diorite  seems  to  predominate  ;  but  west  of  that,  along  the 
entire  stretch  of  shore,  as  far  as  Green  Hill  and  Nantasket,  the 
granite  is  the  prevailing  rock,  with  frequent  inclusions  of  diorite. 
This  is,  structurally,  an  extremely  interesting  section,  and  it  will 
be  described  more  fully  a  little  farther  on.  The  essentially 
patchy  distribution  of  the  diorite  in  the  granite  makes  the  tra- 
cing' of  this  rock  in  the  weathered  and  lichen-covered  inland 
ledges  rather  unsatisfactory,  not  to  say  unprofitable,  and  only 
enough  work  has  been  done  in  this  direction  to  show  that  what 
we  can  see  so  clearly  along  the  shore  is  really  characteristic  of 
the  whole  town.  Thus  it  must  be  evident  to  any  one  observing 
the  ledges  along  the  west  side  of  Little  Harbor  and  on  Forest 
Avenue  that  the  mixed  granite  and  diorite  extends  inland  a  con- 
siderable distance  :  while  in  other  parts  of  the  town  it  is  equally 
clear  that  coarse  and  massive  OTanite  covers  laro'e  areas. 

The  diorite  is  always  dark-colored  and  holocrystalline,  but 
usually  rather  fine-grained,  varying  in  texture  from  compact  or 
aphanitic  to  distinctly  but  not  coarsely  crystalline,  i.  e.,  the  dio- 
rite is  rarely  coarse  in  the  sense  that  the  granite  often  is.  On  the 
other  hand,  it  rarely  resembles  diabase,  except  in  the  most  com- 
pact forms,  the  normal  diflPerence  in  crystalline  structure  or 
habit  being  readily  recognized  in  the  macrocrystalline  diorite. 
Under  the  miscrocope  it  is  usually  seen  to  be  composed  chiefly 
of  plagioclase  and  hornblende.  The  feldspar  is  commonly 
rather  opaque  ;  but  the  hornblende  is,  in  many  cases,  beauti- 
fully clear  and  dichroic,  although  usually  bordered  by  secondary 
biothe,  chlorite,  etc.  ;  while  in  the  more  conjpact  and  highly 
altered  diorite  the  hornblendic  element  is  very  largely  redu.ced 
to  hydrous  silicates,  which  give  the  rock  a  dark  greenish  color. 
Among  the  secondary  minerals  occurring  in  veinlets  and  irregu- 
lar segregations  epidote  is  most  prominent,  but  it  is  frequently 
accompanied  by  chlorite  and  quartz.  The  black  oxides  of 
n'on  (magnetite  and  menaccanite)  are  usually  present  but  rarely 


16 

abundant,  .and  the  rock  also  often  contains  a  small  amount  of 
original  quartz.  The  quartz  diorite  is  especially  interesting  as 
indicating  a  gradual  passage  to  the  more  basic  granites. 

The  diorite  is  undoubtedly  a  plutonic  rock  ;  and  it  is  sufficient- 
ly varied  in  character  to  suggest  that  it  is  possibly  not  all  of  the 
same  age  ;  but  no  facts  have  been  observed  which  point  to  a 
definite  conclusion,  and  all  that  can  be  regarded  as  well  deter- 
mined is  that  its  relations  to  the  granite  are  always  essentially 
the  same. 

The  granite,  on  the  other  hand,  belongs  very  clearly  to  two, 
and  probably  three,   more  or  less  distinct   periods   of  igneous 
activity,  or  successive  phases  of  the  same  period.     First  in  order 
of  time  comes  the  granite  which  is  most  intimately  associated 
with  the  diorite.     This  is  very  abundant,  and  partakes  of  the 
character  of  the  diorite.    It  is  on  the  average  only  a   little  more 
coarsely  crystalline  than  the  diorite  ;   and  usually  contains  suffi- 
cient hornblende  or  black  mica  to   make  it  quite  dark    colored  , 
for   a  granite.       The  quartz   is   often  deficient,   and  the   feld- 
spar is  partly  plagioclase,  so  that  it  would  be  easy  to  mistake 
a  portion'  of  the  rock  for  either  diorite  or  syenite.      The  horn- 
blende shows,  perhaps,  even  more  alteration  than  in  the  diorite, 
being  very  largely  replaced  by  chlorite.        Next  in  order  comes 
the  light  gray  and  pinkish  granite,   which    is  usually  coarsely 
crystalline  and  massive  and  is  as  a  rule  comparatively  free  from 
inclusions  of  diorite.     This  rock   is  rich  in  acid  feldspar  and 
quartz  ;  but  the  hornblendic  element  is  usually  quite  scantily  de- 
veloped ;  and  black   mica  (biotite)    often   partially,   sometimes 
wholly,  replaces  the  hornblende,  wliilc  both  of  these  accessories 
are,  as  a  rule,  largely  altered  to  chlorite  or  other  hydrous  species. 
It  can  be  seen  breaking  through  the  first  granite  and  the  diorite  at 
many  points  along  the  shore,  especially  between  Little  Harbor 
and  Nantasket ;  and  some  of  the  smaller  dikes  of  this  granite  arc 
quite  fine  grained  and  not  easily  distinguished   from   the   third 
type.       Tills  coarse  acid  granite  is  the  only  rock  in  tliis  district 
of  any  particular  economic  interest ;   and  it   has   been   quarried 
only  to  a  very  limited  extent.    There  is  one  small  quarry  on  the 


17 

Scituate  shore,  near  the  Osher  Rocks,  and  anotlicr  on  tlu;  Co- 
hasset  shore,  north  of  Sandy  Cove;  but  inland  notliino-  dosci'v- 
ing  the  name  of  a  quarry  has  been  observed.  The  tliird  n^ranito, 
in  chronological  sequence,  forms  small  and  irregular  dikes 
from  a  fraction  of  an  inch  to  several  feet  in  width  cutting  all  the 
older  rocks,  but  occurring  chiefly  in  the  coarse,  typical  granite 
(No.  2),  from  which  it  appears  to  differ  in  composition  only  in 
containing  less  hornblende  or  mica.  These  dikes  are  always 
fine-grained,  varying  from  a  finely  crystalline  gray  or  pinkish 
oranite  or  micro-granite  to  a  true  felsite. 

The  micro-granite  passes  into  an  acid  felsite  of  the  same  chem- 
ical composition.  But  while  the  micro-granite  is  common,  form- 
ing hundreds  of  small  dikes,  true  felsite,  which  does  not  reveal 
a  holocrystalline  ground -mass  under  the  microscope,  is  rather 
rare.  The  largest  mass  which  has  been  observed  is  at  the 
northeast  corner  of  the  large  ledge  of  granite  on  the  south 
shore  of  Lyford's  Liking,  south  of  Round  Hill.  This  is  a 
brown,  thoroughly  compact  or  felsitic,  structureless  rock;  and 
forms  a  mass  several  yards  across,  just  at  the  water's  edge. 
The  contacts  with  the  granite  are  not  clearly  exposed  ;  but  it  is 
evidently  a  dike  in  the  granite.  Small  dikes  of  a  true  quartz- 
porphyry  have  also  been  observed  on  the  Jerusalem  Road,  eai^t 
of  Green  Hill.  The  great  abundance  of  pebbles  of  felsite  in 
the  Nantasket  conglomerates  indicates  that  this  rock  was  for- 
merly much  more  extensively  developed  in  this  district.  It 
probably  occurred  chiefly  in  the  form  of  broad  surface  flo\As, 
such  as  still  exist  in  other  parts  of  the  Boston  Basin  ;  and 
the  dikes  of  both  felsite  and  micro-granite  probably  date  from 
these  volcanic  eruptions,  being  branches  from  the  main  fissures 
or  necks  throuo-h  which  the  felsite  reached  the  surface. 

Although  these  three  types  of  granite  clearly  reveal  the 
chronoloo-ical  succession  described  above,  it  can  not  be  shown 
that  they  are  widely  separated  in  time  or  belong  to  entirely 
distinct  periods  of  igneous  activity  ;  and  it  is  especially  obvious 
that  the  micro-granite  and  felsite,  although  cutting  the  coarse 

OCCAS.  PAPERS  B.  S.  N.  H.     IV.     2 


18 

granite,  should,  on  account  of  their  resemblance  to  it  in  com- 
position, be  regarded  as,  in  a  general  view,  essentially  syn- 
chronous. 

This  coast  abounds  in  instructive  exposures  of  the  granitic 
rocks,  but  the  nearly  continuous  belt  of  wave-washed  ledges 
along  the  Jerusalem  Road  between  Nantasket  and  Beach  Island 
is  particularly  worthy  of  thorough  study,  illustrating,  as  it  does, 
the  relations  of  all  the  rocks  and  presenting  some  features  which 
are  not  clearly  exposed  elsewhere.  On  the  south  side  of  Beach 
Island,  as  already  noted,  there  is  a  fine  development  of  the 
coarsely  crystalline,  massive  granite  ;  but  from  the  junction  of 
this  with  the  mixed  granite  and  diorite  just  south  of  the  artificial 
harbor  to  Kantasketthe  finer-grained,  dark-colored,  older  granite 
is  the  principal,  and  for  a  considerable  portion  of  the  distance, 
excepting  the  dikes,  almost  the  only,  rock.  This  older  granite 
is  characterized  throughout  the  entire  section  by  a  remarkable 
gneissoid  structure  due  to  the  flowing  of  the  material  while  in  a 
viscous  condition.  At  many  points  this  flow-structure  is  so 
perfectly  developed  that  the  rock  presents  a  distinctly  gneissic 
character  even  in  small  hand-specimens  ;  and  where  the  fluidal 
lines  are  least  obvious,  they  are  still  traceable  on  the  broad, 
clean  surfaces  of  the  ledges.  The  trend  of  the  flow-structure  or 
pseudo-stratification  varies  somewhat.  It  is  about  N.E.— S.W. 
on  Beach  Island  ;  while  farther  west  it  ranges  usually  between 
N.-S.  and  N.W.-S.E.,  but  becomes  N.  60°  W.  as  we  approach 
the  l:)cach  leading  to  Green  Hill.  The  dip  is  usually  nearly 
vertical,  although  sometimes  as  low  as  60°  or  less. 

This  gneissoid  granite  encloses  at  most  points  numerous  frag- 
ments of  diorite  of  varying  lithological  character.  These  exhibit 
a  great  range  in  size  ;  and  they  are  usually  more  or  less  elongated 
in  form,  being  often  distinctly  lenticular  and  indicating  in  their 
smooth  and  somewliat  indefinite  outlines  a  partial  fusion  of  the 
diorite  in  the  melted  granite.  The  elongated  fragments  coincide 
in  direction  with  the  flow-structure  of  the  granite,  and  this  struct- 
ure is  always  most  perfect  where  the  diorite  is  most  abundant. 
The  Ibrm  of  the  diorite  fragments  means,   probably,  that  the 


19 

diorite  also  possessed  a  gneissic  structure  of  some  sort  I^efore  tlic 
eruption  of  the  granite  ;  in  fact  this  can  frequently  be  observed 
in  the  more  lenticular  masses  of  diorite.  The  diorite,  then,  has 
been  injected  by  the  granite  chiefly  along  its  own  original  struct- 
ure-planes ;  and  the  flow-structure  of  the  granite  is  probably  due 
largely  to  its  having  come  up  between  these  parallel  walls  of 
diorite. 

Breaking  through  this  gneissoid  granite  and  the  enclosed  dio- 
rite at  irregular  intervals  are  many  small  and  some  large  masses 
of  the  coarse,  light-colored  granite.  These  usually  conform  in 
direction,  at  least  approximately,  with  the  flow-structure  of  the 
older  granite,  and  exhibit  in  this  direction  a  similar  but,  as  a 
rule,  less  distinct  flow-structure.  We  thus  not  only  have  a 
somewhat  gneissoid  diorite  of  probable  igneous  origin  broken 
through  very  profusely  by,  and  enclosed  in,  a  highly  perfect 
gneissoid  granite  —  a  true  eruptive  rock  which,  but  for  its  rela- 
tions to  the  diorite,  might  readily  be  classed  as  of  sedimentary 
origin,  a  true  gneiss  ;  but  both  of  these  terranes  are  traversed 
again  and  again  in  a  way  possible  only  to  an  igneous  rock,  by 
a  second  and  more  acid  pseudo-gneiss.  Again,  all  the  preceding- 
rocks  are  injected  by  occasional  small  and  irregular  dikes  of  micro- 
granite  and  felsite,  which,  for  the  most  part,  are  devoid  of 
gneissic  structure.  And,  finally,  this  entire  sequence  of  granitic 
eruptions  is  divided  by  a  well-defined  series  of  porphyrite  dikes 
and  no  fewer  than  three  distinct  systems  of  diabase  dikes  ;  all 
of  which  will  be  described  in  connection  with  the  dikes  of  Nan- 
tasket. 

This  shore  undoubtedly  presents  the  best  general  section  of 
the  plutonic  rocks  of  the  Boston  Basin.  Certainly  at  no  other 
point  do  we  find  plutonic  masses  of  so  many  different  ages  so 
clearly  exposed  in  their  normal  relations  ;  and  in  this  connection 
it  should  be  noted  that  the  passage  (in  natural  sequence)  from 
the  diorite  through  the  more  basic  to  the  more  acid  granite 
affords  some  indication  that  the  diorite  is  not  widely  separated 
in  time  from  the  granites,  the  entire  series  constituting  but  one 
complete  igneous  cycle. 


THE  ROCKS  OF  NANTASKET.. 

GENERAL    RELATIOXS    AXD    ORIGIX. 

Although  the  surface  exposures  of  the  rocks  of  the  Nantasket 
area  are  completely  isolated  by  drift  deposits  and  the  sea,  these 
strata  are  probably  continuous  to  the  north  and  west  with  the 
great  body  of  sediments  occupying  the  Boston  Basin.  And  it 
is  certain,  as  will  appear  later,  that  they  are  terminated  on  the 
south  by  profound  dislocations  ;  so  that  the  sharply  defined 
boundary  between  the  Nantasket  sediments  and  the  broad  area 
of  granite  can  not  be  regarded  as  marking  the  true  original 
border  or  maximum  extension  of  the  Boston  Basin  in  this  direc- 
tion. On  the  contrary,  the  facts  point  very  plainly  to  the  con- 
clusion that  the  basin  rocks  formerly  extended  a  considerable 
but  undetermined  distance  beyond  this  line  ;  being  here,  how- 
ever, on  the  upthrow  side  of  the  great  faults,  they  were  lifted 
above  the  present  plane  of  erosion.  We  may,  nevertheless,  en- 
tertain the  hope  that  future  investigation  will  reveal  upon  the 
granitic  plateau  outlying  remnants  of  the  sedimentary  series, 
and  thus  indicate  more  exactly  the  original  limits  of  the  basin. 

The  Nantasket  rocks,  above  the  fundamental  granite,  and 
omitting  the  dikes,  consist  chiefly  of  the  conglomerate  (Roxbury 
pudding  stone)  and  the  interbedded  lavas  and  tuifs.  The 
incompleteness  of  the  Nantasket  section  is  plainly  shown  in  the 
entire  absence  of  the  great  slate  series,  which  elsewhere  in  the 
Boston  Basin  overlies  the  conglomerate  ;  and  it  is  probable,  as 
will  appear  when  the  facts  are  presented,  that  the  upper  members 
of  the  conglomerate  series  are  also  wanting.  The  student  will, 
however,  find  compensation  for  these  deficiencies  in  the  magnifi- 
cent development  of  the  basal  beds  of  the  conglomerate.  At  no 
other  point  are  the  floor  of  the  Boston  Basin  and  the  strata  rest- 


21 


<r[[ 


ing  cTirectly  upon  it  so  clearly  and  instructively  exposed.  On 
the  other  hand,  the  eccentric  position  of  Nantasket  with  reference 
to  the  basin  as  a  whole  renders  it  a  natural  if  not  a  necessary 
inference  that  the  beds  which  are  here  seen  to  re[)ose  upon  the 
granite  are  not  the  lowest  or  oldest  sediments  of  this  series  ; 
but  these  must  be  sought  farther  north,  along  the  axis 
of  the  basin,  which,  according  to  the  view  of  a  progressive  sub- 
sidence during  the  deposition  of  the  conglomerate  and  slate, 
must  be  the  deepest  and  oldest  part  of  the  trough. 

Throughout  the  Nantasket  area  the  true  conglomerate,  as 
distinguished  from  the  tuff,  is  largely  composed  of  granitic 
detritus  ;  but  this  material  is  especially  prominent  where  the 
lower  beds^pf  conglomerate  lie  immediately  upon  the  granite. 
The  volcanic  rocks  intercalated  in  the  sedimentary  series  include 
three  principal  kinds :  melaphyr,  melaphyr-tuff  and  porphy- 
_rite  ;  and,  as  will  appear  in  the  detailed  descriptions,  the  evidence 
for  the  contemporaneous  origin  of  these  igneous  sheets  is  well 
nigh  perfect  at  every  point.  It  has  been  found  impracticable, 
in  the  main,  to  determine  satisfactorily  the  positions  of  the  vol- 
canic vents.  It  appears  probable,  however,  that  the  eruptions 
were  chiefly  submarine ;  while  the  tuffs  would  seem  to  indicate 
that  the  lavas  issued  to  some  extent  from  craters  rather  than 
fissures  ;  and  we  may  well  suppose  that  these  ancient  igneous 
channels,  of  whatever  form,  are  still  buried  beneath  their  own 
ejectamenta.  It  will  be  shown,  however,  that  the  source  of  the 
more  acid  lava,  the  porphyrite,  is  probably  indicated  in  part  at 
least  by  the  dikes  of  porphyrite  along  the  Cohasset  shore  ;  and 
of  the  oldest  melaphyr  by  the  great  dike  of  melaphyr  (2)  in  the 
western  area. 

Briefly  stated,  then,  the  history  of  the  Nantasket  strata  is  sub- 
stantially as  follows  :  The  district  now  known  as  the  Boston  Basin , 
which  had  previously  been  a  land  surface,  experienced  a  gradual 
but  profound  subsidence,  during  the  progress  of  which  the  sea 
slowly  encroached  upon  what  is  now  the  Nantasket  area.  The 
abundant  detritus  which  must  have  resulted  from  the  sub-aerial 
chemical  decay  of  the  granitic  and  other  crystalline  rocks  of  this 
region  during  long  preceding    ages  —  a  thick    sheet    of  seden- 


22 


tary  soil  such  as  may  now  be  observed  in  low  latitudes  —  was 
rapidly  worked  over  by  the  advancing  tide  to  form  extensive 
beds  of  gravel  and  sand  on  the  beaches  and  near  the  shore, 
while  the  clayey  constituent  of  the  soil  was  carried  far  out  into 
the  deeper  waters  of  the  bay. 

Not  long,  apparently,  after  the  formation  of  these  strata 
began,  and  as  a  result,  probably,  of  the  same  agency  that  gave 
rise  to  the  subsidence,  volcanic  phenomena  were  instituted  in 
different  parts  of  the  Boston  Basin  ;  and  for  a  long  time  Mas- 
sachusetts Bay  must  have  borne  some  resemblance  to  the  mod- 
ern Bay  of  Naples.  But  since,  as  already  stated,  the  eruptions 
were  largely  submarine,  this  region  doubtless  exhibited  a  still 
more  perfect  combination  of  aqueous  and  igneous  activity. 
Sheets  of  basic  and  sub-acid  lavas  of  greatly  varying  thickness 
and  extent  were  poured  out  over  the  newly  formed  beds  of  gravel 
and  sand  and  were  in  turn  covered  by  these  ever-growing  de- 
posits, the  overlying  gravel  enclosing  many  fragments  Avorn 
from  the  lava  itself.  The  different  flows  of  lava  represented 
in  the  same  vertical  section  are  usually  separated  by  considera- 
ble beds  of  conglomerate ;  but  sometimes  the  eruptions  suc- 
ceeded each  other  so  rapidly  as  to  produce  composite  beds  of 
lava,  or  lava  and  tuff,  several  hundred  feet  thick  without  any 
interlarded  sediments.  Some  dikes  of  porphyrite  and  at  least  two 
of  melaphyr  probably  date  from  this  period  and  may,  perhaps, 
be  regarded  as  channels  through  which  the  lava  reached  the  sur- 
face. But  the  great  majority  of  the  dikes  traversing  the  conglom- 
erate and  melaphyr  are  diabase  and  appear  to  have  been  formed 
long  subsequently,  when,  after  the  upper  conglomerate  and  a 
great  thickness  of  slate  had  been  quietly  deposited  over  this  area, 
the  strata  were  tilted  up  and  extensively  faulted.  The  geologic 
revolution  marking  the  close  of  this  period  in  the  history  of  the 
Boston  Basin  was  not  attended  by  severe  plication  in  the  Nan- 
tasket  area  ;  but  faulting  is  by  far  the  more  prominent  structural 
feature.  The  subsequent  history  of  this  area,  during  all  the 
long  ages  down  to  the  glacial  epoch,  is  recorded  only  in  the 
quiet  erosion  which  has  swept  away  the  great  slate  series  and, 
at  some  points,  the  entire  thickness  of  the  conglomerate  arid 
interbedded  lavas. 


23 


GENERAL    .STRUCTURE    OF    THE    NANTA8KET    ARRA. 

The  main  structural  features  arc  so  clearly  expressed  on  iIk; 
map  that  this  topic  may  be  treated  very  summarily.  Nantasket 
is  a  inonoclinal  area ;  and,  although  it  has  been  completely 
shattered  by  numerous  faults  and  dikes,  the  strata  exhibit  only 
low  and  little  variable  dips,  being  almost  unique  in  this  I'e- 
spect  among  the  sediments  of  the  Boston  Basin.  Along  the 
northern  margin,  from  Atlantic  Hill  to  Rocky  Neck,  the  pre- 
vailing dip  is  south-southeast  J. 0°  to  20°.  But  southward,  or 
toward  the  granite  border,  the  dip  usually  diminishes,  and 
becomes  more  easterly,  the  beds  immediately  adjoining  the 
granite  being  often  horizontal  or  havino-  a  slio-ht  inclination 
obliquely  away  from  the  granite.  These  gently  inclined  beds, 
together  with  the  granite  floor  on  which  they  rest,  are  broken 
by  longitudinal  and  transverse  faults  into  a  series  of  blocks. 
A  glance  at  the  map,  on  which  the  faults  are  represented  by 
broken  black  lines,  shows  that  these  blocks  are  extremely  vari- 
able in  form  and  size  ;  and  it  is  somewhat  surprising  that  the 
unequal  up  and  down  movements  or  jostling  of  these  numerous 
earth-blocks  could  take  place  without  greater  disturbance  of 
their  bedding-planes. 

The  principal  longitudinal  faults,  such  as  those  immediately 
bordering  the  granite  and  the  profound  fracture  which  skirts 
the  southern  shore  of  Nantasket  Harbor,  traversing  the  entire 
district  from  Strait's  Pond  to  Weir  River  Bay,  downthrow  to 
the  north ;  and  the  displacements  are  so  great  that,  although 
the  bedded  rocks  dip  toward  the  granite,  in  receding  from  the 
granite  northward  we  pass  in  general  from  older  to  newer 
strata.  The  continuity  of  the  strata  is  interrupted  so  frequently 
and  completely,  both  longitudinally  and  transversely,  by  the 
numerous  faults  ;  and  the  conglomerate  is,  in  the  main,  so 
homogeneous  and  the  tuflf  so  local ;  that  we  are  obliged  to 
depend  very  largely  upon  the  lithological  characters  of  the 
interbedded  lavas  in  correlating  adjacent  ledges  as  well  as  the 
more  widely  separated  parts  of  the  field.     The  sequence  of  the 


24 

bedded  rocks  of  the  entire  area  can  not  be  read  off  directly 
from  any  more  or  less  continuous  or  connected  horizontal  sec- 
tions. But  we  find  instead  numerous  isolated  vertical  sections, 
which  can  only  be  correlated,  if  at  all,  by  a  critical  ledge-to- 
ledge  study  and  comparison.  The  best  results  of  such  an  inves- 
tigation are  presented  in  the  subjoined  general  section  of  the 
Nantasket  strata ;  from  which  it  appears  that  above  the  granite 
floor  there  are  no  fewer  than  six  beds  of  conglomerate,  alterna- 
ting with  five  sheets  of  mclaphyr  and  tuff  and  one  sheet  of  por- 
phyrite. 

Table  of  the  Nantasket  Strata.  ■ 

Granite,  bordering  and  underlying  the  bedded  rocks. 

First  Conglomerate,  basal,  resting  on  granite 25-50  feet. 

Rocky  Neck,  Granite  Plateau,  Clifl' Plateau,  etc. 
First  Melaphyr,  compact  and  jaspery 25-40  feet. 

Rocky  Neck,  East  and  West  Porphyrite  Hills  and  Clitf  Plateau. 
Second  Conglomerate,  jaspery 25-50  feet. 

Rocky  Neck,  East  and  West  Porphyrite  Hills,  Great  Hill,  etc. 
Porphyrite 50-100  feet. 

East  and  West  Porphyrite  Hills,  Black  Rock,  etc. 
Third  Conglomerate 50-100  feet. 

Conglomerate  Hill  and  Plateau,  Folsom's  Island.  Kound  Hill 
and  Green  Hill. 
Second  Melaphyr,  green  and  araygdaloidal 20-30  feet. 

Crescent  Hill,  Melaphyr  Plateau,  and  South  Sliore. 

Fourth  Conglomerate 20-30  feet. 

Crescent  Hill,  Marsh  Island,  etc. 
Third  Melaphyr,  green  and  amygdaioichil 35-45  feet. 

Rocky  Neck,  Melaphyr  Peninsula  and  Plateau,  Crescent  Hill 
and  Marsh  Island. 
Fifth  Conglomerate 25-30  feet. 

Rocky  Neck  and  Long  Beach  Rock. 
Fourth  Melaphyr  and  Tuff,  green,  compact  or  brecciated     .     300-450  feet. 

Atlantic  and  Centre  Hills,  Gun  Rock,  Little  Bhick  Rock,  etc. 

Sixth  Conglomerate 10-15  foc'j. 

Rocky  Neck. 
P'^ifth  Melaphyr,  green  and  amygdaloidal  .    , 15-20  feet. 

Rocky  Neck. 

600—960 


25 

The  assigned  thicknesses  arc  in  part  actual  measurements  ; 
but  chicrty  estimates  and  approximations.  They  arc  intended, 
also,  to  give  some  idea  of  the  variations  in  thickness  observed  in 
tracing  the  individual  beds  from  one  [)art  of  the  field  to  another. 

While  it  must  be  admitted  that  the  data  upon  which  this 
table  rests  are  in  some  respects  much  less  conclusive  and  more 
ambiguous  than  could  be  desired,  it  may  be  fairly  considered 
a  conservative  determination  as  regards  the  number  of  distinct 
terranes  involved ;  since  in  all  cases  where  the  indications 
appeared  to  be  evenly  balanced  that  interpretation  of  the  facts 
has  been  preferred  which  avoided  multiplying  the  alternations 
of  conglomerate  and  lava.  It  is  quite  possible,  indeed,  that  in 
some  instances  what  have  been  regarded  as  parts  of  one  continu- 
ous sheet  of  lava  are  really  entirely  distinct  and,  perhaps,  not 
even  synchronous  flows.  This  view  would,  however,  seem  to 
carry  the  further  supposition  that  some  of  the  flows  failed  to 
extend  over  the  entire  field.  This  is  not,  a  priori,  improbable  ; 
though  few  facts  have  been  observed  which  seemed  to  demand 
this  explanation.  Some  of  the  tuff"  beds  are  unquestionably 
very  local,  the  underlying  and  overlying  melaphyrs  coming 
together  abruptly  at  their  margins  ;  but,  although  carefully 
sought,  no  corresponding  indications  have  anywhere  been 
detected  in  the  melaphyr,  except  in  the  case  of  the  flow  on 
Atlantic  and  Centre  Hills  which  appears  to  be  terminated  on 
the  west  by  the  junction  of  the  underlying  and  overlying  tuffs. 
It  is  very  clear,  however,  that,  as  already  stated,  the  melaphyr 
flows  are  composite  in  some  cases,  several  eruptions  having  fol- 
lowed each  other  in  quick  succession  over  the  same  area.  This 
is  conspicuously  true  of  the  fourth  melaphyr,  the  great  sheet 
forming  Atlantic  and  Centre  Hills,  etc. 

Although,  as  the  map  shows,  few  of  the  Nantasket  faults 
can  be  classified  as  distinctly  longitudinal  or  transverse,  the 
greater  number  are  either  approximately  east- west  or  north- 
south  in  direction.  It  is  also  noticeable  that  the  blocks  bounded 
by  the  principal  faults  are  in  several  instances  broken  into 
quite  narrow  wedges  by  a  series  of  parallel  east- west  faults. 


26 

The  arrows  pointing  away  from  the  fault  lines  on  the  map 
indicate  the  direction  of  the  downthrow  in  each  case  ;  and  the 
accompanying  figures,  when  present,  express  the  amount  of 
the  displacement  in  feet  wherever  it  could  be  even  approxi- 
mately determined. 

The  map  also  brings  out  very  clearly  the  pi-incipal  facts  con- 
cerning the  dikes  of  Nantasket  —  their  sizes,  trends,  distribu- 
tion and  correlation  in  different  outcrops.  Mr.  ^Merrill  has 
shown  that  they  consist  almost  exclusively  of  diabase,  and  are 
all  highly  altered  ;  although  they  exhibit  some  notable  differ- 
ences in  texture  and  degrees  of  alteration.  The  dikes  are 
numbered  on  the  special  map,  and  the  numbers  are  repeated 
for  each  important  outcrop,  so  as  to  express  more  clearly  their 
individual  continuity.  The  most  of  the  dikes  are  readily  arranged 
by  their  trends  in  three  distinct  systems.  The  dominant  trend  is 
manifestly  approximately  east-west.  But  these  are  seen  on  closer 
inspection  to  embrace  two  systems,  diverging  20°  to  30°.  The 
normal  trend  of  the  first  or  oldest  system  is  N.  75°  to  80°  E., 
and  of  the  second  S.  75°  to  80°  E.  Their  relative  ages  are  in- 
dicated by  a  slight  difference  in  texture  and  especially  by  a  very 
clear  intersection  on  the  shore  east  of  Gun  Rock  and  several  inter- 
sections on  East  Porphyrite  Hill.  The  third  system  trends 
due  north-south  and  is  the  newest  of  all,  as  shown  by  two 
intersections  of  dikes  of  the  second  system  :  Dike  (32  cuts  dikes 
13,  14  and  15  of  the  second  system  in  Green  Hill  Ledge  very 
clearly  indeed;  and  the  composite  dike  (66)  in  the  gorge 
between  East  Porphyrite  Hill  and  Cliff  Plateau  certainly  cuts 
31  of  the  second  system  which  in  turn  cuts  22  and  24  of  the 
first  system  on  East  Porphyrite  Hill.  Notwithstanding  the 
regularity  of  their  forms  and  trends,  the  dikes,  with  the  partial 
exception  of  the  newest  system,  exhibit  a  remarkable  independ- 
ence of  the  best  developed  systems  of  joint-planes.  This  is 
especially  noticeable  along  the  northern  bases  of  Atlantic  and 
Centre  Hills,  indicating  tiiat  the  older  dikes,  at  least,  probably 
antedate  the  joint-structure  of  the  rocks  to  some  extent.  A 
further  inspection  of  the  map  will  show,  however,  tliat  not  only 


27 

do  some  of  tlic  (likes  eoineide  in  ])0!silion,  but  they  all  eoinciflc 
in  direetion,  witli  the  t";iult-line.s  oi"  tlic  distriet.  Thi,s  ueneral 
coincidence  in  trend  of  the  faults  and  dikes  is  a  very  sinuilicanl 
fact,  indicating  that  the  dikes  date  from  the  period  of  general 
disturbance,  when  the  rocks  were  tilted  and  faulted.  The  ma[) 
also  brings  out  the  fact  that  the  east-west  dikes  occur  in  three 
distinct  zones  or  belts,  marking  the  three  principal  dislocations 
of  the  Nantasket  area. 


LITHOLOGY. 

To  avoid  unnecessary  repetition  in  the  detailed  descriptions 
of  the  Nantasket  ledges  we  may  advantageously  notice  in 
advance  the  general  lithological  characters  of  the  principal 
types  of  rocks.  These  fall  naturally  into  three  classes,  as 
follows  : — 

(1).  Sedimentary  rocks,  consisting  chiefly  of  the  conglom- 
erates. 

(2).  The  older  eruptive  rocks,  embracing  the  diorites, 
granites  and  felsites. 

(3).  The  newer  eruptive  rocks,  including  both  dikes  and 
lava-flows. 

The  third  division  is  by  far  the  most  important ;  and  it  is  a 
matter  for  sincere  congratulation  that  it  has  been  worked  up 
by  a  competent  specialist. 


The  Sedimentary  Rocks. 

The  Nantasket  conglomerate  is,  with  unimportant  exceptions, 
chiefly  composed  of  well  rounded  or  water  worn  fragments  of 
granite,  felsite,  melaphyr  and  porphyrite.  On  account  of  the 
predominance  of  the  granite  and  felsite  debris,  the  conglomer- 
ate is,  in  general,  light-colored,  pinkish  or  reddish.     It  has  in 


28 

the  main  the  medium  texture  of  the  Roxbury  pucldingstone,  the 
pebbles  not  exceeding  four  inches  iu  diameter ;  but  at  some 
points,  such  as  Great  Hill,  Green  Hill  Ledge  and  the  small 
area  north  of  Crescent  Hill,  they  are  in  part  much  larger,  one 
to  two  feet.  The  larger  pebbles  are  chiefly  granite  and  mela- 
phyr,  the  felsite  having  been  always,  as  now,  too  brittle  and 
too  finely  jointed  to  form  many  large  pebbles.  The  conglom- 
erate is  usually  a  typical  [)uddingstone,  presenting  distinctly 
the  character  of  a  breccia  only  where  it  immediately  overlies  the 
granite,  and  is  largely  composed  of  local  granitic  debris. 

It  is  very  noteworthy  that  the  quartzite  pebbles,  Avhich  are 
such  a  prominent  feature  of  the  conglomerate  in  most  parts  of 
the  Boston  Basin,  are  almost  entirely  wanting  in  the  Nantasket 
ledges;  indicating  that  formerly,,  as  at  the  present  time,  the 
ancient  quartzites  had  but  a  slight  development  on  this  side  of 
the  basin.  The  same  reasoning  will  not,  however,  explain  the 
general  absence  of  pebbles  of  diorite  in  the  conglomerate. 
Next  to  granite,  diorite  is  by  far  the  most  abundant'of  the  older 
rocks  bordering  the  Boston  Basin,  covering;  an  aoorreo-ate  area 
of  at  least  one  hundred  square  miles  within  ten  miles  of  the  pres- 
ent margin  of  the  basin.  Since  the  diorite  is  everywhere  inter- 
sected by,  and  therefore  older  than,  the  granites,  it  is  unques- 
tionably also  older  than  the  conglomerate  ;  and  we  are  obliged 
to  suppose  that,  like  the  other  of  our  older  rocks,  it  was  proba- 
bly abundantly  exposed  over  the  surface  of  this  region  during 
the  formation  of  the  conglomerate.  A  few  pebbles  in  the  con- 
glomerate, in  different  parts  of  the  basin,  may  be  doubtfully 
referred  to  the  diorite ;  but,  generally  speaking,  this  rock  has 
contributed  little  or  nothing  to  the  composition  of  the  conglom- 
erate. Bowlders  and  finer  debris  of  the  diorite  are  extremely 
abundant  in  the  glacial  drift  of  this  section  ;  but  it  is  noticeable 
that  the  smaller  masses,  especially,  are  usually  more  or  less 
decomposed,  showing  much  more  alteration  than  similar  frag- 
ments of  granite  or  felsite.  If,  therefore,  as  already  suggested, 
we  may  follow  Mr.  liouxe'  in  attributing  the  formation  of  the 
early  sediments  of  tliis  r(\ni(in  to  the  woi'kingover  by  the  sea  of 

'  Proc.  B.  S.  N.  H.,  xxin.,  2'J-M. 


29 

a  thick  sheet  of  sedentary  soil  or  clu'inical  detritus,  it  is  obvious 
that  the  more  basic  rocks,  hke  dioritc  and  diabase,  must  have 
been  more  deeply  and  thoroughly  decayed  than  the  more  acid 
rocks,  like  granite  and  felsite,  the  latter  being  intermediate  in 
this  respect  between  the  former  and  the  purely  silicious  rocks — 
the  quartzites.  The  debris  of  the  diorite  is,  therefore,  to  be 
sought  in  the  slate  rather  than  in  the  conglomerate  ;  and  we 
must  recognize  the  principle  that  the  occurrence  of  older  rocks 
in  the  conglomerate  is  not  in  the  order  of  their  abundance  so 
much  as  of  their  chemical  stability. 

Limited  layers  of  finer  sediment — grit  and  sandstone — by 
reference  to  which  the  dip  and  strike  may  be  determined,  occur 
rather  sparingly  in  most  of  the  ISTantasket  ledges,  as  in  Long- 
Beach  Rock,  Green  Hill  Ledge  and  Great  Hill ;  although  they 
are  practically  wanting  in  several  of  the  largest  masses,  includ- 
ino;  Cono*lomerate  Plateau  and  Cono-lomerate  Hill.  The  con- 
glomerate  is  throughout  very  firm  and  thoroughly  consolidated, 
breaking,  usually,  without  reference  to  the  contours  of  the 
almost  adamantine  pebbles.  In  fact,  the  paste,  including  the 
intercalated  sandstone  layers,  is  often  so  intensely  hard  as  to 
suoro-est  that  the  volcanic  heat  of  the  interbedded  lavas  has  been 
an  important  agent  in  lithifying  these  ancient  gravels.  This  is 
undoubtedly  a  true  cause,  but  still  it  does  not  afford  an  entirely 
adequate  explanation  of  the  phenomena,  since  the  conglomerate 
beds  are  usually  well  indurated  through  their  entire  thickness, 
even  the  lower  layers  of  gravel,  which  were  deposited  upon  a 
cold  and  eroded  surface  of  lava,  being  sensibly  as  hard  in  most 
cases  as  the  upper  layers,  which  were  covered  by  a  stream  of 
incandescent  lava.  There  are,  on  the  other  hand,  many  facts 
which  indicate  that  the  intense  lithification  is  largely  due  to  the 
deposition  of  secondary  silica  over  and  between  the  grains  com- 
posing the  paste,  thus  virtually  changing  the  arenaceous  matrix 
of  the  pebbles  to  quartzite.  This  interstitial  silica  is  especially 
obvious  where  it  takes  the  form  of  distinct  seo-reo^ations  of  red 
jasper,  as  in  the  western  part  of  Nantasket  and  on  Kocky  Neck. 


30 

Its  source  is  undoubtedly  to  be  found  chiefly  in  the  alteration 
of  these  basic  lavas  ;  and  it  occurs  even  more  abundantly  in  the 
melaphyr  itself  than  in  the  conglomerate,  in  the  form  of  irreg- 
ular segregations  and  veins  of  jasper,  chalcedony  and  vitreous 
quartz. 

The  Older  Eruptive  or  Granitic  Rocks. 

Both  the  diorite  and  the  older,  dark  colored  granite  of  the 
Cohasset  shore  are  practically  wanting  in  the  Nantasket  area  ; 
the  granitic  rocks  being  limited  to  the  coarsely  crystalline,  acid 
type,  excepting  a  few  small,  irregular  and  ill-defined  dikes  of 
finely  crystalline  granite  (micro-granite)  and  felsite.  The  ex- 
traordinary abundance  of  pebbles  of  quartz-porphyry  and  other 
varieties  of  felsite  in  the  conglomerate  is  a  plain  indication  that 
these  rocks  were  far  more  extensively  developed  over  the 
granitic  area  in  early  times  than  now,  forming,  probably,  sur- 
face flows  as  well  as  dikes.  The  coarsely  crystalline,  acid 
granite  of  Nantasket  is  essentially  similar  to  that  of  Cohasset 
and  the  South  Shore  generally  ;  and  little  need  be  added  to  the 
statements  of  the  preceding  pages.  Over  Granite  Plateau  nnd 
.Granite  Point  and  on  Rocky  Neck  the  relations  of  the  basal 
conglomerate  to  the  granite  are  very  intimate  and  intricate  ; 
but  absolutely  no  facts  have  been  observed  which  indicate  that 
the  granite  is  intrusive  in  the  conglomerate,  or  that  any  of  the 
granitic  rocks  of  the  Nantasket  region  are  more  recent  than  the 
sedimentary  rocks.  On  the  contrary,  the  relations  of  the  two 
series,  as  will  appear  later,  are,  at  every  point,  consistent  with 
the  view  that  the  conglomerate  was  deposited  over  an  uneven, 
fissured  surface  of  the  granite,  such  as  that  rock  presents  to-day 
at  many  points. 


The    Newer    Eruptive    Rocks. 

HY    GEORGE    P.    MERRILL. 

The  more  recent  eruptive  rocks  of  Nantasket,  including 
all  those  of  later  date  than  the  diorite,  granite  and  felsite, 
occur  in  the  form  of  (1.)  lava-flows  and  (2)  dikes.  The  lava- 
flows  include  both  melaphyrs  and  porphyrites  and  these  vol- 
canic types  are  also  found  among  the  dikes,  although  the 
latter  consist  chiefly  of  diabase.  The  melaphyrs  and  diabases 
have,  in  many  cases,  undergone  such  complete  alteration  that 
their  correct  identification  is  attended  with  the  greatest  difliculty. 
Indeed,  the  slide  often  shows  not  a  single  recognizable  original 
constituent ;  but  consists  wholly  of  a  compact  aggregate  of 
secondary  minerals,  among  which  are  epidote,  quartz,  kaolin, 
and  sundry  chloritic  and  ferruginous  products  for  which  there 
are,  perhaps,  no  better  names  than  the  terms  virulite  and 
o'pacite,  already  in  general  use. 

Perfectly  satisfactory  results  from  such  materials  could  be 
obtained  only  by  prolonged  and  careful  microscopic  examina- 
tions and  chemical  tests,  accompanied  by  study  in  the  field, 
where  the  rocks  could  be  observed  in  all  their  varying  aspects 
and  in  varying  stages  of  alteration.  Such  study  I  have  natur- 
ally been  unable  to  give  them,  and  my  main  effort  has  been  to 
so  identify  the  rocks  and  note  their  essential  differences,  if  any, 
as  to  aid  Professor  Crosby  in  his  diflficult  task  of  ascertaining 
their  field  relations. 

The  Lava-flows' or  Volcanic  Eocks. 

The  eff^usive  rocks  or  true  lavas  of  Nantasket  embrace,  as 
stated,  two  principal  types  —  melaphyr  and  porphyrite.  The 
melaphyr  is  the  predominant  type  and  forms  several  distinct 
flows  or  sheets,  of  varying  lithological  character  ;  while  the  por- 
phyrite, so  far  as  known,  may  all  be  referred  to  one  widely 
extended  sheet. 


JSh'Jii  plnjr. 

The  melaphyrs  throughout  the  Xantasket  area  differ  from 
the  diabases  in  being  of  a  finer  grain,  in  that  their  feldspars 
are  less  decomposed,  and  in  never,  so  far  as  observed,  contain- 
ing a  recognizable  unaltered  trace  of  either  augite  or  olivine  ; 
also  in  being,  as  a  rule,  more  or  less  amygdaloidal.  They 
also  carry  a  considerable  proportion  of  amorphous  base.  These 
characteristics  are  alone  sufficient  in  every  case  examined  to 
distinguish  the  so-called  melaphyrs  from  the  diabases  (or  the 
surface  flows  from  the  later  dike  rocks).  The  melaphyrs  also 
carry  a  larger  proportion  of  secondary  epidote,  and  less  of  the 
viridite,  these  two  substances  occupying  the  space  of  the  de- 
composed feldspars  and  augites  and  filling  the  amygdaloidal 
cavities.  The  feldspars  have  the  appearance  described  by 
Benton^ ;  and,  althouo-h  still  showinof  twinning'  striae,  are  more 
or  less  kaolinized  or  show  traces  of  the  viriditic  and  epidotic 
alteration  so  common  in  the  dike  rocks.  The  black  material 
of  the  ground-mass  is  in  part  magnetite,  as  shown  by  its  bluish 
reflections  and  by  its  being  strongly  attracted  by  the  magnet ; 
but  it  is  often  accompanied  by  blood  red  scales  of  hematite. 

The  amygdules  in  these  rocks  are  in  part  true  steam-holes 
or  vesicles,  as  shown  by  their  regular  oval  form,  sharply  de- 
fined walls,  and  the  arrangement  of  the  enclosed  and  adjacent 
minerals.  In  other  cases,  however,  they  are  merely  cavities 
left  by  the  decomposition  of  some  constituent  of  the  rock,  and 
are  irregular  in  outline,  poorly  defined  and  show  frequently  the 
remains  of  some  less  easily  decomposed  mineral,  as  a  feldspar, 
projecting  from  the  cavity  wall  inward.  These  ^>.<»e?f^/o-amyg- 
dules,  like  the  true  amygdules,  are  filled  with  a  variety  of 
secondary  minerals,  as  feldspar,  quartz,  epidote  and  chlorite, 
often  without  definite  order,  or  again  showing  a  quite  regular 
concentric  arrangement.  I  have  not,  however,  been  able  to 
find  the  marked  regularity  in  the  arrangement  of  these  minerals 
in  the  pseudo-amygdules  that  I  was  led  to  expect  from  a  peru- 

1  Proc.  B.  S.  N.  IL,  Vol.  xx.,  page  41C. 


33 

sal  of  Mr.  Benton's  paper.  The  chlorite  of  the  amygdules  is 
quite  different  from  the  so-called  viridite  of  the  diabases.  With 
a  low  power  and  by  ordinary  light  it  appears  like  a- continuous 
sheet  of  a  light  green  micaceous  mineral  viewed  perpendicularly 
to  its  cleavage  ;  but  with  a  power  of  400  or  500  diameters,  it 
is  seen  to  be  a  compact  mass  of  minute  scales  which  is  almost 
black  between  crossed  nicols,  but  which  polarizes  faintly  as  the 
stage  is  revolved  and  shows  at  the  same  time  that  the  individual 
foliae  have  a  fan-shaped  arrangement.  It  very  likely  represents 
the  final  stage  of  the  viriditic  alteration,  and  is  presumably  the 
same  as  the  dark  green  compact  chlorite  which  occurs  in  masses 
of  some  size  in  the  amygdules  of  the  melaphyr  at  Brighton. 

Numerous  other  minerals  of  secondary  origin  are  found  in 
the  cavities  and  crevices  of  the  melaphyr ;  but  as  they  occur 
only  in  minute  quantities  in  the  material  at  hand  and  usually 
without  well  defined  crystallographic  outlines,  their  correct  de- 
termination is  a  matter  of  great  difficulty,  and  in  some  cases 
would  be  impossible.  For  this  reason  and  because  they  do  not 
appear  to  have  any  important  bearing  upon  the  problems  under 
consideration  I  have  omitted  mention  of  them  in  my  descriptions. 

Fourth  Melaphyr,  Coastal  Area.  —  This  is  a  fine  grained 
greenish  rock,  with  few  macroscopic  constituents.  True  amyg- 
dules are  rarely  abundant  in  the  melaphyr  of  this  ai^ea,  but 
it  is  very  commonly  distinctly  brecciated,  considerable  masses 
having  a  highly  fragmental  aspect.  Under  the  microscope  it 
is  found  to  consist  of  the  usual  lath-shaped  plagioclases  with 
step-like  ends  imbedded  in  a  grayish  amorphous  ground-mass 
and  showing^  a  marked  fluidal  arrano-ement.  Much  secondary 
epidote  is  present,  occurring  in  scattered  grains,  in  strings  or 
veins  and  in  aggregates  filling  the  numerous  small  cavities  re- 
sulting from  the  decomposition  of  some  of  the  original  constitu- 
ents of  the  rocks.  There  is  also  present  a  considerable  amount 
of  viridite,  which  results  from  the  alteration  of  the  augites, 
and  is  also  derived  in  part  from  the  ground-mass.  This  at 
times  occupies  the  entire  interspaces  of  the  feldspars,  and,  act- 

OCCAS.  PAPERS  B.   S.  N.   H.     IV.  3 


34 

ing  but  faintly  on  polarized  light,  gives  them  the  appearance  of 
beino-  imbedded  in  a  light  greenish  glass . 

In  the  limited  bed  on  the  north  side  of  Atlantic  and  Centre 
Hills,  bomb-like  masses  of  melaphyr  are  highly  amygdaloidal, 
the  amygdules  varying  from  1  to  5  mm.  in  diameter.  These 
I  regard  as  true  amygdules.  They  are  filled  mainly  with 
quartz  and  chlorite,  though  often  a  little  epidote  is  present. 
In  such  cases  the  quartz  occupies  the  outer  zone  with  the 
chlorite  and  epidote  interiorly.  The  ground-mass  of  this  rock  is 
an  extremely  dense,  dark  brownish  gray,  amorphous  material 
bearing  minute  feldspar  microlites. 

The  pseudo-amygdules  of  this  melaphyr  are  commonly  much 
elongated,  and  contain  epidote  and  chlorite  with  more  or  less 
quartz.  The  epidote  occurs  as  minute  crystals,  a  single  row  of 
which  often  forms  a  border  around  the  wall  of  the  cavity  ;  while 
the  entire  central  portion  is  occupied  by  the  very  compact,  light 
green  chlorite  noted  above.  Outside  of  the  amygdules  or  seg- 
regations of  both  kinds,  scattered  through  the  entire  mass  of  the 
rock,  are  the  common  decomposition  products,  ferrite,  opacite, 
viridite  and  kaolin. 

A  typical  sample  of  melaphyr  from  this  area,  but  bearing 
some  secondary  quartz,  which  could  not  be  avoided,  yielded 
50.47  per  cent,  of  silica. 

Second  and  Third  Melaphyrs,  Central  Area. —  Themel- 
aphyrs  of  this  area  are  usually  of  a  deep  greenish  color  and  often 
highly  amygdaloidal ;  but  they  vary  greatly  in  botli  color  and 
texture.  A  common  type  is  a  dark  purplish  gray  rock,  which 
is  also  often  profusely  amygdaloidal.  Under  the  microscope 
its  color  is  seen  to  be  due  to  the  large  amount  of  opacite  and 
magnetite  in  the  ground-mass.  Secondary  epidote  is  abundant 
in  comparatively  large  grains,  which  are  often  surrounded  by  a 
dense  black  border  of  opacite.  The  amygdules  are  filled  with 
quartz,  epidote  and  feldspar.  The  quartz  and  epidote  prevail 
and  occur  in  the  form  of  an  extremely  fine  grained  and  compact 
aggregate  having    a   specific   gravity  of  about  2.!).      The  iron 


35 

oxides,  which  are  much  decomposed,  are  in  part  titan iferous,  as 
shown  by  theii*  peculiar  club-shaped  forms  so  characteristic  of" 
menaccanite.  Moreover,  the  pulverized  rock  gives  a  distinct  re- 
action for  titanium,  after  prolonged  digestion  with  hydrochloric 
acid.  The  green  melaphyr  differs  mainly  in  carrying  a  larger 
proportion  of  epidote  and  in  the  feldspars  having  undergone  the 
viriditic  alteration.  The  amygdules  of  the  rock  show  first  a 
zone  of  epidote,  and  interiorly  quartz,  feldspar  and  chlorite. 
A  typical  sample  of  the  melaphyr  from  this  area,  non-amygda- 
loidal  and  carrying  no  free  quartz,  yielded  on  analysis  41.89  per 
cent,  of  silica. 

First  Melaphyr,  Western  Area. — The  structural  features 
of  this  rock  are  greatly  obscured  by  decomposition,  but  after 
the  examination  of  several  sections  I  am  disposed  to  place  it  with 
the  melaphyrs.  The  microscope  shows  it  to  consist  of  a  large 
number  of  greatly  decomposed  porphyritic  plagioclases  imbed- 
ded in  a  groundmass  so  filled  with  secondary  decomposition 
products  that  its  original  character  is  wholly  obscured.  The  dull 
green  macroscopic  areas  are  epidote  resulting  from  the  feldspathic 
decomposition.  The  original  iron-magnesian  constituents  have 
quite  disappeared,  even  their  outlines  being  no  longer  recogniz- 
able. I  assign  the  rock  to  the  melaphyrs  with  consider- 
able hesitation.  Chemical  analysis  shows  its  basic  character, 
three  samples  from  different  localities  affording  the  following 
percentages  of  silica:  47.29,  47.97  and  51.05:  the  latter,  at 
least,  probably  including  some  free  secondary  quartz. 

Porphyrite. 

The  rock  which  I  here  have  called  porphyrite  differs  from 
the  melaphyrs  in  showing  under  the  microscope  a  larger  pro- 
portion of  irresolvable  ground-mass,  in  being  of  a  more  pro- 
nounced porphyritic  structure  and  in  never  so  far  as  observed 
being  truly  amygdaloidal.  They  are  also  more  compact  and 
fresh  appearing  and   show  in   none   of  the  sections  examined 


36 

traces   of  augite,  olivine  or  other  original  iron-magnesian  sili- 
cates. 

Western  Area. — The  porph^^rite  of  this  area  varies  from 
light  greenish  to  dull  red  and  purplish  in  color  and  is  well  de- 
scribed by  the  term  felsitic.  So  far  as  observed  none  of  its 
mineral  constituents  are  of  such  dimensions  as  to  be  recog- 
nizable by  the  unaided  eye,  nor  is  it  at  all  amygdaloidal,  a 
feature  so  pronounced  in  the  melaphyrs.  The  purple  variety, 
in  the  thin  section,  shows  a  densely  microlitic  base,  with  much 
opacite,  carrying  numerous  porphyritic  feldspars,  which  are 
mostly,  if  not  all,  triclinic.  A  few  of  the  feldspars  show  no 
twinning  stride ;  but  I  cannot  obtain  such  measnrements  as 
would  prove  these  to  be  certainly  monoclinic. 

One  of  the  sections  examined  shows  a  single  corroded  bleb 
of  original  quartz,  carrying  flu  id  al  cavities  and  moving  bubbles. 
Large  dnsky  apatites  are  not  rare.  In  the  greenish  variety 
there  is  also  present  a  little  viridite  from  the  feldspars,  and  a 
few  granules  of  secondary  epidote.  The  porphyrite  presents, 
usually,  a  well-marked  fluidal  structure.  Subjected  to  chemical 
analysis,  the  green  variety  yielded  58.02  per  cent,  of  silica, 
and  had  a  specific  gravity  of  2.73.  The  red  variety  is  more 
felsitic  and  also  more  decomposed.  Two  samples,  light  red, 
and  dark  red  in  color,  yielded  silica  as  follows  :  light,  56. (j()  per 
cent.  ;  dark,  5G.25  per  cent. 

Black  Rock. — Some  of  the  black  rock  porphyrite  more  re- 
sembles the  melaphyrs  in  external  appearance  than  does  that 
of  the  western  area.  Macroscopically  it  is  a  very  compact  and 
hard,  greenish  gray  to  dark  purplish  and  nearly  black  rock, 
bearing  small  whitish  feldspars,  and  secondar}'  epidote  in  crys- 
tals and  granular  aggregates  of  sufficient  size  to  be  distinguish- 
able by  the  naked  eye. 

Thin  sections,  under  the  microscope,  show  it  to  consist  of  a 
dense  ground-mass  of  needle-like  and  short,  stout  feldspar  micro- 
lites,  interspersed  with  numerous   brilliantly  polarizing  epidote 


37 

granules,  and  witli  little  if  any  truly  aniorplious  base.  Im- 
bedded in  this  feldspathic  ground-inass  are  numerous  large,  clear 
and  often  greatly  corroded  plagioclases.  A  few  small  apatites 
are  present.  The  section  shows  a  well-marked  flow-structure. 
Chemical  analysis  yielded  58.25  per  cent,  of  silica  ;  and  an  aver- 
age of  four  determinations  gave  a  specific  gravity  of  2.78. 

The  Dikes  or  Intrusive  Rocks. 

The  dikes,  as  has  been  noted  by  Professor  Crosby,  are  all  of 
normal  form — sharply  defined,  wall-like  masses  making  high 
angles  with  the  horizon  and  invariably  cutting  across  the  strat- 
ified rocks  ;  no  intrusive  beds  or  sheets  having  been  observed  in 
the  Nantasket  district,  owing,  probably,  to  the  absence  of  thin 
bedded  or  slaty  sediments.  As  previously  stated,  the  dikes 
consist  mainly  of  diabase ;  but  they  also  include  an  important 
series  of  porphyrite  dikes  and  at  least  one  large  dike  of  mela- 
.  phyr.  The  dikes  of  melaphyr  and  porphyrite  are  believed  by  Pro- 
fessor Crosby  to  be  contemporaneous  with  the  surface  flows 
of  those  rocks  ;  while  he  finds  that  the  numerous  diabase  dikes 
are  certainly  newer  in  most,  and  probably  in  all,  cases  than  the 
melaphyr  and  porphyrite.  Hence,  following  the  chronological 
order,  we  begin  with  the  last  named  rocks. 

The  Melaj^hyr  Dikes. 

The  dike  of  melaphyr  ( 1 )  on  the  north  side  of  Centre  Hill  is 
so  evidently  identical  with  the  melaphyr  which  it  intersects  that 
it  was  not  examined  microscopically.  The, large  dike  (2)  in 
the  western  area,  which  Professor  Crosby  regards  as  contem- 
poraneous with  the  first  flow  of  melaphyr,  is,  like  that  flow,  of 
doubtful  character.  Macroscopically,  it  bears  a  marked  resem- 
blance to  this  flow,  and  is  contrasted  in  appearance  with  all  the 
diabase  dikes  of  the  district.  Under  the  microscope  it  is  seen 
to  be  a  fresher  and  more  crystalline  rock  than  the  melaphyr, 
showing  in  the  section  a  ground-mass  of  feldspar  microlites  and 


38 

granules  injected  with  epidote  and  the  usual  ferruginous  decom- 
position products.  In  this  ground-mass  are  numerous  phen- 
ocrysts  of  feldspar  and  an  occasional  nearly  colorless  augite. 
There  are  numerous  granular  areas  of  secondary  epidote  ;  but 
no  trace  of  olivine  or  its  decomposition  products  ;  and  1  am 
inclined  to  doubt  its  presence  as  an  original  constituent.  Rec- 
ognizing the  uncertainty  attending  the  determination  of  such 
rocks  from  a  single  section,  I  can  only  say  that  I  am  inclined 
to  believe  the  rock  more  nearly  related  to  the  augite-porphyrites 
or  olivine-free  diabases  than  to  the  melaphyrs.  It  is  classed 
here  with  the  melaphyrs  simply  because  the  true  nature  of  the 
rock  is  not  yet  free  from  doubt,  and  that  arrangement  best 
accords  with  its  field  relations  as  worked  out  by  Professor 
Crosby.  The  analysis  of  a  single  specimen  gave  54.47  per 
cent,  of  silica. 

The  Porphyi'ite  Dikes. 

The  porphyrite  dikes,  so  far  as  observed,  are,  lithologically, 
essentially  similar  to  the  great  flow  of  porphyrite  on  Black 
Rock,  rendering  a  detailed  description  unnecessary. 

The  Diabase  Dikes. 

These  dikes  are,  as  a  rule,  exceedingly  compact,  tough  and 
hard,  of  a  dark  gray,  nearly  black  or  more  frequently  greenish 
color,  and  carry  few  if  any  original  macroscopic  constituents, 
although  pyrite  and  secondary  epidote  are  often  developed  in 
granules  and  aggregates  of  sufficient  size  to  be  detected  by  the 
unaided  eye.  To  this  secondary  epidote  and  the  abundant  viri- 
dite  is  due  in  all  cases  the  green  color  of  the  rocks. 

I  find  nothing  whatever  in  the  slides,  with  possibly  a  single 
exception,  to  indicate  that  the  different  dikes  belong  to  distinct 
periods  of  eruption.  Slides  from  dikes  which  plainly  belong  to 
the  same  system  often  show  all  the  variations  in  texture,  struct- 
ure and  stages  of  decomposition  to  be  ol)served  in  those  which 


39 

from  their  position  belong  iinmistrtkahly  to  distinct  systems. 
Tn  grouping  the  rocks  as  below  I  have,  therefore,  but  followed 
the  notes  of  Professor  Crosby. 

.n      ■[  First  Series. 

Dike  3,  — Rock  finely  porphyritic  with  greenish  feldspars  ; 
macroscopic  pyrite  ;  color  greenish.  This  is  a  very  typical 
diabase,  with  irregular  and  sharply  wedge-shaped  augites,  lath- 
shaped  plagiocTases  and  numerous  grains  of  iron  oxide.  The 
augites  are  undergoing  a  chloritic  alteration  ;  and  the  feldspars 
are  so  badly  kaolinized  that  the  twinning  strife  are  completely 
obscured.  The  iron  oxides  are  in  part  magnetite  and  in  part 
show  the  whitish  alteration  characteristic  of  menaccanite.  Pyrite 
occurs  in  brassy  yellow  irregular  clumps.  Small,  irregular, 
brown  and  strongly  dichroic  scales  attached  to  the  altered  au- 
gites are  evidently  secondary  hornblende.  The  porphyritic 
feldspars  are  so  thoroughly  kaolinized  that  nothing  whatever  can 
be  learned  of  their  original  nature.  Scattering  grains  of  epi- 
dote,  kaolin,  opacite  and  viridite  complete  the  list  of  recognizable 
constituents. 

Dike  5 — Very  fine  grained  and  compact ;  macroscopic 
pyrite  ;  color  greenish.  This  presents  no  distinctions  in  any 
way  essential.  Both  augites  and  feldspars  are  more  decom- 
posed ;  and  there  is  a  corresponding  increase  in  the  propor- 
tions of  epidote,  viridite,  calcite,  etc.  A  few  small  apatites 
appear  in  this  slide. 

Dike  22 — Medium  fine  in  texture  but  distinctly  crystalline, 
and  color  greenish.  This  differs  from  the  preceding  only 
in  being  slightly  coarser  in  texture.  The  iron  oxides,  judged 
by  their  irregular  forms  and  whitish  decomposition  products  are 
largely  titaniferous. 

Dike  25. — Very  fine-grained  and  compact ;  a  few  greenish 
feldspars  porphyritically  developed  ;   and  color  greenish.       The 


40 

aiigite  is  in  most  cases  completely  altered  ;  and  there  is  much 
viridite,  epidote  and  iron  oxide.  This  rock  is  undoubtedly  a 
diabase,  although  I  am  unable  to  find  a  particle  of  recognizable 
augite  or  even  hornblende,  the  viriditic  alteration  being  complete 
in  all  the  sections  examined. 

The  viridite  is  very  abundant  and  is  considered  of  undoubted 
augitic  derivation,  from  its  sharp  straight  and  very  angular 
outlines,  which  are  so  characteristic  of  the  augites  in  this  class 
of  rocks.  Minute  epidotes  are  in  some  cases  very  abundant 
and  impart  to  the  rock  a  yellowish  green  stain.  The  section  is 
traversed  by  veins  of  secondary  quartz. 

Dike  37, —  Very  fine  grained  and  compact ;  macroscopic 
pyrite  ;  color,  greenish.  This  is  of  finer  grain  than  any  of 
the  preceding  and  so  highly  altered  that  none  of  its  original 
constituents  are  now  recognizable,  excepting  by  pseudomorphs. 
The  section  shows  only  a  fine,  compact,  fibrous  or  scaly  ag- 
gregate of  viridite,  opacite,  epidote,  calcite  and  kaolin,  with 
numerous  grains  of  iron  oxide.  Although  so  highly  altered, 
I  have  no  hesitancy  in  referring  this  rock  to  the  diabase  group. 

Second  Series. 

Dike  12. — Very  fine  grained  and  compact ;  no  macroscopic 
constituents  ;  color,  dark  greenish  gray.  Under  the  micro- 
scope it  is  seen  to  be  highly  altered,  but  with  portions  of  augite 
still  fresh  and  showing  pleochroism.  Iron  oxide  very  greatly 
altered  to  a  grayish  amorphous  product.  Other  alteration  pro- 
ducts as  in  preceding  sections. 

Dike  11. — Differs  from  12  only  in  being  of  slightly  coarser 
texture  and  in  having  suffered  more  from  alteration  ;  augites 
completely  changed. 

Dike  10. — Fine  grained  and  compact ;  porphyritic  with  small 
greenish  feldspars.      Plainly  a  diabase,  although  the   feldspars 


41 

are  so  miuldiod  by  decomposition  products  as  to  he  scarcely 
recognizable.  A  few  augite  [)articles  arc  still  unchanged.  No 
essential  differences. 

Dike  9. — Like  10,  but  a  triffe  coarser  in  texture.  The  hand 
specimen  shows  small  black  segregations  ot"  what  is  a])parently 
hornblende  ;  but  these  do  not  appear  in  the  section. 

Dike  8. — Very  compact,  with  no  macroscopic  constituents  ; 
color,  dark  gray,  nearly  black.  The  sample  was  taken  from  the 
edge  of  the  dike  at  the  contact  with  melaphyr.  It  offers  no 
characters  worthy  of  note  to  distinguish  it  from  9. 

Dike  7. — Coarse  and  distinctly  crystalline  ;  color,  greenish. 
This  is  one  of  the  coarsest  rocks  of  the  series.  Under  the  micro- 
scope it  shows  large  plates  of  reddish  brown  augites,  often  en- 
closing the  lath-shaped  plagioclases  and  producing  the  typical 
ophitic  structure.  The  feldspars  here  are  sufficiently  fresh  to 
show  twinning  stria?.  Iron  oxide  is  very  abundant,  in  large 
grains,  with  much  pyrite  and  some  apatite.  The  section  shows  to 
excellent  advantage  the  various  stages  of  augitic  alteration  into 
viridite.  The  rock  resembles  22  more  closely  than  any  of  the 
preceding. 

Dike  26. — Very  fine,  compact  and  homogeneous.  Color,  dark 
gray,  somewhat  greenish.  This  rock  is  greatly  altered  ;  being 
essentially  an  aggregate  of  viridite  scales,  and  fibres  and  gran- 
ules of  iron  oxide,  interspersed  with  epidote.  A  few  cavities  or 
pseudo-amygdules  occur,  the  cavity  wall  being  lined  with  vir- 
idite, while  the  central  portion  is  occupied  by  calcite.  The  feld- 
spars are  very  obscure  ;   and  the  iron  oxide  in  part  titaniferous. 

Dike  29. — Texture  of  medium  fineness,  with  large  segregations 
of  black  hornblende.  Color,  uneven,  black,  greenish  and  yellow- 
ish green  ;  the  last  tint  being  due  to  epidote  veins.  The  augite 
is  greatly  altered  ;  and  there  is  much  viridite  and  epidote,  the 
latter  in  scattered  granules  and  veins.     The  feldspars  are  ob- 


42 

scure  ;  and  quite  large  grains  of  iron  oxide  and  pyrite  occur. 
The  segregation  patches  already  alluded  to  are  found,  under  the 
microscope,  to  consist  largely  of  deep  brown,  strongly  dichroic 
hornblende,  in  compact  and  well-defined  crystals  ;  sometimes 
perfectly  fresh  or  again  somewhat  altered  into  the  inevitable 
viridite.  I  am  not  able  from  the  sections  prepared  of  this  rock 
to  state  whether  this  hornblende  is  original  or  a  product  of  para- 
morphic  alteration  from  the  augite. 

Dike  31. — This  is  of  medium  fineness,  with  macroscopic  pyrite 
and  segregations  of  black  hornblende.  The  color  is  greenish, 
the  rock  being  blotched  with  epidote.  The  augite  is  almost 
completely  altered  to  viridite,  although  a  few  remnants  are  still 
recognizable.  This  rock  also  contains,  as  stated,  coarser  grained 
segregations  in  which  black  hornblendes  are  readily  distinguished 
by  the  unaided  eye.  In  thin  sections  these  are  of  a  deep  brown 
color  and  strongly  dichroic.  Some  of  the  individuals  are  so 
compact  and  well  defined  in  crystalline  outline  as  to  indicate 
that  they  are  original  constituents  of  the  rock  ;  while  in  other 
cases  the  presence  of  a  hornblende  border  of  varying  width  about 
an  augitic  core  indicates  unmistakably  a  paramorphic  origin. 
The  hornblende  has,  in  its  turn,  undergone  in  certain  cases  the 
viriditic  alteration.  This  is  the  only  one  of  the  Nantasket  rocks 
in  which  I  have  been  able  to  find  paramorphic  hornblende  so 
distinctly  characterized  as  to  leave  no  doubt  of  its  origin,  al- 
though chis  is  suggested  by  sections  from  dike  29. 

A  few  small,  grayish,  wedge-shaped  crystals  of  sphene  are 
present.  Feldspars,  in  most  part,  are  considerably  altered ; 
but  in  a  few  instances  are  still  clear  and  show  the  twinning 
striae  very  plainly.  There  is  much  viridite,  which  obscures 
everything. 

Dike  34. — Differs  in  no  essential  particulars  from  29  and  31, 
though  the  section  does  not  show  the  hornblendic  segreo^ations. 

Dikes  40  and  41 — These  two  dikes  are  without  doubt  iden- 
tical with  the  preceding,  though  nothing   in  the  present  compo- 


43 

eition  goes  to  show  tliat  tlioy  were  not  originally  diorites  rather 
than  diabases.  So  far  as  examined,  not  a  particle  of"  fresh  au- 
gite  remains  ;  but  an  occasional  fragment  of  brownish  hornblende 
still  exists  so  far  unchanged  as  to  be  recognizable.  The  lijjht 
green  viridite  is  everywhere  present,  interspersed  with  magnetite 
grains,  particles  of  epidote  and  apatite  needles.  A  few  of  the 
feldspars  are  still  fresh,  though  here  also  the  viriditic  alteration 
has  prevailed.  My  reasons  for  calling  the  rocks  diabase  is  that 
the  products  of  decomposition,  their  form  and  their  arrangement, 
are  identical  with  those  of  the  augitic  rocks  throughout  the  en- 
tire area  under  examination . 

Third  Series. 

Dike  62. — Very  fine  grained  and  compact ;  nearly  hlack. 
Under  the  microscope  this  rock  shows  a  marked  deviation  from 
the  prevailing  types  of  structure  as  already  described.  In  the 
thin  section  it  shows  beautifully  fresh  and  clear  porphyritic 
plagioclases,  in  single  lath-shaped  forms  and  in  cruciform  aggre- 
gates :  tosrether  Avith  altered  olivines  and  occasional  clear  but 
corroded  augites,  imbedded  in  a  dense  fine-grained  ground-mass 
of  small  plagioclases,  augites  and  iron  oxide.  The  olivine,  though 
changed  completely  to  a  chloritic  product,  is  readily  recognized 
by  its  form  and  irregular  fracture-lines.  The  augites  are  con- 
fined almost  wholly  to  the  ground-mass  ;  but  the  section  shows 
a  few  corroded  porphyritic  forms,  as  above  noted.  All  are 
fresh  and  free  from  enclosures.  The  iron  oxide  occurs  abun- 
dantly in  rod-like  forms,  crossing  one  another  at  nearly  right 
angles,  producing  grate-like  or  barred  structures.  This  dia- 
base is  the  least  altered  of  all  that  has  been  examined. 

Dike  66. — Of  medium  coarseness,  and  to  the  naked  eye  ap- 
parently granular  crystalline  and  fresh.  This  rock  is  quite  dif- 
ferent in  general  appearance  from  any  yet  described. 

Macroscopically  it  is  a  well  compacted,  quite  fresh-appearing 
rock,  of  a  deep  purplish  black  color,  in  which  hornblende  or  an- 


44 

gite  is  apparently  the  chief  constituent.  Under  the  microscope, 
however,  the  section  is  found  to  present,  aside  from  the  apatite 
needles,  not  a  sino-le  orisrinal  mineral  in  a  fresh  and  unaltered 
state  ;  but  the  entire  mass  of  the  rock  is  filled  with  small  amor- 
phous grains  and  dust-like  opaque  particles  of  a  black  color,  in- 
terspersed throughout  a  dirty  grayish  groundmass,  with  only 
here  and  there  a  small  fragment  of  an  augite  crystal  or  colorless 
portion  of  feldspar,  in  which  in  a  few  instances  twinning  stria? 
were  still  apparent.  Scattering  grains  of  epidote  and  shreds  of 
a  brownish  mineral,  evidently  hornblende,  complete  the  list 
of  determinable  minerals.  The  black  opacite  and  the  gray 
amorphous  material  are  evidently  derived  from  the  decomposed 
augite  ;  it  being  not  infrequent  to  find  the  border  of  a  crystal 
irregularly  outlined  by  the  larger  grains,  while  interiorly  is  the 
gray  material  and  other  black  grains.  The  form  and  size 
of  the  black  grains  is  such  as  to  render  their  identification  by 
the  microscope  alone  impossible.  The  pulverized  rock  is,  how- 
ever, strongly  magnetic  ;  and  after  long  digestion  with  hydro- 
chloric acid  the  solution  gives  a  fiiint  reaction  for  titanium. 
The  aug^itic  alteration  would  in  this  case  seem  to  be  similar 
to  that  of  the  hornblende  in  certain  New  Hampshire  diorites 
described  by  Hawes.'  That  the  altered  mineral  in  this  case  is 
augite  and  not  hornblende  is  proved  by  a  few  unchanged  parti- 
cles still  remaining  and  the  form  of  the  outlines  still  preserved. 
Viridite,  so  abundant  in  the  other  rocks  described,  is  here  en- 
tirely lacking. 


DETAILED  STRUCTURE  OF  NANTASKET. 

The  main  purpose  of  this  section  is  to  set  forth  as  fully  as 
may  seem  desirable  the  facts  upon  which  the  generalizations  of 

'Geol.  of  New  Hampshire,  Vol.  III.,  part  IV.,  pages  4H  and  (5(i. 


45 

the  ])reccding  sections  are  based ;  not  forgetting-  that  ihc 
highest  interest  attaches  to  the  relations  and  cspeeially  to  the 
contact  phenomena  of"  tlie  bedded  rocks — conglomerates  and 
lavas.  For  present  convenience,  as  well  as  in  the  interest  of 
fiituro  students  of  the  Nantasket  ledges,  each  principal  mass  or 
area  will,  as  regards  the  sedimentary  rocks  and  associated  lavas, 
be  described  somewhat  independently  and  in  topographic  order, 
reserving  the  general  correlation,  the  complete  elucidation  of 
the  stratigraphy,  until  the  close  of  this  systematic  itinerary. 
The  structural  details  of  the  dikes,  or  the  intrusive  igneous  rocks, 
will  then  be  presented,  in  both  chronologic  and  topographic  order. 

The  A^tlantic  Shore  oi'  Coastal  Area. 

This  area,  extending  from  Nantasket  Beach  to  Black  Rock, 
includes  the  most  accessible  and  the  most  frequently  visited  of 
the  Nantasket  ledges  ;  and  it  is  a  fortunate  circumstance  that, 
although  the  melaphyr  largely  predominates  here,  some  of  the 
more  characteristic  features  of  Nantasket  geology  are  well  ex- 
hibited at  this  natural  starting  point.  The  granite,  however, 
is  wholly  wanting ;  and  its  interesting  relations  to  the  con- 
glomerate cannot  be  observed  in  this  part  of  the  field. 

Long  Beach  Rock  Conglomerate. — The  most  northerly 
part  of  Long  Beach  Rock  is  a  somewhat  isolated,  half-tide 
ledge  of  conglomerate.  It  contains  many  pebbles,  mostly 
small,  of  granite  and  felsite,  and  also  many  larger  pebbles  of 
different  varieties  of  melaphyr.  From  the  latter  we  may  infer 
that  this  bed  is  underlain  by  melaphyr,  and  that  it  is  probably 
not  the  lowest  or  basal  conglomerate.  Eight  or  ten  feet  below 
the  top  of  the  conglomerate  is  an  intercalated  layer  of  fine, 
hard,  red  sandstone,  eighteen  inches  thick  ;  and  conformably 
overlying  the  conglomerate,  are  six  to  twelve  inches  of  a  beau- 
tifully banded,  greenish  slate  of  flinty  hardness.  The  sandstone 
and  slate  show  that  the  strike  is  N.65°  E.\  and  the  dip  S.E. 
15°-20.° 

1  All  azimuth  directions  given  in  this  paper  are  referred  to  the  true  meridian. 


46 


CO 


'c^ 


:■/::::■:■ 


:x:;^ 


•!':/.'■ 


■o'jiDllJr-jol 


-,-..'..vco"(l.?W, 


J>'0.»Oo 


o'o  Ooi 


Melaphyr  and  Tuff  .—The  Long 

Beach  Kock  sediments  are  overlam 

conformably   by    a     compact  green 

melaphyr.     The   contact  of  the  mel- 

^  aphyr  and  slate  is  exposed  for  about 

^  one  hundred  feet  along^  the  strike  : 

^  and  at  several  points  there  are  fine 

y        transverse   sections    of  the  contact, 

g        which  IS  accessible  only  at  low  tide. 

§.       Fig.  1  is   a  general  section  of  both 

II        the     conglomerate    and     melaphyr. 

5  ^    The  irregularities  of  the  actual  con- 

2  I    tact,  and    especially  the  bending  of 

y  ^    the  subjacent  laminas  of  slate,  as  well 

£     <  ■?    as  the  intense  induration  of  this  rock, 

■^      '  ►§    which  has  been  baked  to  a  o-ood  semi- 

•^       •  .... 

li     u  ^    porcelainite,  indicate  that  it  was  but 

^  ^  imperfectly   consolidated,  if  not  en- 

g  g  tirely    plastic,    when    the  melaphyr 

M   g  flowed  over  it.      But,  on    the  other 

o  ^  hand,  the  exquisitely  beautiful  brec- 

°  :g  ciation    and   miniature  faultins^  ex- 

X  ^  hibited  on  some  of  the  wave-washed 

■J}     "Vi 

§  ^    surfaces    show  that   before  the  dis- 

o    § 

^  ■::j    turbance    ceased  the    clay  had    be- 
2  >^    come  brittle  enouo-h  to  break.      This 
^  ^    fascinating     exposure     is    the    gem 
of    Nantasket   geology ;    and    it   is 
"^        hoped  that  it  may  long  remain  un- 
^     ^        mutilated,   to    delight  and    instruct 
S  future  students.      That  the  erujjtion 

I  was  submarine,  or  at  least  essentially 

^  contemporaneous  with  the  deposition 

^  of  the  sediments,  is  beyond  reasona- 

ble doubt ;  although  it  is  probable 
that  the  thin  layer  of  green  slate  on 
which     the     melaphyr   immediately 


Occas.  pspers  Bost    Soc,  Nat.  Hist.  IV. 


Plate  4. 


Wave-worn  surface  of  the  intensely  hard,  greenish,  banded  slate  or 

TUFF    separating    THE    CONGLOMERATE    AND    MELAPHYR    ON    LoNG    BeACH 

Rock.     Linear  scale,  one  half  natural  size. 
[Reduced  by  photography  from  a  tracing  made  directly  from  the  stone.] 


Tliis  impalpably  fine  sediment  was  deposited  quietlj'  and  nniformlj' over 
the  uneven,  pebbly  surface  of  the  conglomerate;  and  consequentlv  where 
erosion  iias  cut  down  nearly  to  the  top  of  the  conglomerate,  each  pebble 
determines  the  center  of  a  series  of  concentric  lines,  the  outcropping 
edges  of  the  laminae  of  slate.  In  the  portion  of  the  eroded  surface  here 
represented,  the  banded  slate  has  been  worn  away  sufficiently  to  actually 
expose  only  two  pebbles  of  the  conglomerate. 

In  the  upper  part  of  this  thin  bed  of  tuff,  where  it  experienced  more  dis- 
tinctly the  mechanical  stresses  of  the  overflowing  lava,  the  layers  are, 
to  some  extent,  finely  brecciated  and  faulted,  as  shown  in  the  illustrations 
on  the  next  plate. 


Occas.  papers  Bost.  Soc.  Nat.  H'St.  IV. 


Plate  5. 


o    ^ 


Z     ^ 


'^  3; 


o  o 


47 

rests  should  be  regarded  as  an  exceedingly  fine  volcanic  tufi, 
the  eruption  of  liquid  lava  having  been  preceded  by  an  out- 
burst of  volcanic  ashes.  The  influence  of  the  lava  flow  has  also 
penetrated  the  conglomerate,  the  layer  of  sandstone  eight  feet 
below  being  almost  as  thoroughly  indurated  as  the  slate. 

The  breadth  of  the  melaphyr  is  fully  two  hundred  feet  and  its 
actual  thickness  probably  not  far  from  sixty  feet.  It  is,  for  the 
most  part,  rather  compact  and  of  a  dark  green  color.  But 
toward  the  top  especially  it  is  full  of  irregular  segregations  of 
quartz  and  shows,  in  a  rather  large  way,  a  very  distinct  and 
somewhat  wavy  flow-structure,  resembling  bedding  and  parallel 
with  the  stratification  of  the  underlying  conglomerate.  Irreg- 
ular veins  and  masses  of  compact  and  impure  epidote,  occa- 
sionally of  considerable  size,  also  occur  in  the  upper  part  of  the 
melaphyr.  It  is  very  plain  that  the  entire  thickness  of  mela- 
phyr is  to  be  regarded  as  forming  one  simple  and  normal  flow 
homogeneous  and  massive  below,  but  superficially  brecciated 
and  scoriaceous. 

This  great  lava-flow  is  undoubtedly  continued  eastward  in  the 
outermost  part  of  the  ledge  exposed  at  low  tide  north  of  Centre 
Hill ;  and  it  appears  probable  that  further  east  the  line  of  strike 
changes  so  as  to  connect  Little  Black  Rock  with  the  same  bed. 
This  islet,  which  is  about  four  hundred  feet  long  and  half  as 
broad,  is  entirely  composed  of  melaphyr  very  similar  to  that  on 
Long  Beach  Rock,  except  that  it  is  in  part  amygdaloidal. 

The  prominent  ledge  at  the  northwestern  base  of  Atlantic 
Hill,  and  partially  isolated  by  the  sands  of  Nantasket  Beach,  is 
chiefly  a  distinctly  and  evenly  bedded  greenish  gray  sandstone, 
alternating,  especially  in  the  upper  part,  with  layers  of  greenish 
and  rather  indistinct,  small-pebbled  conglomerate.  This  con- 
glomerate is  made  up  of  much  smaller  pebbles,  and  is  in  every 
way  very  distinct  from  that  on  Long  Beach  Rock.  The  peb- 
bles are  mainly  more  or  less  angular  fragments  of  melaphyr, 
are    not  well    assorted,  the    coarsest  and   finest   material  being 


48 

mingled  indiscriminately,  and,  with  the  exception  of  a  rare 
pebble  of  felsite  or  granite,  the  entire  ledge,  sandstone  as  well 
as  conglomerate,  appears  at  first  to  have  been  derived  from  the 
Long  Beach  Rock  flow  of  melaphyr.  There  is  much,  however, 
in  both  the  composition  and  texture  of  this  mass  to  suggest  that 
it  may  be  a  true  volcanic  tuff,  a  record  of  explosive  volcanic 
action  either  at  the  close  of  the  eruption  already  described  or 
at  the  inception  of  the  similar  littoral  or  submarine  eruption  by 
which  this  bed  was  covered.  It  will  be  observed  that  this  in- 
terpretation is  adopted  on  the  map,  the  lava-flows  being  thus 
supposed  to  have  followed  each  other  so  closely  over  this 
marine  area  as  to  preclude  the  deposition  of  any  appreciable 
amount  of  the  ordinary  or  normal  sediments  between  them. 

This  bed  of  tuflP  strikes  N.  65°  E.,  and  dips  S.E.  15°.  Its 
full  breadth  appears  to  be  exposed,  viz.,  120  feet,  equal  to  a 
thickness  of  about  30  feet.  It  can  be  traced  for  about  300  feet 
along  the  base  of  Atlantic  Hill,  rising  at  the  highest  point  some 
20  feet  above  the  beach.  The  line  of  strike  carries  it  directly 
across  the  shingle  beach  connecting  Long;  Beach  Rock  and  At- 
lantic  Hill.  I  have  been  unable  to  positively  identify  it  on  the 
low-tide  ledge  north  of  Centre  Hill ;  but  it  seems  impossible  to 
doubt  that  it  crosses  here,  although  probably  with  diminished 
thickness.  It  appears  necessary,  also,  to  suppose  that  the  tuff 
overlies  conformably  the  melaphyr  of  Long  Beach  Rock,  al- 
though there  is  no  exposure  of  the  contact.  If  the  tuff  extends 
so  far  to  the  eastward,  it  must  pass  to  the  north  of  Gun  Rock 
and  between  the  two  Black  Rock  islets. 

This  old  ash-bed  is  overlain  clearly  and  conformably  by  a 
second  bed  of  melaphyr,  the  contact  being  well  exposed  at  sev- 
eral points  along  the  northern  base  of  Atlantic  Hill.  The  con- 
tact is  similar  to  that  between  the  green  slate  and  overlying 
melaphyr  on  Long  Beach  Rock  —  conformable  in  the  general 
view  and  yet  with  many  minor  irregularities.  Fig.  2  shows 
the  contact  as  it  appears  at  the  edge  of  the  beach,  below  the 
bath  house  and  near  the  foot  of  the  Atlantic  House  steps.  It 
is  clearly  such  a  contact  as  would    naturally    result    from    the 


49 


flowing  of  liquid  lava  over  uncousolidated  sediments.  And  the 
attention  of  those  who  may  see  in  the  minor  uneonformities  of 
the  contact  evidence  that  the  melaphyr  is  really  intrusive  is 
called  to  the  fact  that  no  detached  fragments  of  the  tuff  or  sedi- 
ment are  observed  in  the  base  of  the  melaphyr. 


M£i4^p'^y^ 

J. 

M 

7? 

"^^^TT! 

j^* ' *  \  *'  I '"V*  '  • 

^ .  J 

**^-  •• 

/C\  ■•.*.■.*.•.■>*•'•  •*•> 

^.'' 

.•'•".^". 

/•  Vf-^' !"  •'  *  „•  -',•*•*.  • 

\^ 

• .'  •  •  • 

*  ^^1*  • 

'.^H* 

\ 

/■'*■'•'.  '■  ',•'"■'.  •^'  .*  •*.  •*  • 

» .  • 

•*.*•' 

/     *  •■•.-•*       •-,•<•'    '.    fc** 

*  '.* 

*  •  *  •  • 

Tuff        /S??4^ 

--'  *•••       ,,.           ."*•'       ..*'*•" 

'•5~^ 

"'t"-sl. 

^-<^2^-$'Se~ 

^5s-c^.'.  \  •  r*  I''*-  •  \ '  *.'4 

v.*. 

I .' .  •* 

Beac^ 

^ 

Fig.    2. —  Contact   of   Tuff    and    Melaphyr  at   the 

northern  base  of  atlantic  hill. 

Scale,  i  inch  =  12  feet. 

The  thickness  of  the  melaphyr,  near  the  bath  house,  is  about 
twenty  feet ;  and  it  appears  to  increase  eastward.  This  is  very 
clearly  a  second  lava-flow,  and  not  an  intrusive  bed  ;  and  in 
lithological  character  it  is  very  similar  to  the  first,  being  chiefly 
dark  green  and  compact.  It  is  sometimes,  however,  more  brec- 
ciated  ;  but  exhibits  the  same  ill-defined  segregations  and  veins 
of  quartz  and  epidote.  This  flow  can  be  traced  eastward  along 
the  base  of  Atlantic  Hill  to  Valley  Beach,  between  Atlantic  and 
Centre  Hills  ;  and  it  undoubtedly  forms  the  landward  end  of  the 
low-tide  ledge  north  of  Centre  Hill. 

Valley  Beach  is  bounded  by  two  good  transverse  sections  of 
the  formations.  Atlantic  Hill  ends  here  in  a  small  clifl',  which 
shows  the  following  thicknesses  from  below  upwards  (Fig.  3)  : 
The  second  melaphyr,  already  described,  about  30  feet; 
volcanic  agglomerate  and  tuflf,  7  feet ;  melaphyr,  9  feet ;  ag- 
glomerate and  tuif,  25  feet;  and  then  melaphyr  extending 
southward  indefinitely,  or  beyond  the  end  of  the  section. 

The  first  fragmental  bed  or   tufl"  in  this  section   measures  7 

OCCAS.  papers.  B.  8.  N.  H.    IV.  4. 


50 


L 

V. 

> 

\i- 

X 

~i 

1 

ft. 

V 

1 

-«. 

JiMI 

/< 

A 
VA 

d 

IE 

Fig.  3. — Section  on  the  west 

SIDE  OF  Valley  Beach. 

Scale,  i  inch  =  20  feet. 


51 

feet  on  the  cliff,  but  grows  narrower  westward  and  appears  to 
die  out  entirely  between  200  and  250  feet  from  the  cliff.  It  is 
probable,  however,  that  the  tuff  really  persists  until  it  passes 
the  edge  of  the  overlying  melaphyr,  and  is  merged  with  the  next 
bed  of  tuff,  as  shown  on  the  map. 

The  boundaries  of  this  tapering  edge  of  the  fragmental  bed 
are  quite  irregular.  This  is  especially  true  of  its  lower  surface, 
where  it  rests  upon  the  second  melaphyr.  The  form  of  this 
contact  for  130  feet  west  from  Valley  Beach,  or  as  far  as  it  is 
clearly  exposed,  is  shown  in  Fig.  4  ;  for  the  sake  of  convenience, 
however,  the  curves  are  represented  as  following  each  other 
more  nearly  in  the  same  direct  line  than  is  actually  the  case. 
The  upper  surface  of  the  melaphyr  presents  smoothly  rounded 
hemispherical  protuberances  or  swellings  one  to  thi-ee  or  four 
feet  in  diameter.  The  actual  boundaries  of  adjacent  protu- 
berances may  usually  be  traced  below  the  surface  of  the  mela- 
phyr ;  and  some  of  these  remarkably  regular  and  graceful  curves 
thus  describe  two-thirds  or  three-fourths  of  a  complete  circle. 
Probably  the  majority  of  the  curves  are  really  sections  of  rounded 
ridges  or  rolls, — the  surface  flow-structure  of  the  lava.  But  it  is 
very  noticeable  that  these  prominences  exhibit  semicircular  pro- 
files to  some  extent  on  both  the  transverse  and  longitudinal  sec- 
tions of  the  bed,  suggesting  hemispherical  tumefactions  or 
superficial  bubbles  formed  on  the  liquid  lava. 

It  seems  impossible  to  regard  these  features  as  the  product  of 
erosion  ;  they  must  be  entirely  original ;  the  actual  surface  of 
the  fresh  submarine  lava-flow,  which  was  covered  almost  in- 
tact either  by  the  ashes  accompanying  a  second  flow  or  by  the 
debris  worn  from  adjacent  masses.  The  fragments  are  mainly 
angular,  ranging  in  size  from  dust  to  three  inches  in  diameter  ; 
and  this  is  clearly  a  thin,  local  deposit  on  the  surface  of  the 
sheet  of  lava. 

The  well-marked  north-south  depression  separating  Atlantic 
and  Centre  Hills,  and  terminating  at  the  lower  end  in  Valley 
Beach,  does  not  seem  to  be  occupied  by  a  dike,  for  the  melaphyr 
shows  half  way  across  the  bottom  of  it,  and  there  is  not  a  ves- 


52 

tige  of  dike  rock  to  be  seen  in  situ  ;  nor  does  it  appear  to  be 
chiefly  due  to  faulting,  but  ratiier  to  rapid  erosion  along  a  series 
of  close  parallel  joints.  There  has  been  a  little  slipping,  how- 
ever ;  for  on  tracing  the  thin  bed  of  tuff  across  the  one-hundred 
feet  of  beach,  we  find  its  outcrop  shifted  about  twenty  feet  to 
the  south,  indicating  an  upthrow  on  the  east  of  perhaps  five 
feet.  The  melaphyr  between  the  two  walls,  may,  of  course, 
have  dropped  down  almost  any  amount,  producing  two  con- 
vero'inof  and  compensatino-  faults.  East  of  the  beach  the  bed  of 
tufi"  already  described  crosses  the  ledges  diagonally  to  the  shore 
in  about  one-hundred  feet,  passing  out  of  sight  under  the  water 
without  any  sensible  change  of  dip  or  thickness. 

Returning  to  the  west  side  of  Valley  Beach,  the  melaphyr, 
nine  feet  in  thickness,  overlying  this  tuif  is  found  to  be  quite 
conformable,  the  contact  showing  only  the  minor  irregularities 
that  would  naturally  be  developed  where  a  stream  of  lava  flows 
over  unconsolidated  sand  and  gravel.  The  contact  is  not 
always  a  sharply  defined  line,  but  the  lava  is  enough  mixed 
with  the  sand  so  that  the  two  rocks  are  blended  through  a 
thickness  of  several  inches.  The  swelling  curves  charactei'izing 
the  lower  surface  of  the  tufi"  are  not  observed  above  it. 

The  structure  of  this  melaphyr,  which  is  really  the  third 
flow,  reckoning  from  the  conglomerate  on  Long  Beach  Rock,  is 
quite  peculiar.  A  compact,  greenish  matrix  encloses  iri'egu- 
larly  rounded  amygdaloidal  masses  from  two  inches  to  two  feet 
in  their  longest  dimensions.  The  amygdules  are  usually  ar- 
ranged in  concentric  lines  or  zones  parallel  with  the  exterior ; 
and  the  coarsest  amygdules  are  sometimes  towards  the  periphery 
and  sometimes  in  the  centre.  These  masses  are  quite  clearly 
distributed  in  irregular  lines  parallel  with  the  bed  ;  and  this  tact, 
as  well  as  the  great  number  of  the  masses,  is  decidedly  unfavor- 
able to  the  view  that  they  are  true  volcanic  bombs.  But  the 
best  explanation  which  has  occurred  to  my  mind  is,  perhaps, 
not  wholly  satisfactory,  viz.,  that,  during  the  flowing  of  the 
lava,  vesicular  layers  and  crusts  were,  by  the  unequal  flowing 
and  revolving  motions,  broken   up  and  the  fragments  rounded 


53 

into  the  forms  we  now  see.  East  of  Valley  Beach,  along  the 
north  front  of  Centre  Hill,  this  melaphyr  expands  rather  rap- 
idly ;  and  it  is  very  obvious  that  while  the  rounded  amygdaloid- 
al  masses  or  pseudo-bombs  are  thickly  and  pretty  uniformly 
scattered  through  the  middle  and  upper  parts  of  the  flow,  they 
are  almost  entirely  wanting  in  the  lower  part.  They  continue 
to  form  a  prominent  feature  of  the  upper  part  of  the  melaphyr 
about  half  way  across  Centre  Hill,  and  then  die  out  rather 
gradually.  Beyond  this  point  the  identity  of  the  melaphyr  as  a 
separate  flow  is  lost,  since  it  is  indistinguishable  from  the  over- 
lying melaphyr,  with  which  it  is  here  in  direct  contact.  That 
the  third  melaphyr  actually  extends  quite  across  Centre  Hill,  if 
not  across  Gun  Rock,  there  can  be  no  reasonable  doubt.  This 
flow  is  of  exceptional  interest,  not  alone  on  account  of  the 
pseudo-bombs,  but  also  because  it  aflbrds,  perhaps,  the  only 
instance  in  the  Nantasket  area  where  the  tapering  edge  of  a  bed 
of  lava  can  be  clearly  traced.  It  certainly  thins  out  rapidly 
toward  the  west,  and,  as  stated,  can  not  be  followed  much 
more  than  two  hundred  feet  west  of  Valley  Beach,  the  under- 
lying and  overlying  tuffs  appearing  to  come  together  at  this 
point.  It  will  not  escape  attention  that  the  amygdaloidal 
masses  are  thus  found  where  we  should  most  naturally  look  for 
them  —  in  the  marginal  and  superficial  portions  of  the  flow. 

The  second  bed  of  tuff  in  the  Valley  Beach  section,  or  that 
overlying  the  amygdaloidal  melaphyr,  is  lithologically  similar 
to  that  below  the  melaphyr ;  but  it  is  three  times  as  thick,  and 
can  be  traced  westward  the  entire  length  of  Atlantic  Hill,  with 
no  sensible  change  of  dip  or  thickness,  passing  under  the 
beach  on  the  west  where  the  County  or  Beach  road  first 
reaches  the  base  of  the  hill.  It  is  well  exposed  near  the  Atlan- 
tic House  steps,  just  above  the  bath  house ;  from  here  it  con- 
tinues around  the  hill  a  little  above  the  middle  of  its  height ; 
and  then  gradually  descends  to  the  level  of  the  beach  on  the 
east.  The  continuity  of  the  bed  is  clear ;  but  its  strike  must 
change  from  N.  65^  E.  on  the  west  to  at  least  N.  75°  or  80°  E. 
on  the  east  side  of  the  hill.     Its  upper  and  lower   contacts  are 


54 

clearly  exposed  at  several  points,  and  are  favorable  to  the  view 
that  the  melaphyrs  are  contemporaneous.  It  consists,  like  the 
other  beds  of  tuff,  almost  w^holly  of  more  or  less  angular  and 
imperfectly  assorted  fragments  of  melaphyr  imbedded  in  a  dis- 
tinctly stratified  cement  of  the  same  character.  On  the  west 
side  of  the  hill  it  rests  directly  upon  the  second  melaphyr  ; 
while  in  the  vicinity  of  Valley  Beach  the  second  tuff  and  the 
third  or  amygdaloidal  melaphyr  are  interposed,  as  already  ex- 
plained. East  of  Valley  Beach  it  is  scantily  exposed,  and  can 
not  be  traced  much  beyond  the  Waverly  House.  But  it  is 
here,  seemingly,  somewhat  interstratified  or  mingled  with  the 
underlying  amygdaloidal  melaphyr. 

Above  the  last  described  or  third  tuff  comes  the  great  body 
of  melaphyr  forming  all  the  remaining  portions  of  Atlantic  and 
Centre  Hills  and  extending  south  to  Conglomerate  Plateau,  as 
shown  on  the  map.  The  outcrops  are  so  numerous  over  this 
area  that  there  is  absolutely  no  room  to  doubt  the  essential 
continuity  of  the  melaphyr.  Its  breadth,  measured  from  the 
third  tuif,  along  Valley  Beach  Avenue,  is  nearly  1400  feet,  cor- 
responding to  a  probable  thickness  of  more  than  300  feet.  Or, 
classing  the  three  beds  of  tuff  as  fragmental  lavas,  as  the  facts 
appear  to  warrant,  and  thus  regarding  the  volcanic  series  as 
essentially  continuous  back  to  the  conglomerate  on  Long  Beach 
Rock,  the  breadth  of  the  entire  series  of  three  tuffs  and  four 
melaphyrs  is,  in  round  numbers,  at  least  1800  feet,  and  thick- 
ness 450  feet. 

The  great  mass  of  melaphyr  above  the  third  tuff  is  fairly 
uniform  lithologically.  It  is  usually  more  or  less  brecciated, 
with  numerous  highly  iiTcgular  and  limited  segregations  of 
vitreous  quartz  and  chalcedony,  as  well  as  epidote.  But 
sometimes  it  is  much  more  distinctly  brecciated,  especially  im- 
mediately above  the  third  tuff.  On  Atlantic  and  Centre  Hills 
it  is  rarely  amygdaloidal  or  quite  compact  in  texture.  Near 
the  southern  border  of  the  melaphyr  area,  on  the  north  side  of 
Willow  Ledge  Hill,  it  encloses  about  twenty  feet  in  thickness 
of  greenish  tuff  and  agglomerate.     The  green  arenaceous  tuff  ia 


55 

interstratified,  especially  in  the  lower  part  of  the  section,  with 
a  bright  red,  slaty  rock,  which  is  somewhat  contorted  ;  and  in 
the  upper  part  of  the  section  with  the  agglomerate,  which  is 
chiefly  composed  of  quite  small,  but  mostly  angular  fragments 
of  melaphyr.  These  beds,  which  are  very  distinctly  stratified 
throughout,  strike  about  N.  80°  E.,  dip  S.  15'^,  and  ai-e  overlain 
conformably  by  the  melaphyr ;  but  the  lower  contact  is  not  ex- 
posed. On  the  west  they  are  quite  certainly  cut  oflTby  afault,  and 
this,  as  shown  on  the  map,  is  probably  also  their  fate  on  the  east. 
At  any  rate,  the  melaphyr  clearly  crosses  their  strike  in  this  direc- 
tion, and  it  is  impossible  to  trace  them  for  more  than  three  hun- 
dred feet.  A  red  slate  or  tuff  similar  to  that  found  here,  crops 
out  just  west  of  the  Rockland  House,  accompanied,  appai'cntly, 
by  agglomerate,  although  this  may  be  only  the  ordinary  con- 
glomerate or  puddingstone.  The  correlation  of  these  red  out- 
crops would  require  us  to  postulate  dislocations  which  are  not 
indicated  on  the  map  and  for  which  there  is  no  independent 
evidence.  It  appears  wiser,  therefore,  to  regard  them  as  local 
and  non-synchronous  accumulations  of  volcanic  dust  and  lapilli. 

The  melaphyr  immediately  northeast  of  the  Rockland  House 
is  similar  to  that  on  Atlantic  Hill,  and  is  undoubtedly  a  con- 
tinuation of  it :  and  the  same  is  true  of  the  melaphyr  forming 
Gun  Rock  and  its  neighboring  ledges.  The  bedsof  tuff  on  the 
north  side  of  Atlantic  Hill  cannot  be  traced  west  of  the  abrupt 
western  slope  of  the  hill,  perhaps  for  want  of  outcrops  ;  nor  can 
they  be  followed  eastward  more  than  half  way  across  Centre 
Hill.  But  in  this  direction  the  various  flows  of  melaphyr  are, 
apparently,  united  ;  and  in  the  Gun  Rock  district  there  is  not  a 
trace  of  any  sedimentary  rock,  but  the  true  conglomerate  and 
the  fragmental  lava  or  tuff  are  alike  wanting.  It  would  be 
easy,  of  course,  to  explain  the  absence  of  the  tuffs  by  faulting, 
but  it  is,  perhaps,  more  probable  that  these  fragmental  lavas 
were  never  spread  over  this  area.  The  submerged  or  half-tide 
ledge  east  of  Gun  Rock  (see  map)  is  about  three  hundred  feet 
long  at  low  tide,  and  consists  wholly  of  melaphyr  similar  to 
that  of  Gun  Rock. 

There  is,  apparently,  no  reason  to   doubt  that  all  the  mela- 


56 

phyr  described  up  to  this  point,  with  the  inchided  tuffs,  on 
Rockland,  Atlantic,  and  Centre  Hills,  Gun  Rock,  the  sub- 
merged ledges  and  Little  Black  Rock,  belongs  to  one  great  bed 
or  sheet  having  an  approximately  east-west  trend  and  southerly 
dip  and  overlying  the  conglomerate  forming  the  north  end  of 
Long  Beach  Rock.  Throughout  this  great  mass,  however, 
there  are  many  indications  that  it  is  really  composite,  consisting 
of  a  succession  of  flows  ;  which  either  were  not  submarine,  or 
followed  each  other  so  rapidly  as  to  preclude  the  formation  of 
any  sensible  thickness  of  intervening  sediments.  The  included 
beds  of  tuff  show  this  very  clearly.  The  zones  of  brecciated 
melaphyr,  the  fractures  being  marked  by  segrations  of  silica 
and  impure  epidote,  seem  to  show  it.  And  there  are  often 
curving  lines,  such  as  have  been  described  at  the  base  of  the 
second  tuff  (Fig.  4),  which  appear  to  mark  the  contact  of  two 
successive  flows. 

On  the  north  side  of  Centre  Hill,  near  the  end  of  Centre 
Hill  Avenue,  is  another  proof,  in  the  form  of  a  dike  of  mela- 
phyr (No.  1,  on  the  map),  that  this  great  bed  is  not 
throughout  of  the  same  age.  Its  compactness  and  general 
resemblance  to  the  enclosing  melaphyr,  as  well  as  the  irreg- 
ular contacts,  mark  this  dike  as  probably  contemporaneous  with 
some  later  flow  of  this  immediate  series,  although,  of  course, 
of  later  date  than  the  flow  which  it  is  seen  to  intersect. 

The  general  absence  of  the  amygdaloidal  texture  in  this  mel- 
aphyr, except  in  the  flow  separating  the  second  and  third  tuffs, 
is  very  noticeable  ;  and  this  negative  character,  as  well  as  the 
prevalent  brecciation,  contrasts  it  strongly  with  the  most  of  the 
other  masses  of  melaphyr  or  basic  lava  in  the  Nantasket  area. 
In  fact,  as  Mr.  Merrill's  descriptions  show,  both  the  structure 
and  composition  of  this  rock  assign  it  a  position  between  the 
ultra  basic  lava  or  typical  melaphyr,  and  the  more  acid  lava  or 
porphyrite  of  this  region. 

Porphyrite. — The  remaining  rocks  and  islets,  east  of  Gun 
Hock,  constitute  a  very  typical  area  of  porphyrite,  bounded   on 


57 

the  north,  apparently,  by  the  eastward  extension  of  the  i^reat 
belt  of  melaphyr  indicated  by  Little  Black  Rock,  and  on  the 
south  by  the  conglomerate  of  Green  Hill.  Although,  as  Mr. 
Merrill  states,  this  rock  resembles  the  melaphyrs  more  in  its 
general  aspect  than  does  the  porphyrite  of  the  western  part  of 
the  Nantasket  area ;  chemical  analysis  shows  that  it  is  as  acid 
as  any  porphyrite  in  this  district.  It  is  somewhat  variable  in 
its  microscopic  features,  but  usually  presents  a  compact,  or 
apparently  felsitic,  dark  gray  or  purplish  to  nearly  black,  base, 
enclosing  numerous  porphyritically  developed  feldspars  or 
minute  aggregates  of  epidote.  It  is  never  amygdaloidal,  and 
rarely  distinctly  brecciated  ;  but  exhibits  at  several  points  a 
well-developed  striping  or  flow-structure. 

Black  Rock,  which  is  about  seven  hundred  feet  east  and  west, 
and  half  as  broad,  is  a  continuous  and  almost  perfectly  bare  mass 
of  porphyrite.  Fluidal  lines  are  very  plainly  marked  in  a  por- 
tion of  the  porphyrite,  with  usually  an  approximately  east-M'^est 
trend  and  a  gentle  southerly  dip,  thus  proving  that  the  normal 
attitude  of  the  rocks,  so  clearly  exhibited  about  Atlantic  Hill, 
is  preserved  to  the  very  eastern  limit  of  the  Nantasket  area. 
Near  the  middle  of  the  west  half  of  the  island,  a  considerable 
mass  of  porphyrite  is  split  up  by  close ,  parallel  east-west  and 
nearly  vertical  joints  so  as  to  present  a  very  shaly  appearance. 
The  character  and  especially  the  attitude,  or  dip  and  strike,  of 
this  structure  indicates  that  it  is  possibly  true  cleavage  rather 
than  jointing,  and  analogous  to  the  foliation  of  the  diabase  on 
Calf  Island.^  The  small  island  north  of  Green  Hill  (Green  Hill 
Rock)  is  another  mass  of  gray  and  purplish  porphyrite,  with  flu- 
idal lines ;  and  the  half-tide  ledge  between  this  and  Black  Rock 
exhibits  a  similar  constitution.  Since  the  porphyrite  of  this 
area  is  exposed  only  in  these  three  insular  masses,  it  is  mani- 
festly impossible  to  determine  with  certainty  either  its  bounda- 
ries, its  thickness  or  its  relations  to  the  bordering  melaphyr  and 
conglomerate.  If,  however,  we  may  follow  the  lithologic  indica- 
tions and  correlate  it  with  the  porphyrite  of  the  western  area,  it 
must,  as  will  appear  later,  be  referred  to  a  horizon  below  this 

iProc.  B.  S,  N.  H.  xxni.,    455, 


58 

melaphyr  and  conglomerate  and  thus  owe  its  exposure  to  exten- 
sive faulting.  The  map,  it  will  be  observed,  has  been  constructed 
in  accoi'dance  with  this  view  ;  and  reference  to  the  table  of  Nan- 
tasket  strata  on  page  24  will  afford  some  idea  of  the  magnitude 
of  the  displacements  which  it  involves, — at  least  50  feet  on  the 
side  toward  the  conglomerate  and  400  feet  on  the  side  toward 
the  melaphyr. 

Indications  are  not  wanting  that  Black  Rock  is  near  the  point 
or  centre  of  emission  of  the  Nantasket  porphyrites.  One  evi- 
dence of  this  is  the  seemingly  great  thickness  of  the  porphyrite 
in  the  Black  Rock  area,  and  its  more  crystalline  character  as 
compared  with  the  porphyrite  in  the  western  part  of  Nantasket. 
The  most  important  fact  pointing  to  this  conclusion,  however, 
is  the  series  of  porphyrite  dikes  on  the  Cohasset  Shore  east  of 
Green  Hill.  The  most  casual  observation  shows  that  the  ap- 
proximately north-south  dikes  on  the  Cohasset  Shore  can  not 
all  be  referred  to  the  third  system  of  diabase  dikes  ;  but  besides 
the  three  well-defined  systems  of  dark -colored,  finely  crystalline 
diabase  dikes,  there  is  evidently  a  fourth  system  having  a  gen- 
eral north-south  trend,  consisting  of  a  distinctly  greenish  rock, 
which  varies  in  texture  from  apparently  felsitic  and  porphyritic 
to  visibly  holocrystalline  or  nearly  so.  The  dikes  of  this  kind 
are  not  only  lithologically  but  chronologically  distinct,  for  they 
are  repeatedly  cut  by  both  the  east-west  and  north-south  dia- 
base dikes ;  and  hence,  although  agreeing  approximately  in 
trend  with  the  newest  series  of  dikes,  they  must  be  regarded  as 
the  oldest  system  exposed  on  this  shore. 

My  interest  in  this  oldest  system  of  dikes  was  not  arovised  so 
much  by  the  clear  proof  of  their  age  as  by  the  marked  litholog- 
ical  resemblance  which  they  bear  to  the  more  acid  lavas  or 
porphyrites  of  Nantasket,  and  especially  to  the  porphyrite  of 
the  Black  Rock  area.  Except  that  the  porphyrite  fiows,  al- 
though often  visibly  epidotic,  arc  rather  rarely  distinctly  green- 
ish in  color,  while  the  dikes  of  this  system  arc  always  so  ;  and 
that  the  dikes  are  more  crystalline  than  the  effusive  rocks,  as 
we  should  naturally  expect ;   while  the  latter  exhibit  commonly 


69 

a  striping  or  flow-structure  which  is  wanting  in  the  dikes  ;  the 
superficial  resemblance  is  certainly  very  obvious,  suggesting  at 
once  that  the  dikes  may  be  the  channels  or  vents  througli  which 
the  effusive  porphyrite  reached  the  surface,  or  at  least  referred 
to  the  same  eruptions — the  same  period  of  volcanic  activitv. 

Mr.  Merrill  has  found  (page  38)  that,  although  somewhat 
more  crystalline,  these  dikes  are  essentially  similar  in  micro- 
scopic characters  to  the  Black  Rock  poryhyrite  ;  and  in  order  to 
further  test  the  validity  of  this  hypothesis  a  typical  example 
from  one  of  these  dikes  was  submitted,  through  the  kindness  of 
Dr.  T.  M.  Drown  and  Mr.  G.  F.  Eldridge  of  the  Massachusetts 
Institute  of  Technology,  to  partial  chemical  analysis,  with  the 
following  result,  the  mean  of  four  accordant  determinations  : 
silica,  60.84  per  cent.  ;  alumina  and  ferric  oxide,  20.12  per 
cent.  ;  with  which  may  be  compared  the  percentage  of  silica 
— 58.25  per  cent. — afforded  by  the  Black  Rock  porphyrite.  As 
in  the  case  of  the  melaphyr  (page  38),  we  find  that  the  dike  is 
slightly  more  acid  than  the  effusive  portion  of  the  eruption  ; 
its  more  crystalline  character  causing  it,  apparently,  to  yield 
less  completely  to  the  deep-seated  alteration,  thus  reversing  the 
normal  relative  proportions  of  silica  observed  in  recent  erup- 
tive rocks. 

That  the  classification  of  these  dikes  by  the  superficial  char- 
acters is  unsafe  is  shown  by  a  second  analysis,  by  Mr.  Eldridge, 
from  another  dike  of  the  series.  This  afforded,  as  the  mean  of 
four  accordant  determinations,  silica,  48.47  per  cent.  ;  alumina 
and  ferric  oxide  31.53  per  cent.  ;  and  if  not  a  true  diabase,  it 
should,  apparently,  be  associated  with  the  more  basic  melaphyrs 
of  Nantasket.  Obviously,  then,  a  systematic  microscopic  and 
chemical  examination  of  these  dikes  will  be  required  for  their 
accurate  classification  ;  and  we  can  now  only  assume  that,  as 
appearances  indicate,  they  are  chiefly  porphyrite. 

A  more  detailed  description  of  these  dikes  will  be  presented 
later,  in  the  systematic  account  of  the  dikes  of  this  region  ;  and 
all  that  I  desire  now  is  simply  to  call  attention  to  the  following 
points  ; —  (1)   That  they  are  probably  of  the  same  age  as  the 


60 

surface  sheet  of  porphjrite  and  connected  witli  the  vents  through 
which  it  was  extruded.  (2)  That  no  dikes  of  this  character 
have  been  discovered  west  of  Green  Hill  or  more  than  one  mile 
east  of  it.  (3)  That  they  all  have,  without  exception,  a 
northerly  trend,  i.  e.,  they  either  run  toward  the  Black  Rock 
porphyrite  or  so  as  to  intersect  the  eastward  extension  of  its 
line  of  strike.  And  yet  (4)  none  of  them  have  been  found 
breaking  through  the  porphyrite  itself,  these  dikes  being  thus 
comparable  with  the  great  melaphyr  dike  (No.  2),  since  in  both 
cases  the  complete  erosion  of  a  portion  of  the  effusive  accumu- 
lations was  essential  to  expose  the  intrusive  masses  —  the  roots 
of  the  eruptions  —  in  the  underlying  granite. 

G-reen  Hill  Conglomerate. — The  prominent  ledge  of  con- 
glomerate which  has  been  laid  bare  by  the  wearing  away  of  the 
northern  slope  of  Green  Hill,  is  probably,  as  the  fault-lines  of 
the  map  indicate,  a  part  of  the  great  bed  forming  Folsom's 
Island  and  Conglomerate  Plateau  ;  but  it  appears  more  con- 
venient to  describe  it  in  its  geographic  than  in  its  geologic 
sequence.  The  ledge  is  about  four  hundred  feet  long  from 
northwest  to  southeast,  and  shows  throughout  a  southeasterly 
dip  of  20°  to  30°,  giving  an  apparent  thickness  of  perhaps  175 
feet.  Beginning  on  the  southeast,  there  are  10  or  12  feet  of 
medium  conglomerate  with  distinct  layers  of  reddish  sandstone, 
showing  the  bedding  well.  This  is  underlain  by  about  60  feet 
in  thickness  of  very  coarse  conglomerate,  many  pebbles,  espec- 
ially of  granite,  ranging  from  six  to  eighteen  inches  in  diameter. 
The  remainder  of  the  section  is  conglomerate  of  a  more  normal 
character,  varying  from  medium  to  rather  coarse  in  texture. 
The  conglomerate  is  composed  throughout  of  well  rounded 
masses  of  granite  (coarse  and  pinkish),  felsite  of  different  vari- 
eties, and  porphyrite ;  but,  appai-ently,  little  true  melaphyr. 
The  porphyrite  is  mainly  of  compact,  dark  gray  and  brown  va- 
rieties. 


61 


The   Central  Area. 

This  area  embraces  all  that  part  of  Nantasket  east  of  the  rail- 
road and  south  of  Atlantic  and  Centre  Hills.  The  oeoloj-ical 
relations  of  the  central  area  to  the  preceding  or  coastal  area  can 
be  most  easily  traced  in  Willow  Ledge  Hill,  which  belongs  in 
part  to  each.  The  summit  and  main  mass  of  this  elevation  is  a 
nearly  square  block  of  melaphyr  and  tuff.  These  rocks  have 
been  described  in  the  preceding  section,  and  the  limits  of  this 
block  are  sharply  defined  on  the  map  by  the  bounding  fault- 
lines.  The  similar  block  of  conglomerate  on  the  west  lies  at  a 
lower  level,  and  its  outcrops  are  separated  from  the  volcanic 
block  by  a  small  artificial  pond. 

Both  of  these  blocks  are  separated  from  the  narrow  ridge  of 
conglomerate  between  them  and  Hull  Street  by  the  well-marked 
east- west  depression  occupied  by  Spring  Valley  Road.  This 
depression  is  an  evident  fault-line,  for  the  beds  of  conglomerate 
are  so  nearly  horizontal  that  they  must  abut  directly  against  the 
melaphyr ;  and  the  contact  is  undoubtedly  oblique  to  the  strike 
of  the  conglomerate.  As  indicated  on  the  map,  this  is  one  of 
the  great  faults  of  the  Nantasket  region,  traversing  the  entire 
area,  from  Strait's  Pond  to  Weir  River  Bay.  Few  of  the  dis- 
locations of  this  district  are  more  obvious,  and  none  are  of 
greater  structural  importance.  East  of  Valley  Beach  Avenue 
it  is  marked  by  the  well-defined  escarpment  terminating  the 
melaphyr  of  Centre  Hill  on  the  south,  the  conglomerate  lying 
at  a  lower  level  here,  under  a  meadow  ;  while  to  the  westward 
it  coincides  with  the  similar  escarpment  along  the  north  side  of 
Melaphyr  Plateau,  and,  crossing  the  bay,  defines  approximately 
the  abrupt  northern  shore  of  the  western  area.  This  line  of 
fracture  is  thus  clearly  the  natural  boundary  line  between  the 
coastal  and  central  areas.  But  when  we  pass  to  the  considera- 
tion of  the  direction  and  amount  of  the  displacement  along  this 
line,  and  the  stratigraphic  relations  of  the  two  areas,  a  much 
more  difficult  problem  confronts  us.      The  escarpments,   which 


62 

face  now  to  the  south  and  then  to  the  north,  evidently  signify 
nothing  but  unequal  erosion,  appearing  always  on  the  side  of 
the  harder  and  more  resistant  rock,  which  is  usually  melaphyr. 
Facts  will,  however,  be  accumulated  in  the  following  pages 
pointing  to  the  correlation  of  the  conglomerate  of  Conglomerate 
Plateau  with  a  horizon  below  that  of  the  conglomerate  on  the 
extremity  of  Long  Beach  Rock  ;  and  hence  to  the  conclusion 
that  the  downthrow  is  not  only  on  the  north,  but  that  it  must 
exceed  the  entire  thickness  of  the  melaphyr  series  of  Atlantic 
and  Centre  Hills, —  four  hundred  if  not  five  hundred  feet. 

Although  melaphyr  occurs  abundantly  in  the  central  as  well 
as  the  coastal  area,  it  is  of  a  diflferent  and  distinctly  more  basic 
type.  It  is  very  generally  and  often  profusely  amygdaloidal, 
rarely  distinctly  brecciated  and  never,  so  far  as  observed,  ac- 
companied by  beds  of  tuff.  The  abundance  of  conglomerate 
south  of  the  great  fault,  as  well  as  the  numerous  faults,  still 
further  contrasts  the  two  areas.  It  is,  of  course,  quite  possible, 
if  not  probable,  that  some  of  the  faults  of  the  coastal  area  have 
escaped  discovery  on  account  of  the  uniform  character  of  the 
melaphyr  in  Atlantic  and  Centre  Hills  ;  and  the  great  apparent 
thickness  of  this  melaphyr  ought,  perhaps,  to  be  regarded  as 
largely  due  to  repetition  by  faulting. 

The  alternations  of  the  conglomerate  and  melaphyr  in  the 
central  area  are  so  frequent  that  it  will  be  more  convenient  to 
describe  these  rocks  together,  following  the  order  of  the  ledges 
or  t02:)ographic  features. 

East  shore  of  Nantasket  Bay,  Crescent  Hill  and  Marsh 
Island. — These  localities,  forming  a  narrow  belt  across  the 
western  end  of  the  central  area,  between  the  railroad  and  Mela- 
phyr Plateau,  may  be  conveniently  described  together,  since,  as 
the  map  shows,  they  present,  as  the  result  of  parallel  faulting, 
simply  repetitions  of  certain  beds  of  conglomerate  and  melaphyr. 
This  tract  is  bordered  by  two  well-marked  north-south  faults, 
and  this  primaiy  fault-block  is  divided  by  no  fewer  than  twelve 
transverse  fractures,  witli  the  slips  alternating  in  direction,  and 
indicating  the  unequal  rising  and  sinking  of  V-shaped  blocks 


63 


(Fig.  8).  Commencing  at  the  north  end,  the  first  ledges  of 
conglomerate  and  melaphyr  rise  somewhat  abruptly  from  the 
marsh  and  the  bay  immediately  south  of  the  great  east-west 
fault,  and  about  five  hundred  feet,  following  the  railroad,  south- 
west of  the  broad,  low  outcrop  of  red  and  green  slate  or  slaty 
tuff  already  described ;  and,  as  the  map  shows,  the  section  is 
almost  uninterrupted  along  the  east  side  of  the  railroad  from 
this  point  to  the  south  side  of  Marsh  Island. 


Melaphyr. 


Conglomerate. 


■  Fig.  5. — Section  showing  the  fault-dike  (25)  and  the  contact 
between  the  third  conglomerate  and  second  melaphyr, 
south  of  the  fault.     scale,  i    inch  =^  30  ft. 

It  begins  with  the  typical  melaphyr  of  the  central  area,  a 
dark  green,  evidently  basic  and  highly  amygdaloidal  variety. 
The  amygdules  are  not  uniformly  distributed ;  but  they  are 
especially  abundant  in  a  superficial  layer  of  the  rock  from  two 
to  three  feet  thick.  They  are  usually  somewhat  elongated  and 
have  a  distinctly  linear  arrangement  parallel  with  the  layer ;  so 
that  the  flow-structure  of  the  melaphyr  as  a  whole  is  very  marked. 
This  mass  of  melaphyr  has  an  exposed  breadth  of  about  forty  feet 
north  and  south  and  is  divided  midway  by  dike  26  ;  while  it  is 
abruptly  terminated  on  the  south  by  dike  25.  This  dike  un- 
doubtedly marks  an  east-west  fault ;  for  it  is  bordered  by  con- 
glomerate on  the  south  (Fig.  5).  The  fault  probably  hades 
with  the  dike  to  the  north,  and  the  melaphyr  is  clearly  on  the 
downthrow  side.  Not  more  than  eight  feet  in  thickness  of  the 
conglomerate    are  exposed  before  it    is  overlain  by  melaphyr. 


64 

The  contact  is  very  sharply  defined,  and  most  admirably 
exposed ;  and  it  is  conformable  to  the  flow-structure  in  the 
melaph^'r,  dipping  south-southeast  5°-l(y.  This  melaphyr 
appears  to  be  identical  with  that  north  of  the  fault,  the  amyg- 
dules  and  flow-structure  being  even  more  pronounced  ;  and  they 
may  be  fairly  regarded  as  parts  of  the  same  flow.  The  displace- 
ment here  has  been  suflficient  to  conceal  the  conoflomerate  and 
to  prevent  its  reappearance  at  the  northern  edge  of  the  mela- 
phyr, perhaps  fifteen  feet  as  a  minimum.  It  should  be  stated, 
however,  that  the  melaphyr  on  the  north  or  downthrow  side 
appears  to  be  about  horizontal  or  to  have  a  slight  northerly  dip. 
Above  the  conglomerate,  on  the  south  side  of  the  fault,  the 
melaphyr  has  a  breadth  across  the  strike  before  we  come  to  the 
overlying  conglomerate  of  nearly  one  hundred  feet  and  an  appar- 
ent thickness  of  less  than  twenty  feet.  The  true  thickness,  how- 
ever, is  pi'obably  not  less  than  twenty-five  or  thirty  feet,  the 
apparent  diminution  being  due  to  faults  with  the  downthrow 
to  the  south  accompanying  dikes  29  and  31.  Southwesterly  or 
parallel  with  the  railroad  the  sloping  edge  of  the  melaphyr  flow 
is  exposed  broadly  and  almost  continuously  as  far  as  dike  35. 
Near  the  railroad  this  dike  marks  the  contact  of  the  melaphyr 
and  overlying  conglomerate.  The  exposure  is  not  very  satis- 
factory ;  but  there  seems  no  reason  to  doubt  that  the  melaphyr 
passes  under  the  conglomerate  conformably  and  wit'nout  appre- 
ciable faulting.  Following  the  contact  to  the  northeast,  it 
sweeps  around  in  a  curve  concave  toward  the  melaphyr.  The 
two  rocks  are  seen  near  together,  but  not  in  actual  contact,  un- 
til we  come  to  the  large  inclined  dike  (31).  Here  a  slight 
fault  with  the  upthrow  to  the  north  brings  the  melaphyr  into 
sight  directly    under  the    conglomerate    (Fig.  6). 

This  conglomerate  overlying  the  melaphyr  is  similar  to  that 
underlying  it.  It  is,  however,  coarser  and  more  irregular  in 
composition,  containing  many  rounded  pebbles  of  granite  and 
felsite  6  to  12  inches  in  diameter  ;  and  it  is  especially  distin- 
guished by  holding  many  large  pebbles  of  melaphyr,  some  of 
which  are  coarsely  amygdaloidal  and  apparently  identical  with 


or, 


that  on  which  the  conglomerate  rests.  The  conglomerate  outcrops 
broadly  south  of  dike  35,  forming  a  low  hill ;  and  a  few  streaks 
of  sandstone  show  that  the  bed  dips  southeast  about  10°,  so 
that  the  thickness  can  scarcely  exceed  25  feet.  It  is  distinctly 
overlain  on  the  southeast  corner  of  the  outcrop  by  a  second 
flow  of  amygdaloidal  melaphyr.  The  surface  of  the  ledge 
slopes  with  the  dip,  and  thus  a  thin  layer  of  melaphyr  covers 
quite  a  breadth  of  the  conglomerate.  A  small  triangular  patch 
of  the  overlying  melaphyr  is  also  seen  resting  on  the  conglom- 
erate immediately  north  of  dike  35,  indicating,  apparently,  a 
slight  fault  with  the  downthrow  to  the  north  on  the  line  either 
of  this  dike  or  of  the  small  dike  (3G)  which  is,  probably,  a 
branch  of  the  main  dike. 


Comrlotnerate.  ^ 


Melaphyr.  Ca/j^-'omerafe. 

Fig.  6. — Section  showing   the   fault-dike  (31)    and  the  contact 

BETWEEN     THE     SECOND      MELAPHYR    AND    THE    FOURTH     CONGLOM- 
ERATE,   NORTH    OF    THE    FAULT.       ScALE,     I    INCH  =  30    FEET. 

Between  dikes  29  and  31  the  conglomerate  crosses  the  north- 
south  fault  bounding  Melaphyr  Plateau  and  extends  eastward 
into  the  melaphyr  about  100  feet  with  a  breadth  of  75  feet. 
This  extension,  being  on  the  upthrow  side  of  the  fault,  is  ele- 
vated above  the  rest  of  the  bed  some  20  feet.  At  its  south-east 
corner  the  extension  is  overlain  by  amygdaloidal  melaphyr  of  the 
same  character,  in  precisely  the  same  manner  and  with  the  same 
southeasterly  dip  (10'-15°)  as  at  the  southeast  corner  of  the 
lower  area.  The  extension  is  undoubtedly  cut  off  b}^  a  fault 
on  the  south,  with  the  downthrow  in  that  direction,  so  that 
the  contact  of  the  conglomerate  and  overlying  melaphyr  in  the 

OCCAS.  PAPERS.  B.  S.  N.   H.  IV.  6. 


f.f. 

extension  can  not  be  traced  far  in  the  direction  of  the  same 
contact  in  the  main  area.  The  conglomerate  extension  is  bonnd- 
ed  on  the  north  by  dike  29,  which  clearly  marks  a  fault  with 
the  downthrow  also,  probably,  to  the  south.  Hence  the  mela- 
phyr  north  of  the  conglomerate  extension  appears  to  be  that 
normally  underlying  the  conglomerate,  and  therefore  distinct 
from,  and  separated  by  a  fault  from,  that  overlying  the  con- 
glomerate. 

The  small  meadow  or  marsh  sepai'ating  the  conglomerate 
south  of  dike  35  from  Crescent  Hill  must  cover  a  fault  with  the 
downthrow  to  the  north,  as  indicated  on  the  map  ;  for  the  bed 
of  conglomerate  first  described  and  the  underlying  and  over- 
lying melaphyrs  are  repeated,  with  a  stronger  topographic  re- 
lief and  fewer  dikes,  in  the  hill.  .  The  conglomerate  forms  the 
summit  of  the  northwestern  part  of  the  hill  and  extends  en- 
tirely around  the  northern  and  western  slopes,  its  outcrop  thus 
havino;  the  form  of  a  crescent.  It  is  the  irreo-ular  or  mixed 
coarse  and  fine  conglomerate,  containing  pebbles  of  amygda- 
loidal  melaphyr  as  Avell  as  of  granite  and  felsite.  The  contact 
with  the  overlying  melaphyr  is  very  plainly  exposed  on  the 
summit.  It  is  a  true  effusive  contact  and  has  a  southeasterly 
dip  of  10° — 20°,  being  conformable  with  the  bedding-planes  of 
the  conglomerate.  This  overlying  melaphyr  shows  several  dis- 
tinct zones  of  amygdules,  as  well  as  other  indications  of  flow- 
structure,  and  altogether  exposes  a  thickness  of  25  to  35 
feet.  The  contact  is  also  clearly  exposed  for  fifteen  feet  at  the 
base  of  the  hill  on  the  east  side.  The  thickness  of  the  con- 
glomerate can  scarcely  exceed  30  or  35  feet.  The  underlying 
melaphyr  exposes  a  thickness  of  about  fifteen  feet  at  the  north- 
west corner  of  the  hill,  the  contact  descending  both  east  and 
south.  Near  the  middle  of  the  west  side  of  the  liill  the  under- 
lying melaphyr  appears  again,  being  elevated  some  ten  feet 
above  the  railroad  by  an  east-west  fault  with  the  upthrow  to 
the  north  (Fig.  7).  This  fault  gives  the  conglomerate  and 
underlying    melaphyr   a    distinctly  synclinal    structure    in    the 


fiT 


north-south  line,  the  axis  of  the  syncline  clipping  east  10°  or 
more  ;  which  prevents  the  mclaphyr  from  reappearing  on  the 
east  side  of  the  hill. 


Consrlomerate. 


Melafhyr. 


Fig.  7- — Section  across  the  fault  On  the  west  side  of  crescent 

HILL.   scale,  I  inch  =  50  FEET. 

Crescent  Hill  is,  in  fact,  divided  by  a  series  of  east- west 
faults.  Commencing  on  the  north,  the  first  fault  is  65  feet  from 
the  northwest  corner  of  Crescent  Hill  and  exactly  opposite  the 
noi'thcrn  end  of  Great  Hill.  It  downthrows  to  the  north  about 
eicjht  feet,  iogofino-  the  contact  of  the  cono-lomerate  and  lower 
melaphyr  ;  and,  since  it  does  not  now  cut  the  upper  melaphyr, 
the  effect  is  to  increase  the  apparent  thickness  of  the  conglom- 
erate. The  second  fault  is  tliirty  feet  farther  south,  it  hades 
S.  25°  and  downthi^ows  in  the  same  direction  about  eight  feet, 
being  compensating  with  reference  to  the  first  fault.  Twenty- 
five  feet  farther  south  is  a  small  dike  (43)  and  the  only  one 
observed  on  the  hill.  Accompanying  the  dike,  but  not  coin- 
ciding in  plane  with  it,  is  the  third  fault,  which  hades  and 
downthrows  to  the  south,  the  throw  being  about  eight  feet,  and 
the  beds  sloping  steeply  from  the  fault  on  the  north.  The  next 
or  fourth  fault,  seventy  feet  farther  south,  is  that  already  re- 
ferred to,  on  the  west  side  of  the  hill.  This  is  one  of  the 
plainest  and  most  instructive  faults  in  the  Nantasket  district. 
Figure  7  represents  a  general  view  of  the  fault  as  seen  from 
the  railroad,  taking  in  the  whole  height  of  the  hill.  The 
fracture  is  still  a  well-marked  and  somewhat  open  fissure  ;  and 
the  jogging  of  the  contact  of  the  conglomerate  and  upper  mela- 
phyr shows    that   the    downthrow  is  to  the    south    and  about 


68 

eighteen  feet.  The  lower  nielapliyr  very  clearly  rises  to  the 
south  or  toward  the  fault,  and  again  to  the  north,  giving  the 
syncline  already  described.  The  fault  trends  N.  75°  E.,  being 
in  line  with  dike  37  in  Great  Hill ;  and  it  can  be  readily  traced 
directly  across  the  entire  breadth  of  the  hill,  following  approxi- 
mately the  boundary  line  between  the  conglomerate  and  upper 
melaphyr,  and  breaking  this  contact  almost  as  distinctly  on  the 
eastern  as  on  the  western  face  of  the  hill.  About  midway  be- 
tween this  fault  and  the  southern  end  of  the  conglomerate  is  a  fifth 
fault,  which  is  the  counterpart  of  the  last,  the  downthrow  being  to 
the  north  and  about  15  feet.  It  plainly  jogs  the  contact  between 
the  conglomerate  and  upper  melaphyr;  and  trends  N.  75°  W., 
being  in  line  with  dike  44  on  Great  Hill.  Finally,  at  the  ex- 
treme southern  end  of  the  conglomerate  there  appears  to  be  a 
sixth  fault,  probably  parallel  with  the  last,  but  with  the  down- 
throw to  the  south.  The  amount  of  the  throw,  if  to  the  south, 
is  certainly  10  if  not  15  feet,  being  enough  to  throw  the  con- 
glomerate entirely  out  of  sight.  If  the  throw  were  to  the  north 
it  would  need  to  be  25,  and  more  probably  50,  feet  to  do  this. 

The  fact  that  the  melaphyr  under  Crescent  Hill,  although 
rising  to  the  west,  can  not  be  found,  not  even  the  slightest  trace 
of  it,  on  Great  Hill,  is  alone  sufficient  to  prove  an  important 
north-south  fault  between  the  two  hills,  with  the  downthrow  to 
the  east ;  and  the  further  fact  that  not  one  of  the  dikes  in  Great 
Hill  can  be  traced  in  Crescent  Hill  points  to  the  same  conclu- 
sion. Indeed,  the  contrast  presented  by  the  opposite  walls  of 
this  narrow  defile  is  one  of  the  especially  surprising  and  puz- 
zling features  of  Nantasket  geology.  The  very  abrupt  way  in 
which  the  conglomerate  of  Crescent  Hill  terminates  on  the  east 
proves  that  the  hill  is  bounded  in  this  direction  also  by  an  im- 
portant north-south  fault,  the  downthrow  probably  being  as 
before  on  the  side  of  Crescent  Hill,  or  to  the  west. 

Marsh  Island  is  a  large  ledge  of  conglomerate  and  melaphyr 
completely  isolated  by  the  eastern  marsh.  Geologically  it  is 
but  a  repetition,  at  a  lower  level  and  without  important  dis- 
placements, of  Crescent  Hill.     The  conglomerate,  which  forms 


60 


the  main  mass  of  tlie  ledge,  is  of  the 
coarse,  irregular  type  seen  in  Crescent 
Hill  ancVon  the  east  shore  of  the  bay,  ■^  n 
containing  many  large  pebbles  of  gran-  m  ^ 
ite,  felsite  and  melaphyr.  The  breadth 
of  the  conglomerate  outcrop,  from 
northwest  to  southeast,  is  about  140 
feet ;  and  the  thickness  of  the  bed,  the 
upper  and  lower  surfaces  of  which  are 
clearly  exposed,  is  about  25  or  30  feet. 
It  is  underlain  on  the  northwest,  at  the 
very  base  of  the  ledge,  by  a  highly 
scoriaceous  and  amygdaloidal  green 
melaphyr,  of  which  only  a  slight  thick- 
ness is  exposed ;  and  is  overlain  on  the 
southeast  by  a  very  similar  flow  of 
melaphyr,  which  at  one  point  reaches 
up  over  the  conglomerate  nearly  to  the 
top  of  the  ledge.  These  are  undoubt- 
edly true  effusive  contacts,  and  the  up- 
per one  especially  shows  the  dip  of  the 
conglomerate  to  be  S.  E.  10° — 20°. 
The  melaphyr  contains  abundant  seg- 
regations and  veins  of  epidote  and  red 
jasper.  Since  there  appears  to  be  no 
reason  whatever  to  question  the. strati- 
graphic  equivalence  of  Crescent  Hill 
and  Marsh  Island,  it  is  necessary  to 
suppose  that  the  marsh  between  them 
conceals  an  important  fault ;  the  down- 
throw being  to  the  north  and  ,  fully 
equal  to  the  combined  thickness  of  the 
conglomerate  and  upper  melaphyr, 
about  50  or  60  feet. 

The  foreo-oino;  details,  which  are  summarized  in  the  north- 
south  section   (Fig.    8),   show  that  the   various  stratigraphic 


1) 


70 

features  of  this  belt  of  ledges  are  readily  explained  by  one  bed 
of  conglomerate  between  two  very  similar  flows  of  melaphyr, 
with  only  such  dislocations  as  are  actually  seen  or  may  be  rea- 
sonably inferred,  until  we  come  to  the  conglomerate  underlying 
the  lower  melaphyr  south  of  dike  25.  This  is  a  partial  expo- 
sure only  of  a  lower  and  older  conglomerate  which  does  not 
appear  elsewhere  in  the  section. 

Although  the  transverse  faults  of  this  block  are  frequently  re- 
versed in  throw  and  compensating,  the  northerly  throw  clearly 
prevails,  the  sum  of  all  the  displacements  giving  161  feet  to 
the  north  and  71  feet  to  the  south.  Hence  the  entire  series  is 
equal  to  one  northerly  slip  of  90  feet. 

Melaphyr  Plateau. — This  rectangular  block  of  melaphyr 
is  sharply  defined  by  the  bounding  fault  lines  and  escarpments, 
and  may  be  regarded  as  a  very  characteristic  feature  in  Nan- 
tasket  geology.  With  the  exception  of  the  small  patch  of  con- 
glomerate on  the  western  edge,  it  consists  throughout  of  the 
most  typical,  basic,  green  variety  of  melaphyr,  with  frequent 
highly  amygdaloidal  layers.  The  flow-structure  is  very  perfect 
at  many  points,  and  shows  that  the  sheets  or  flows  are  still 
nearly  horizontal,  with  slight  undulations  to  north  and  south  and, 
perhaps,  a  very  gentle  general  dip  to  the  southeast.  The  facts 
already  stated  in  connection  with  the  mass  of  conglomerate  pro- 
jecting into  the  Avestern  edge  of  the  plateau  indicate  that  the 
plateau  embraces  both  of  the  flows  of  melaphyr  observed  in  the 
Crescent  Hill  section.  The  lower  flow  forms  the  northern  bor- 
der of  the  plateau,  or  that  portion  north  of  the  conglomerate  ex- 
tension :  while  all  the  remaining  area,  or  fully  three-fourths  of 
the  whole,  must  be  referred  to  the  upper  melaphyr.  The  con- 
glomerate, which  normally  separates  the  two  flows,  has  been  en- 
tirely cut  out  of  the  section,  so  far  as  the  surface  development 
is  concerned,  by  the  oblique  strike  ftiults.  At  the  extreme  east- 
ern end  of  the  plateau  there  are  indications,  in  the  form  of  thin 
outliers  of  sandstone,  that  the  upper  melapliyr  was  also  for- 
merly covered  by  conglomerate.     It  will  appear  later  that  this 


n 

conglomerate  overlying  the  upper  mclaphyr  should  not  be  cor- 
related with  that  forming  the  adjacent  ledges  of  Conglomerate 
Plateau,  but  rather  with  that  outcropping  on  Long  Beach 
Rock  and  underlying  the  melaphyr  series  of  Atlantic  Hill. 

Conglomerate  Plateau. — This  broad  mass  of  conglomerate? 
the  largest  in  the  Nantasket  district,  is  of  the  normal  type,  well 
rounded  pebbles  of  felsite  and  granite  predominating,  although 
pebbles  of  porphyrite  and  compact  melaphyr  are  also  common. 
The  texture  varies  from  moderately  coarse  conglomerate  to 
sandstone  ;  but  the  bedding  can  be  made  out  at  only  a  few  points 
as  on  the  north  side  of  Spring  Valley  Road.  The  dip  seems, 
however,  to  be  always  gentle  (5° — 10°)  to  the  south-southeast, 
diminishing  toward  the  granite.  The  prominent  ledges  ex- 
hibit some  well  developed  joint-planes  ;  and  the  plateau  is  un- 
doubtedly bounded  on  all  sides,  and  not  alone  on  the  north,  by 
fault-lines.  It  rises  abruptly  from  the  marsh  on  the  west ;  Hull 
Street  and  Spring  Valley  Road  traverse  it  in  narrow  defiles  ; 
while  the  straight  and  solid  wall  of  conglomerate  15  to  30  feet 
high  in  which  it  terminates  on  the  south  is  an  exceptionally  typ- 
ical fault  scarp.  Facts  will  be  presented  later  tending  to  show 
that  the  downthrow  in  the  case  of  all  of  these  faults  is  on  the  side 
away  from  the  plateau,  and  hence  that  this  conglomerate  is  older 
than  and  normally  underlies  the  surrounding  formations  ;  but,  as 
already  indicated,  the  displacement  is  mticli  greater  on  the  north 
side  than  in  any  other  direction.  Of  the  thickness  of  the  con- 
glomerate it  is  impossible  to  speak  with  certainty,  since  neither 
the  base  nor,  probably,  the  original  summit  of  the  bed  are  ex- 
posed. It  can  not  be  less  than  the  height  of  the  plateau,  about 
fifty  feet,  and  may  exceed  one  hundred  feet,  this  being,  prob- 
ably, the  thickest  of  all  the  Nantasket  beds. 

Round  Hill. — This  hill,  which,  as  the  map  indicates,  is  named 
for  its  curved  outline,  is  a  solid  mass  of  conglomerate  rising 
very  abruptly  from  the  eastern  marsh  on  the  north  and  west  to 
to  a  height  of  fiftj  feet  and  sloping  gently  down  to  the  south- 


72 

east  beneath  a  bed  of  melapliyr.  The  conglomerate  of  Round 
Hill  is  connected  with  that  of  the  plateau  by  essentially  contin- 
uous outcrops  along  the  eastern  border  of  the  marsh,  and  must 
be  regarded  as  a  part  of  the  same  bed.  It  is  of  medium  texture 
above,  passing  in  the  lower  part  of  the  hill  into  fine  conglom- 
erate and  a  considerable  thickness  (10  to  20  feet)  of  reddish  sand- 
stone ;  while  in  the  ledges  rising  from  the  marsh  on  the  southwest 
we  have  a  coarse  and  irregular  conglomerate  with  layers  of 
sandstone,  which  show  that  in  these  outlying  ledges  as  well 
as  in  the  hill  the  dip  is  mainly  to  the  east  or  east-southeast  and 
very  gentle  — 5°  to  10°,  but  rarely  exceeding  5°.  The  exposed 
thickness  of  the  conglomerate,  including  the  outlying  ledges,  is 
probably  between  75  and  100  feet.  The  low  ledge  of  melaphyr 
protruding  from  the  marsh  on  the  north  side  of  the  hill  is  prob- 
ably a  part  of  the  bed  underlying  the  mai'sh  ;  and  the  fault  sep- 
arating this  melaphyr  from  the  conglomerate  is  thus  given  a 
general  east-west  course.  Since  both  this  fault  and  that  bound- 
ing the  marsh  on  the  east  cut  across  the  strike  of  the  conglom- 
erate, the  breadth  of  the  outcrop  of  tliat  x'ock,  as  the  map  shows, 
has  been  g-reatlv  diminished  at  this  southeast  ansfle  of  the  marsh 
where  the  fault-lines  meet.  According  to  the  view  here  pre- 
sented, the  melaphyr  forming  the  floor  of  the  marsh  belongs  to 
the  same  bed  as  that  overlying  the  conglomerate,  and  hence  the 
conolomerate  must  also  underlie  the  marsh,  but  at  a  sfreater 
depth  than  the  melaphyr.  More  probably,  however,  as  the  iso- 
lated ledge  of  conglomerate  in  the  channel  east  of  Marsh  Island 
suggests,  the  entire  section  of  Marsh  Island  and  Crescent  Hill  is 
represented  beneath  the  marsh,  the  latter  being  bounded  by  faults 
on  all  sides  and  separated  from  Round  Hill  by  a  slip  of  at  least 
150  feet. 

South  of  the  hill,  on  the  bank  of  the  river  (Lyford's  Liking), 
the  granite  is  well  exposed,  with  conglomerate  and  sandstone 
resting  against  and  upon  it.  The  sandstone  is  really  a  fine,  red- 
dish and  intensely  hard  quartzite  ;  and  both  it  and  the  conglom- 
erate appear  to  fill  fissures  and  depressions  in  the  surface  of  the 
granite,  in  the  same  manner  as  the  basal  conglomerate  of  the 


73 

western  area  to  be  described  later.  Tf  we  make  tbis  correlation, 
then,  since  it  is  reasonably  certain  that  other  beds  of"  niela|tliyi- 
as  well  as  conglomerate  underlie  the  coni2;lomerate  of  Hound 
Hill,  and  we  can  not  connect  the  latter  with  the  basal  c<mglom- 
erate  on  the  granite  ;  it  appears  necessary  to  suppose  that  an 
important  fault,  with  the  downthrow  to  the  norths  separates  tlie 
granite  from  the  hill, — a  continuation  with  a  different  direction 
of  the  fault  extending  from  Granite  Point  on  Weir  River  Bay 
southeasterly  across  Great  Hill. 

The  melaphyr  on  the  southeast  side  of  Round  Hill  is  nowhere 
seen  in  satisfactory  contact  with  the  conglomerate  ;  but  the  slope 
of  the  hill  on  this  side  is  nearly  parallel  with  the  bedding  of  the 
conglomerate,  and  all  the  indications  are  favorable  to  the  view 
that  the  melaphyr  overlies  the  conglomerate  conformably.  This 
is  the  typical  green  and  amygdaloidal  melaphyr  of  the  central 
area,  and  cannot  be  distinguished  from  that  outcropping  in  the 
marsh.  The  flow-lines  dip  gently  to  the  southeast,  as  do 
the  bedding  lines  of  the  conglomerate.  To  the  north,  this  bed 
of  melaphyr  approaches  very  near  the  southeast  corner  of  the 
marsh,  as  already  explained  ;  and  we  here  reach  the  turning 
point  of  a  marked  and  rather  abrupt  change  in  the  strike  of  both 
the  conglomerate  and  melaphyr.  The  normal  dip  of  the  Round 
Hill  beds  appears  to  be  about  east-southeast,  and  of  the  strata  of 
Conglomerate  Plateau  and  the  district  directly  south  of  it,  about 
south-southeast . 

The  District  between  Round  Hill  and  Hull  Street. — 

This  tract  consists  of  rocky  fields  sloping  southward  from  the 
escarpment  marking  the  southern  border  of  Conglomerate 
Plateau  to  the  marshes  of  Lyford's  Liking.  The  melaphyr 
overlying  the  conglomerate  on  the  southeast  slope  of  Round 
Hill  bends  to  the  east,  as  already  explained,  to  follow  the 
southern  border  of  Conglomerate  Plateau.  Dikes  40  and  41 
aie  evidently  attended  by  some  faulting  ;  and  the  narrow  band  of 
conglomerate  in  the  melaphyr  may  be  accounted  for  in  this  way  ; 
but  whether  it  belongs  to  the  conglomerate  below  the  melaphyr 


74 

or  to  that  above  it  is  not  quite  clear.  The  conglomerate  south 
and  east  of  the  melaphyr  undoubtedly  overlies  it,  although  there 
are  no  satisfactory  exposures  of  the  contact.  The  limited  mass 
of  melaphyr  between  the  conglomerate  and  the  granite,  although 
possibly  not  in  sitic,  is  probably  a  reappearance  of  the  melaphyr 
separating  the  two  beds  of  conglomerate.  It  is  topographically 
below  the  upper  conglomerate.  An  important  east-west  fault,  a 
continuation  of  the  great  border  fault  already  referred  to,  must 
separate  the  conglomerate  and  melaphyr  from  the  adjoining 
ledofes  of  o^ranite  ;  and  the  latter  here  exhibit  no  traces  of  the 
basal  conglomerate.  The  rather  abrupt  northward  jog  of  the 
granite  border  may  be  explained  by  a  southward  extension  with  a 
slight  displacement  of  the  fault  between  Conglomerate  Plateau  and 
the  marsh,  as  shown  on  the  map.  Farther  east  the  melaphyr 
rises  and  broadens  out,  the  overlying  conglomerate  having  been 
carried  above  the  present  plane  of  erosion  ;  and  the  underlying 
conglomerate  fails  to  outcrop  along  the  southern  base  of  Con- 
glomerate Plateau.  In  fact  indications  are  not  wanting  that  the 
melaphyr  here  covers  the  entire  area  between  the  plateau  and 
the  marsh.  This  would  be  readily  explained  without  increasing 
the  thickness  of  the  melaphyr  by  simply  regarding  the  beds  as 
horizontal  here  ;  and  the  assumption  is  fully  justified,  even 
though  the  contact  of  the  melaphyr  and  conglomerate  can  not 
be  seen,  by  the  fact  that  the  flow-structure  is  well-marked  in 
this  melaphyr  and  quite  horizontal. 

The  district  between  Hull  Street  and  Strait's  Pond. — 

This  area  is  separated  from  the  last  by  the  fault,  probably  of  no 
irreat  magnitude,  terminatino-  Cono-lomerate  Plateau  on  the  east. 
It  is  topographically  similar,  and  composed,  apparently,  of  the 
same  conglomerate  and  melaphyr,  although  extending  north  to 
the  great  east-west  fault  and  the  melaphyr  of  Centre  Hill. 
The  chief  point  of  difference  is  found  in  tlie  frequent  alternations 
of  conglomerate  and  melaphyr.  These  alternations  are  best  ex- 
plained by  repeated  east-west  faults,  dividing  the  district  into 
eeven  or  eight  narrow  blocks,  as  shown  on  the  map.     No  satis- 


75 

factory  contacts  have  been  observed  ;  but  the  le(l<T;es  are  so  re- 
lated as  to  indicate  that  the  contacts  must  mark  f'aidts  in  most  cases 
at  least.  The  melaphyr  is  similar  to  tliat  west  of  Hull  Street ;  and 
the  conglomerate  is  probably  that  imderlying  the  melaphyr,-  the 
cono^lomerate  of  Round  Hill  and  Conoflomerate  Plateau  :  although 
it  would  be  difficult  to  prove  that  the  overlying  conglomerate  is 
not  also  represented  here.  Dike  31,  the  only  one  observed  in 
this  district,  separates  conglomerate  and  melaphyr  and  is  un- 
questionably a  fault-plane.  On  the  south,  near  the  Hull  Street 
bridge,  the  granite  outcrops  boldly,  with  no  traces  of  conglom- 
erate or  melaphyr  upon  it.  The  melaphyr  outcrops  very  near  the 
granite  on  the  north  ;  but,  although  the  contact  is  not  exposed,  it 
seems  impossible  to  doubt  that  a  fault  divides  them  here,  as  at 
points  farther  west.  Folsom's  Island  probably  lies  on  the  far- 
ther side  of  a  moderate  north-south  fault,  which  terminates  these 
naiTOw  fault-blocks  and  defines  the  abrupt  eastern  face  of  Cen- 
tre Hill.  There  seems  to  be  no  reason  to  doubt  that  this  is 
the  conglomerate  underlying  the  melaphyr ;  and  since  a  careful 
search  has  failed  to  discover  any  melaphyr  here,  we  may  fairly 
suppose  that  it  is  unbroken  by  east-west  faults,  that  the  bedding 
planes  are  essentially  horizontal,  and  that  it  has  been  elevated 
sufficiently  by  the  north-south  fault  just  referred  to  to  carry  the 
melaphyr  wholly  above  the  present  plane  of  erosion.  Crossing 
Strait's  Pond  at  this  narrowest  point,  we  find  what  appears  to 
be  the  same  conglomerate  outcropping  abundantly  between  the 
pond  and  Jerusalem  Road-  It  extends  quite  up  to  the  street, 
and  immediately  opposite  are  the  ledges  of  granite.  The  beds 
sti'ike  about  northwest  and  dip  northeast  30°  to  40°.  It  is 
plain  that  the  conglomerate  runs  diagonally  against  the  wall  of 
granite,  from  which  it  must  be  separated  by  a  fault.  The  con- 
glomerate probably  forms  the  whole  of  this  rectangular  area  north 
of  Jerusalem  Road  and  it  actu  ally  outcrops  again  at  the  extreme 
eastern  end,  where  the  small  promontory  projects  eastward  into 
the  pond.  In  fact,  when  we  consider  in  this  connection  the 
straightness  of  the  granite  border  and  the  important  development 
of  conglomerate  at  the  base  of  Green  Hill,  it  appears  at  least 


76 

probable  that  almost  the  entu'e  basin  of  Strait's  Pond  is  under- 
lain by  this  conglomerate.  It  is  this  view  of  the  geological 
stucture  that  is  intended  to  be  expressed  in  the  arrangement  of 
the  fault-lines  on  the  map.  Of  course,  if  the  conglomerate  is 
carried  down  to  any  considerable  depth  by  the  rather  high  dip 
which  it  shows  near  Jerusalem  Road,  it  is  very  probably  covered 
in  the  central  part  of  the  basin  by  the  overlying  melaphyr. 

The    Western  Area. 

This  area  comprises  all  that  part  of  Nantasket  proper  west  of 
the  railroad.  It  is  bounded  on  the  south  and  west  by  Weir 
River  Bay,  and  on  the  north  by  Nantasket  Harbor.  We  have 
already  met  the  fundamental  granite  along  the  southern  margins 
of  the  central  and  coastal  areas  ;  but  it  is  in  the  western  area 
that  we  first  encounter  what  appear  to  be  the  oldest  beds  of 
conglomerate  and  melaphyr  in  the  Nantasket  district.  The 
geological  structure  is  quite  as  varied  and  complex  as  in  the 
central  area,  and  as  before,  the  descriptions  may  most  conven- 
iently follow  the  topographic  order. 

G-ranite  Plateau. —  This  is  the  name  given  to  the  largest 
mass  of  granite  in  the  Nantasket  peninsula.  It  forms  the  large 
southwestern  angle  of  the  westei'n  area,  and  is  a  compact  group 
of  ledges  having,  for  the  most  part,  an  elevation  of  from  twenty 
to  forty  feet.  The  granite  is  of  the  normal  type,  and,  exhibiting 
but  slight  variations,  requires  no  special  description  ;  but  the 
outliers  of  conglomerate  scattered  over  the  plateau  are  a  feature 
of  particular  interest.  The  more  important  of  these  are  indicated 
on  the  map  ;  and  the  best  and  clearest  exposures  are,  naturally, 
to  be  found  along  the  shore.  The  conglomerate  is  most  abun- 
dant around  the  northeast  corner  of  the  plateau,  where  it  shows 
east  and  southeast  dips  of  5° — 10",  and  a  maximum  tliickness  of 
perhaps  15  feet.  Elsewhere  it  is  hardly  thick  enough  to  conceal 
the  granite  ;  and  everywhere  it  is  largely  composed  of  angular 
and  unassorted  debris  of  precisely  the  same  granite,     This  is 


Occas.  papers  Bost.  Soc.  Nat.  Hist.  IV. 


Plate  6. 


Section  of  an  irregular  fissure  in  the  surface  of  Granite 
Plateau  filled  with  conglomerate.      Scale, 
one  fifth  natural  size. 


■jrvX+V-jt^^. 


Plan  of  fissures  in  the  surface  of  Granite   Plateau 
filled  with  sandstone  and  fine  conglomerate. 
Scale,  one  fifth  natural  size. 


77 

alone  sufficient  to  prove  that  the  conglomerate  is  nevvei-  than, 
and  was  deposited  upon  the  granite  ;  although  at  first  glance, 
or  in  a  general  view,  the  opposite  relation  might  be  regarded  as 
well  sustained,  the  conglomerate  appearing  as  Isolated  masses 
enclosed  in  the  granite.  But  an  examination  of  the  contact 
between  the  two  rocks  removes  all  doubt  as  to  the  true  history 
of  Granite  Plateau.  The  conglomerate  clearly  rests  upon  the 
granite  and  fills  the  irregularities  of  its  surface.  Instead  of  the 
granite  being  intrusive  in  the  conglomerate,  the  appearances  are 
just  the  reverse,  narrow  cracks  in  the  granite  being  filled,  dike- 
like, with  conglomerate  and  sandstone.  These  fragmental  dikes, 
the  roots  of  a  once  continuous  bed  of  conglomerate,  are  exposed 
repeatedly  along  the  shore,  as  well  as  over  the  surface  of  the 
plateau  ;  and  they  seem  to  prove  conclusively  that  erosion  has 
cut  just  to  the  base  of  the  conglomerate,  clearly  exposing  the 
original  granite  floor,  but  leaving  it  essentially  intact.  It  is  very 
evident  that  the  conglomerate  was  deposited  over  such  a  broken 
and  fissured  surface  as  the  granite  presents  along  our  shore  to- 
day. 

Besides  the  granitic  debris,  the  conglomerate  includes  numer- 
ous pebbles  of  felsite  and  a  large  proportion  of  melaphyr,  show- 
ing that  volcanic  action  had  been  instituted  in  this  region  before 
the  formation  of  this  bed  of  conglomerate  and  hence  that,  al- 
though resting  directly  upon  the  granite,  it  is  not  the  true  basal 
conglomerate.  Probably,  as  a  rule,  each  successive  bed  reached 
farther  inland,  the  overlapping  edges,  only,  being  in  immediate 

contact  with  the  granite.      The  cono-Iomerate  in  the  more  south- 
ed o 

ern  outlier  on  the  eastern  edge  of  Granite  Plateau  is  curiously 
involved  with  irregular  masses  of  a  compact  eruptive  rock,  which 
can  not  be  referred  directly  to  either  an  underlying  or  overlying 
mass  of  lava  but  which  proves  on  examination  to  be  identical, 
lithologically,  with  the  first  f  cvr  to  be  described  in  other  parts 
of  the  western  area.  The  significance  of  its  occurrence  at  this 
point  will  be  explained  after  we  have  studied  the  flow  to  which 
it  is  related. 

The  granitic  area   south  and  southeast  of  the  west  marsh, 


embracino-  the  isolated  ledi^es  of  o-ranite  in  the  marsh  and  the 
southern  slope  of  Great  Hill,  is  essentially  similar  in  every  re- 
spect, including  the  outliers  of  conglomerate ;  but  the  interven- 
ing marsh  is  probably  underlain  wholly  by  conglomerate,  which 
has  been  depressed  by  faulting.  The  principal  exposures  of 
conglomerate  are  on  the  borders  of  the  marsh,  directly  south  of 
Great  Hill,  where  it  is  seen  almost  in  contact  with  but  not  di- 
rectly upon  the  granite,  dipping  away  from  the  latter  or  south- 
east 5°  to  10°.  The  outlying  mass  near  the  bank  of  the  river 
is  almost  certainly  in  siUi,  and  indicates  that  a  considerable 
body  of  conglomerate  is  concealed  here.  Traces  of  conglomer- 
ate have  been  observed  on  the  outlying  ledge  of  granite  in  Weir 
River  Bay  ;  and  the  preceding  description  might  be  repeated  for 
Granite  Point,  although  it  is  separated  from  the  plateau  by  the 
boundary  fault,  wliich,  witli  the  downthrow  to  the  north,  marks 
the  northern  limit  of  the  granite  at  most  points. 

Granite  Point  is  a  knob  of  granite  capped  with  conglomerate 
dipping  slightly  to  the  east  and  northeast,  the  beginning  of  an 
important  bed  extending  into  Cliff  Plateau.  The  relations  of 
the  granite  and  conglomerate  are  especially  clear  on  the  south 
side  of  the  point  ( Fig  9 ) .  A  large  and  irregular  hollow  or 
ravine  in  the  granite  is  filled  with  conglomerate  largely  composed 
of  granite  debris  and  including  several  lars^e  bowlders  of  o-ran- 
ite,  each  of  which  is  precisely  similar  in  charactei-  to  the  nearest 
part  of  the  enclosing  granite.  This  conglomerate  is  from  one- 
half  to  two-thirds  melaphyr,  often  in  large  and  angular  frag- 
ments. Although  this  is  so  clearly  not  the  basal  conolomerate 
of  the  Boston  Basin,  it  will  appear  in  the  sequel  that  it  is  prob- 
ably the  lowest  bed  now  exposed  in  any  of  the  Nantasket  sec- 
tions ;  and  it  will,  for  convenience,  be  designated  hereafter  the 
first  or  basal  conglomerate.  It  is  interesting  mineralogically 
on  account  of  the  numerous  segregations  of  bright  red  jasper 
which  occur  in  it.  These  are  exceedingly  irregular  in  outline, 
apparently  replacing  the  paste  of  the  conglomerate.  The  origin 
of  the  jasper  is,  doubtless,  to  be  found  in  the  alteration  of  the 
melaphyr ;   and  attention  has  already   been  called  to  the  prob- 


'9 


able  fact  that  the  intense  hardness  of  the  Nantasket  confflonier- 
ates  is  due  to  interstitial  silica  derived  from  the  same  soiu'ce. 


A-yjr  V  V  VV  \\  iry  \- 


Fig  9. — Section  of    the    granite  and   overlying   conglomerate 
ON  Granite  Point. 
Scale,  i  inch  =10  feet. 

Cliff  Plateau. — Cliff  Plateau  comprises  the  triangular  group 
of  conglomerate  and  melaphyr  ledges  between  Granite  Plateau 
and  the  shore  on  the  north.  The  average  elevation  is  about 
thirty  feet,  but  two  points  rise  above  fifty  feet.  The  name  re- 
fers to  the  well-marked  cliflp  25  to  40  feet  high  which  bounds  it 
on  the  north,  extending  from  Granite  Point  to  dike  6(i.  Gran- 
ite Point,  as  already  stated,  is  overlain  by  the  basal  conglom- 
erate, with  thin  beds  of  sandstone  showing  nearly  horizontal 
dips,  the  main  slope  being  to  the  northeast.  One  bed  of  sand- 
stone two  to  three  feet  thick  runs  the  entire  length  of  the  cliff 
facing  West  Porphyrite  Hill,  until  it  meets  dike  33,  meeting  it 
about  five  feet  above  the  beach  and  thus  sloping  approximately 
one  foot  in  ten.  It  is  tlirown  up  about  three  feet  by  this  dike 
and  then  continues  with  the  same  inclination,  but  o'rowino;  less 
distinct,  to  dike  31,  beyond  which  it  cannot  be  traced,  probably 
passing  under  the  water.  The  eastern  half  of  the  cliff,  i"ising 
abruptly  from  Nantasket  Harbor,  between  dikes  33  and  6G,  is 
composed  entirely  of  this  conglomerate,  with  the  exception  of  a 
narrow  slice  of  the  overlying  melaphyr  which,  toward  the  east- 
ern end,  has  been  let  down  by  a  fault  trending  N.  80°  E.  with 
the  downthrow  to  the  north.  In  fact,  as  the  map  shows,  the 
basal  conglomerate  forms    throughout    the  main   body   of  the 


80 

plateau,  having  for  the  greater  part  of  the  area  a  southeasterly 
dip  of  5° — 10".  It  is  overlain  on  the  east  and  southeast  by  two 
considerable  patches  of  melaphyr,  detached  areas  of  what  must 
originally  have  been  a  continuous  flow.  This  melaphyr  is  a 
greenish  to  purplish,  compact  or  finely  crystalline,  non-amygda- 
loidal  variety,  containing  epidote  in  minute  crystallizations  and 
also  larger  segregations  of  both  epidote  and  red  jasper.  It  is 
distinctly  different  from,  and  undoubtedly  older  than,  any  mel- 
aphyr in  the  Central  or  Coastal  areas,  although  bearing  some 
resemblance  to  the  more  basic  porphyrites. 

The  contact  between  the  basal  conglomerate  and  the  south- 
eastern patch  of  melaphyr  is  very  clearly  exposed,  especially  on 
the  west  side  of  the  melaphyr,  and  the  melaphyr  unquestionably 
overlies  the  conglomerate.  The  contact  appears  on  the  map  as 
a  reg-ular  curve  convex  to  the  northwest,  indicating  that  the 
melaphyr  lies  in  a  shallow  trough.  This  ap})earance  is  due 
partly,  however,  to  the  very  plain  east-west  fault  which  marks 
the  northern  edge  of  the  melaphyr,  with  a  downthrow  to  the 
south  of,  perhaps,  ten  feet.  The  melaphyr  is  distinctly  and 
comformably  overlain  on  the  southeast  by  two  small  patches  of 
what  must  be  a  second  bed  of  conglomerate,  since  the  melaphyr 
separates  it  from  the  basal  conglomerate.  These  outlines  owe 
their  preservation  chiefly  to  the  two  east-west  faults  shown  on 
the  map  and  downthrowing  to  the  south  ten  and  fifteen  feet, 
approximately.  The  boundary  fault  separating  these  rocks 
from  Granite  Plateau  is  an  absolute  necessity  at  this  point ;  for 
their  nearly  level  beds  are  vis-a-vis  with  the  abrupt  ledges  of 
granite.  The  displacement  here  cannot  be  less  than  twenty- 
five  nor,  probably,  more  than  fifty  feet ;  for  the  basal  conglom- 
erate, which  caps  the  granite  on  the  south  side  of  the  fault,  is 
at  least  forty  and  possibly  fifty  feet  thick,  and  the  overlying 
melaphyr  is  twenty-five  to  thirty  feet. 

The  northern  area  of  melaphyr  appears  also  to  occupy  a 
shallow  trough,  and  the  intervening  portion  of  the  basal  con- 
glomerate must  be  slightly  anticlinal  in  structure  ;  altliough  tlie 
ridge  coinciding  with  dikes  22  and  34,  and  slightly  capped  with 


81 


melapliyr  at  one  point,  rises  so  abruptly  from  beneath  the  mel- 
aphyr  on  the  north  as  to  suggest  an  intervening  fault,  which  is 
indicated  on  the  map,  with  the  downthrow  to  the  north.  Layers 
of  sandstone  in  the  eastern  end  of  this  rido-e  show  the  usual 
easterly  dip.  The  contact  between  the  basal  conglomerate  and 
this  northern  patch  of  mclaphyr  is  clearly  exposed  along  the 
brink  of  the  cliff  and  in  the  gorge  formed  by  dikes  66,  where  the 
dip  is  southeast  perhaps  10" ;  and  it  can  be  traced  quite  around 
the  west  end  of  the  melaphyr,  which  everywhere  plainly  over- 
lies the  conglomerate.  The  northern  patch  of  melaphyr,  like 
the  southern,  is  overlain  on  the  southeast  by  a  thin  cap  of  the 
second  conglomerate,  this  upper  contact  being  parallel  with  the 
lower.  The  normal  succession  of  all  the  rocks  in  Cliff  Plateau 
is  clearly  exhibited  in  the  accompanying  section. 


w. 


Cliff  Plateau. 


Dikes  bb.  E.  Porphyrite  Hill. 


l5^^?vv«yA'SrV;-x\";^  v^^\mi&$%i  l^^^-^v?^ 


Granite. 


Conss'lomerate. 


M''aphyr. 


mm 

Porfihyrite. 


Fig.  io. — -Section  from  Granite    Point    across    Cliff  Plateau  and  East 
Porphyrite  Hill.     Scale,  i  inch  =  400  feet. 

East  Porphyrite  Hill.  — This  hill,  which  forms  the  promi- 
nent northeast  angle  of  the  western  area,  is  separated  from  Cliff 
Plateau  by  the  group  of  north-south  dikes  {^'o)  and  the  depi-es- 
sion  or  gorge  due  to  their  erosion.  It  is  surprising  to  find  that 
these  repeated  fractures  are  accompanied  by  no  appreciable  dis- 
placement ;  but  the  basal  conglomerate,  the  melaphyr,  and  the 
second  or  overlying  conglomerate  can  all  be  traced  across  the 
dikes  without  the  least  slip  (Fig.  10).  The  basal  conglomerate 
just  barely  shows  at  the  base  of  the  cliff  on  the  east  side  of  tiie 
dikes  and  does  not  reappear  beyond,  the  easterly  slope  of  all  the 
beds    being  very  noticeable.      The   melaphyr  can  be   followed 

OCCAS.    PAPERS    B.   S.    N.    H.   IV.  C. 


82 

along  the  shore  as  far  as  dike  27,  when  it  also  disappears,  and 
the  second  conglomerate  becomes  the  prevailing  rock  on  the 
west  side  of  the  hill.  This  conglomerate,  which  is  largely  a 
fine-grained  grit  and  sandstone,  and  hence  plainly  stratified,  is 
characterized  by  segregations  of  red  jasper,  but  in  a  less  degree 
than  the  basal  conglomerate.  The  dips  are  rather  high,  E. 
about  20°  on  the  west  side  of  the  hill  and  S.  E.  27°  near  the 
overlying  porphyrite  at  the  north  base  of  the  hill.  The  con- 
glomerate is  conformably  overlain  by  the  great  mass  of  porphy- 
rite which  forms  the  summit  and  eastern  and  southern  slopes  of 
the  hill,  the  actual  contact  being  particularly  well-exposed  on 
the  north  side.  As  a  whole  this  porphyrite  is  similar  to  that  of 
the  Coastal  area,  but  it  ia  rather  more  felsitic  in  texture,  redder 
in  color,  non-epidotic ;  and  segregations  of  red  jasper  occur  in 
it  to  a  limited  extent.  The  base  of  the  flow  is  mainly  of  a 
greenish  gray  color  changing  upward  to  a  dull  reddish  tint ;  and 
the  upper  part  is  largely  of  a  deep  dull  red,  often  resembling  at 
a  little  distance  a  dark  red  sandstone.  Near  the  contact  with 
the  underlying  conglomerate  it  shows  a  distinct  flow-structure, 
parallel  with  the  contact,  as  well  as  a  coarse  and  fine  breccia- 
tion,  the  latter  resembling  or  passing  into  a  genuine  "toad- 
stone"  or  spherulitic  structure.  Dike  27,  which  crosses  the  hill 
approximately  at  right  angles  to  the  strike,  is  accompanied  by 
an  important  fault.  It  downthrows  to  the  north  and  shifts  the 
contact  of  the  conglomerate  and  porphyrite  about  175  feet,  as 
shown  on  the  map,  equivalent  to  a  vertical  slip  of  fidly  50  feet. 
This  hill  is  a  complex  of  dikes,  and  since  all,  apparently,  are 
attended  by  some  displacement,  the  continuity  of  the  contact 
between  the  conglomerate  and  porphyrite  is  lost.  This  confu- 
sion makes  it  difficult,  also,  to  determine  the  exact  thickness  of 
both  the  porphyrite  and  conglomerate.  It  is  quite  certain, 
however,  that  the  entire  thickness  of  the  porphyrite,  as  well  as 
of  the  conglomerate,  is  exposed  here  ;  for  it  is  undoubtedly 
overlain  conformably  by  the  small  patclies  of  conglomerate  on 
the  southeast  corner  of  the  hill  (Fig.  10). 


83 

These  patches  must  belong  to  a  third  bed  of  conglomerate,  and 
since  they  are  parallel  with  the  second  or  underlying  conglom- 
erate, the  separation  of  the  two  beds  measures,  approximately, 
the  thickness  of  the  porphyrite,  which  cannot  well  be  less  than 
fifty  feet  and  may  be  more.  I  have,  however,  observed  no  dis- 
tinct indications  that  it  is  a  composite  flow  ;  but  it  is  from  bot- 
tom to  top  one  solid  and  essentially  homogeneous  mass.  The 
underlying  conglomerate  must  be  nearly  as  thick  as  the  por- 
phyrite. 

West  Porphyrite  Hill. — This  approximately  isolated  and 
well-defined  rock-mass  is,  in  both  form  and  structure,  an  al- 
most exact  I'epetition  of  East  Porphyrite  Hill,  except  that  the 
structure  is  but  slightly  complicated  by  faults  and  dikes.  In 
short  it  is  a  solid  bed  of  conglomerate  underlain  by  melaphyr 
and  overlain  by  porphyrite  ;  and  none  of  the  Nantasket  ledges 
present  a  clearer  or  more  instructive  section  (Fig.  11).  The 
underlying  melaphyr  is  very  compact  for  the  most  part,  green- 
ish and  purplish,  non-amygdaloidal,  and  not  conspicuously 
brecciated  or  scoriaceous,  except  near  the  original  surface  of  the 
flow,  where  it  is  in  contact  with  the  conglomerate.  It  shows 
many  minute  crystallizations  and  some  whitish  veinlets  of 
epidote,  and  numerous  small,  irregular  segregations  of  bright 
red  jasper.  In  other  words,  it  is  essentially  identical  with  the 
melaphyr  which  we  have  found  overlying  the  basal  conglomer- 
ate on  Cliff"  Plateau  and  passing  beneath  the  second  conglom- 
erate on  East  Porphyrite  Hill.  The  outcrop  begins  on  the 
north  at  the  end  of  the  beach  leading  to  Melaphyr  Peninsula 
and  skirts  the  base  of  the  hill  to  the  middle  of  the  west  end. 
The  contact  with  the  overlying  conglomerate  is  exposed  con- 
tinuously for  several  hundred  feet,  but  the  base  of  the  melaphyr 
is  not  seen,  the  thickness  exposed  in  the  hill  not  exceeding  15 
feet.  A  greater  thickness  is  proved,  however,  by  the  outlying 
half-tide  ledges,  which  extend  nearly  half-way  across  Weir  River 
Bay.     These  were  carefully  examined  ;  and  all  that   are  marked 


84 

as  melapliyr  on  the  map  are  of  precisely  the  same  kind  as  that 
just  described,  and  are  plainly  parts  of  that  flow.  The  tops  of 
the  ledges  sometimes  show  traces  of  the  overlying  conglomerate  ; 
and  these  prove  a  very  perceptible  westerly  dip,  as  if  the  bay 
occupied  here  a  faintly  marked  syncline. 


Melaphyr 
N.    Peninsula. 


W.  Porphyrite 
Hill. 


Cliff  Plateau. 


Granite 
Plateau. 


tvifxi  ^i^ss^  N:[a^:/il  F/!^/g-i;-<g| 

Granite.  Conglomerate .  Melaphyr.  Porphyrite. 

Fig.  II. —  Section  from  Nantasket  Harbor  across  Melaphyr  Peninsula, 
West  Porphyrite  Hill,  and  Cliff  Plateau  to  Granite  Plateau. 
Scale,  i  inch  =  400  feet.  The  dikes  are,  from  north  to  south, 
nos.  28,  31,  34,  AND  22.     Dike  no.  29  is  omitted. 

The  conglomerate  is  mainly  of  medium  texture,  and  becomes 
finer  upward  ;  but  at  the  base  it  is  in  a  large  part  very  coarse 
and  irregular.  The  contact  with  the  melaphyr,  although  so 
perfectly  exposed,  is  somewhat  obscure  and  indefinite,  because 
the  conglomerate  fills  the  numerous  cracks  and  marked  inequal- 
ities which  naturally  characterize  the  upper  surface  of  the  mela- 
phyr flow ;  and  also  because  at  its  base  the  conglomerate  is 
largely,  in  some  cases  almost  wholly,  composed  of  melaphyr 
debris.  Very  few  of  the  fragments  and  pebbles  of  melaphyr 
are  amygdaloidal ;  and  they  can  nearly  all  be  certainly  referred 
to  the  subjacent  bed.  Near  the  base  of  the  conglomerate,  es- 
pecially, many  of  tlie  component  fragments  are  exactly  like  the 
melaphyr  which  it  rests  upon.  It  would,  perhaps,  be  diflicult 
to  find  a  clearer  example  of  a  contemporaneous  lava  covered  by 
newer  sediments.  The  relations  of  the  two  rocks  are  simply 
inexplicable  on  the  theory  that  the  melaphyr  is  intrusive. 
Along  the  contact,  especially,  granite  is  very  abundant  in  the 
conglomerate ;  and  several  masses  were  noticed  from  one  to 
three  feet  in  diameter,  the  largest  ones  resting  directly  upon 
the  melaphyr,  or  seeming  to  be   almost  imbedded  in  it.      The 


85 

C^nmite  is  chiefly  roiimlcd,  hut  sonic  hlocks,  lik(;  niiiiiy  oC  (lie 
melaphyr  fragments,  are  rather  aiig'uhu*.  Tlic  segregations  of 
red  jasper  occur  in  all  parts  of  the  conglouierate,  but  most  abun- 
dantly in  the  lower  part.  In  none  of  the  Nantasket  rocks  is  it 
more  plentiful  or  more  favorably  exposed  for  observation.  In- 
tercalated layers  of  sandstone  show  that  the  conglomerate  is 
nearly  horizontal,  but  undulating,  along  the  north  side  of  the 
hill ;  while  around  the  western  end  the  contact  with  the  mela- 
phyr, and  the  strings  of  sandstone,  dip  gently  (5°— lO'"^)  to  the 
east.  The  true  thickness  of  the  conglomerate  is  probably  not 
over  25  feet  and  almost  certainly  less  than  the  same  bed  on 
East  Porphyrite  Hill.  The  outlying  ledge  of  conglomerate 
west  of  Granite  Point,  instead  of  belonging  to  the  basal  con- 
glomerate, as  one  might  have  judged  from  its  position,  is  pre- 
cisely like  the  conglomerate  of  West  Porphyrite  Hill,  being 
coarse  and  largely  made  up  of  the  underlying  melaphyr,  and 
containing  numerous  large  fragments  of  granite,  etc.  The 
channel  between  this  conglomerate  and  the  ledges  of  melaphyr 
north  of  it  probably  mai'ks  a  slight  fault  ( 10  to  20  feet)  with 
the  downthrow  to  the  south. 

The  overlying  porphyrite,  of  which  about  25  feet  in  thick- 
ness still  remain ,  is  very  similar  to  the  lower  half  of  that  on 
East  Porphyrite  Hill.  It  is  compact  (felsitic),  not  distinctly 
crystalline,  porphyritic  or  amygdaloidal,  and  quite  homogeneous, 
except  for  the  scattering  and  mostly  small  segregations  of  red 
jasper.  The  colors  are  chiefly  pale  green  and  red  tints,  weath- 
ering usually  light  pink.  The  absence  of  the  dark  red  variety 
which  is  so  prominent  on  East  Porphyrite  Hill  is  due  to  erosion, 
the  entire  upper  half  of  the  porphyrite  having  been  removed 
from  the  western  hill,  while  the  eastern  hill  still  shows  a  com- 
plete section  of  the  flow.  The  porphyrite  is  seen  near  the  con- 
glomerate at  several  points  on  the  north  and  west  sides  ;  but  the 
only  clear  and  satisfactory  exposure  of  the  actual  contact  is 
afforded  by  the  rather  precipitous  eastern  face  of  the  hill. 
The  contact   here  shows   an   exceptionally  steep   southerly  dip 


86 

of  about  25" ;  and  this  is  still  farther  augmented  by  a  fault  with 
a  downthrow  to  the  south  of  perhaps  ten  feet,  which  skirts  the 
northern  edge  of  the  porphyrite  and  crosses  the  level  platform 
of  conglomerate  to  the  western  base  of  the  hill,  as  shown  on  the 
map,  thus  explaining  the  abrupt  termination  of  the  melaphyr 
midway  of  the  breadth  of  the  hill.  Along  the  contact  with  the 
conglomerate  the  porphyrite,  as  on  the  eastern  hill,  is  some- 
what brecciated,  and  shows  a  distinct  flow-structure  parallel  with 
the  contact,  as  well  as  a  coarse  sort  of  spherulitic  or  concretionary 
structure.  On  the  southwest  corner  of  the  hill  the  cono^lomer- 
ate  is  seen  to  pass  beneath  the  porphyrite,  but  the  contact  is 
not  clearly  exposed.  The  southern  face  of  the  porphyrite  is 
precipitous,  matching  the  opposing  face  of  Cliff  Plateau  ;  and 
there  can  be  no  reasonable  doubt  that  this  narrow  pass  marks 
an  important  fault  with  the  downthrow  to  the  north  ;  for  the 
porphyrite  is  vis  a  vis  with  the  basal  conglomerate,  and  the 
melaphyr  exposed  at  the  northwestern  base  of  West  Porphyrite 
Hill  is  the  equivalent  of  that  forming  the  southeastern  summit 
of  Cliff  Plateau.  The  displacement  must  be  equal  to  the  com- 
bined thickness  of  the  porphyrite  and  the  underlying  conglom- 
erate and  melaphyr,  i.e.,  all  the  beds  in  West  Porphyrite  Hill,  or 
75  feet  as  a  minimum.  This  fault  probably  passes  southwest 
between  the  outlying  ledges  of  conglomerate  on  the  north  and 
granite  on  the  south  ;  and  the  boundary  fault  between  Granite 
Plateau  and  Cliff  Plateau,  which  certainly  diminishes  rapidly 
in  this  direction,  appears  to  terminate  on  this  line,  as  repre- 
sented on  the  map. 

We  now  have  above  the  granite,  in  regular  sequence,  the 
following  beds  :  (1)  the  first  or  basal  conglomerate,  on  Granite 
Plateau,  Granite  Point,  and  Cliff  Plateau;  (2)  the  first 
melaphyr,  on  Cliff  Plateau  and  East  and  West  Porphyrite 
Hills ;  (3)  the  second  conglomerate,  on  Cliff  Plateau  and 
East  and  West  Porphyrite  Plills  :  (4)  the  porphyrite,  on  the 
Porphyrite  Hills  ;  and  (5)  the  third  conglomerate,  on  the  south- 
east  corner  of  East  Porphyrite  Hill,     From   Granite   Point  to 


87 

the  bay  east  of  East  Porphyrite  Hill  the  section  is  coiitiiiuouH 
and  complete,  all  five  of  the  beds  being  represented  (Fig.  10)  ; 
but  the  evidence  along  this  line  is  greatly  strengthened  at  the 
weakest  point  by  the  very  clear  and  simple  section  afforded  by 
West  Porphyrite  Hill  (Fig.  11).  The  lithological  comparisons 
are  almost  perfect  and  certainly  safe  for  so  small  an  area ;  and 
the  succession  may  be  said  to  rest  at  every  step  on  the  approved 
canons  of  stratigraphy. 

The  general  structural  relations  of  the  rocks  in  those  portions 
of  the  western  area  already  described  are  clearly  exhibited  in 
the  two  accompanying  sections :  one  (Fig.  10)  east  and 
west,  and  the  other  (Fig.  11)  north  and  south.  For  the  sake 
of  greater  clearness  the  east-west  dikes  are  not  shown  in  the 
first  section. 

Conglomerate  Hill. — This  completely  isolated  mass  of  con- 
glomerate is,  perhaps,  the  simplest  of  all  the  structural  blocks 
composing  the  Nantasket  area.  It  is  all  conglomerate,  with 
the  exception  of  occasional  thin  layers  of  hard  red  sandstone, 
which  show  a  dip  of  not  more  than  from  5°  to  10°  to  the  east  or 
a  little  south  of  east.  Some  parts  of  the  conglomerate  are  very 
coarse,  holding  pebbles  from  six  to  twelve  inches  in  diameter ; 
while  other  parts  are  very  fine  and  arenaceous.  The  red 
jaspery  porphyrite  of  East  Porphyrite  Hill  is  a  very  conspicuous 
constituent ;  but  the  red  jasper  is  also,  to  a  very  limited  extent, 
an  indio-enous  constituent,  formino-  irreo-ular  segregations  in  the 
cement  of  the  conglomerate.  The  hill  appears  to  be  entirely 
unbroken  by  either  faults  or  dikes ;  and  the  most  diligent 
search  has  failed  to  reveal  a  vestige  of  eruptive  rock  of  any 
kind,  except  in  the  form  of  pebbles.  There  are  no  overlying 
rocks ;  and  the  base  of  the  conglomerate  is  not  exposed. 
Hence  the  total  thickness  of  this  bed  is  a  matter  of  conjecture  ; 
but  since  the  eastern  slope  of  the  hill  is  approximately  parallel 
with  the  dip,  the  exposed  thickness  must  be  at  least  fifty  feet. 
The  hill  is  surrounded  on  all   sides  by  grass  land  and  marsh  ; 


and  about  the  only  clue  to  its  stratigraphic  position  is  that 
afForcled  by  the  enclosed  pebbles  of  jaspery  porphyrite.  There 
can  be  little  or  no  doubt  that  these  represent  the  flow  on  East 
Porphyrite  Hill ;  and  the  inference  appears  to  be  justified  that 
this  is  the  third  conglomerate,  a  small  outlier  of  which  has 
already  been  observed  resting  upon  the  porphyrite.  This  view 
of  its  position  means  that  Conglomerate  and  East  Porphyrite 
Hills  are  separated  by  an  east-west  fault  with  a  downthrow  on 
the  south  of  fully  fifty  feet.  Following  this  fault  west  to  the 
valley  between  these  hills  and  Cliff  Plateau,  we  find  that  it 
must  terminate  in  a  north-south  fault,  for  it  cannot  be  traced 
across  the  plateau ;  and  this  transverse  fault  must  downthrow 
to  the  east,  for  otherwise  there  would  be  insufficient  room 
beneath  Conglomerate  Hill  for  the  porphyrite  and  the  second 
conglomerate.  Conglomerate  Hill  thus  appears  as  a  rectangu- 
lar block  which  has  dropped  down,  the  downthrow  being- 
greatest  at  the  junction  of  the  two  faults,  at  the  northwest  cor- 
ner, which  explains  the  fact  that  the  dip  is  less  than  in  East 
Porphyrite  Hill  and  more  easterly.  Tlie  north-south  fault 
probably  coincides  with  dikes  66  and  extends  with  greatly 
diminished  throw  nearly  or  quite  to  the  north  shore,  terminating 
all  the  minor  faults  crossing  Cliff  Plateau.  East  Porphyrite 
Hill  and  Cliff  Plateau  are  thus  two  broken  fault  blocks  which 
have  risen  relatively  to  Conglomerate  and  West  Porphyrite 
Hills  ;  while  all  these  blocks  must  have  dropped  with  reference 
to  Granite  Plateau. 

Grreat  Hill- — This  is  the  highest  point  in  the  western  area, 
rising  very  abruptly  74  feet  above  the  western  marsh,  and  it  is 
the  boldest  and  most  impressive  topographic  feature  in  all 
Nantasket.  With  the  exception  of  the  granite  which  supports 
the  hill  on  the  southwest,  it  is  another  isolated  mass  of  conglom- 
erate ;  and  we  are  obliged  to  depend  wholly  upon  internal 
evidence  in  determining  its  stratigraphic  relations.  The  entire 
Jack  of  correspondence  between  Great  and  Crescent   Hills   lias 


89 

already  hcen  cited  as  provhiii;  a  dis])lacciricnt  between  tliein  ; 
and  the  same  reasoning  applies  with  equal  force  when  we  pass 
from  Great  Hill  to  Conglomerate  Hill.  The  Great  Hill  con- 
glomerate is  extremely  irregular,  varying  abruptly  and  re- 
peatedly from  fine  sandstone  to  a  mass  of  granite  and  melaphyr 
bowlders  one  to  three  feet  in  diameter.  Granite  pebbles  and 
bowlders  are  numerous,  but  usually  i-oundcd.  Fclsite  and 
melaphyr  are  also  abundant,  but  the  latter  is  never  amygda- 
loidal.  The  conglomerate  is  jaspery  at  most  points — red  jasper, 
but  so  highly  ferruginous  as  to  be  perceptibly  less  bright  and 
hard  than  that  in  the  more  western  hills.  It  occurs,  however, 
in  the  same  way,  and  lends  support  to  the  view  that  this  con- 
glomerate should  be  correlated  with  one  of  the  three  lower  or 
basal  beds.  The  conglomerate  is  thoroughly  indurated  through- 
out, although  this  is  most  noticeable  in  the  sandstone  layers, 
which  are  as  hard  as  quartzite,  but  always  red,  felsite  debris 
largely  predominating.  The  dip  is  flat  as  a  whole,  but  vari- 
able,— horizontal  at  some  points  and  inclined  15°-20°  at  others, 
especially  near  the  granite.  The  normal  dip  appears  to  be  east 
to  southeast  and  not  above  5° ;  and  the  exposed  thickness  of  the 
conglomerate  must  be  nearly  the  height  of  the  hill  or  at  least 
sixty  feet. 

There  is  no  overlying  and  there  appeared  at  first  to  be  no 
underlying  melaphyr  or  porphyrite  ;  and  the  exact  correlation  of 
this  conglomerate  seemed  a  hopeless  task,  until  I  came  to  study 
in  detail  the  phenomena  presented  near  the  contact  with  the 
granite,  on  the  southwest  corner  of  the  hill.  It  is  evident  at 
a  glance  that  the  main  body  of  conglomerate  does  not  rest 
upon  the  granite,  but  is  faulted  down  against  it,  the  boundary 
fault  crossing  the  hill  at  this  point,  and  with  essentially  the 
same  result  as  on  the  west  side  of  the  marsh.  The  granite  is, 
however,  capped  by  several  feet  of  conglomerate  at  this  point : 
and  the  fault,  which  is  very  clearly  exposed,  hades  irregularly 
to  the  north.  Following  it  down  cai-efully  we  come  finally, 
some  six  feet  below  the  top   of  the  granite    and  about  fifteen 


90 

feet  above  the  marsh,  to  the  base  of  the  conglomerate.  It 
is  seen  to  rest  upon  a  compact  greenish  and  purphsh  eruptive 
rock,  which  proves  to  be  identical  with  the  first  melaphyr  of 
Cliff  Plateau  and  West  Porphyrite  Hill.  It  can  be  traced  north- 
ward about  one  hundred  feet,  the  contact  gradually  sloping 
down  to  the  level  of  the  marsh.  Its  contact  with  the  granite 
is  clearly  a  fault  with  the  hade  and  throw  to  the  north,  and 
considerable  fragments  of  the  melaphyr  appear  to  lie  along  the 
fault-fracture  above  the  base  of  the  conglomerate,  testifying 
further  to  the  direction  of  the  slip.  In  short,  we  have  exposed 
at  this  point,  on  the  south  side  of  the  fault,  the  granite  capped 
by  conglomerate  ;  and  several  feet  below  this  contact,  on  the 
north  side,  the  first  melaphyr  overlain  by  the  main  body  of 
conglomerate  forming  Great  Hill.  The  conglomerate  on  the 
south  is  probably  the  first  or  basal  bed,  as  indicated  by  its  re- 
lations to  the  granite :  and  that  on  the  north  must  be  the 
second  bed,  as  proved  by  its  relations  to  the  melaphyr.  This 
makes  the  displacement  equal  to  the  combined  thickness  of  the 
basal  conglomerate  and  the  first  melaphyr,  perhaps  75  feet  in 
all. 

It  is  a  fact,  however,  that  the  fault-fracture  cannot  be  traced 
upward  through  the  conglomei"ate  ;  but  the  conglomerate  over- 
lying the  granite  is  essentially  continuous  with  that  north  of 
the  fault-line  ;  and  it  would,  perhaps,  be  best  to  refer  all  of  the 
Great  Hill  conglomerate  to  the  first  or  basal  bed,  regarding  the 
underlying  melaphyr  as  a  lower  fiow  than  has  been  seen  else- 
where in  the  western  area.  This  view,  which  would  dispense 
with  the  fault  between  the  granite  and  conglomerate,  derives 
further  support  from  the  fact  that  the  basal  conglomerate 
on  Granite  Plateau  and  elsewhere  is  crowded  with  the  debris  of 
similar  melaphyr  ;  but  it  is  not  followed  in  the  construction  of 
the  map,  simply  to  avoid  multiplying  lava-flows  where  the 
facts  do  not  absolutely  require  it.  Still  another  interpretation 
is  consistent  with  all  the  observed  facts;  viz.,  that  the  con- 
glomerate belongs  wholly  to  the  second  bed  and  that  the  sup- 


91 

posed  fault  is  really  an  overlap,  the  conglomerate  having 
covered  a  boss  of"  granite  which  rose  like  an  island  throiigli  the 
first  or  underlying  melaphyr. 

Following  the  fault-contact  across  to  the  southeast  corner 
of  the  hill,  we  again  find  the  conglomerate  on  the  north  under- 
lain by  the  compact  melaphyr ;  and  the  granite  outcrops  at  a 
little  distance  on  the  south  without  any  capping  of  conglomer- 
ate, erosion  having  cut  deeper  here.  It  is  evident  now  that 
Great  Hill  must  be  on  the  upthrow  side  of  the  fault  bounding 
it  on  the  northwest ;  and  that  the  displacement  is  more  than 
equal  to  the  combined  thickness  of  the  porphyrite  and  third  con- 
glomerate,  or  more  than  100  feet. 

Correlation  of  the  First  Melaphyr.  —  Passing  down 
the  long  southwestern  slope  of  Great  Hill,  we  come  at  last, 
in  the  ledges  of  granite  isolated  by  the  western  marsh,  to 
the  large  dike  of  melaphyr,  No.  2  of  the  map  and  the  descriptive 
list  of  dikes.  This  dike,  which  is  strongly  contrasted  in 
character  with  all  the  diabase  dikes  of  this  district  and  certainly 
is  not,  as  it  appears  to  be  at  first  glance,  a  southward  continu- 
ation of  the  composite  dike  (66)  separating  Cliff  Plateau  and 
East  Porphyrite  Hill,  demands  our  attention  at  this  time  on  ac- 
count of  its  important  and  interesting  relations  to  the  first  mel- 
aphyr, the  only  melaphyr  of  the  western  area.  The  microscopic 
resemblance  is  certainly  very  marked,  and  in  his  brief  descrip- 
tion of  the  microscopic  characters  (page  37),  Mr.  Merrill 
has  cited  no  fact  inconsistent  with  the  view  that  the  dike  is  a 
more  crystalline  and  less  altered  form  of  the  same  original  rock 
as  the  flow.  Attention  was  first  attracted  to  this  dike,  as  re- 
gards its  relations  to  the  effusive  melaphyr,  in  seeking  an  ex- 
planation of  the  melaphyr  which  is  mingled  irregularly  with  the 
first  or  basal  conglomerate  on  the  eastei-n  edge  of  Granite 
Plateau.  This  ledge  of  conglomerate  and  melaphyr  is  only  a 
few  feet  west  of  the  direct  course  of  the  dike  ;  and  the  question 
naturally  arose,    Why  may    not  these  irregular  intrusions  be 


92 

regarded  as  branches  from  this  dike?  To  actually  trace  the 
connection  is  impossible,  on  account  of  the  marsh  ;  but  I  have 
endeavored  to  show  its  probability  by  demonstrating  more  fully 
the  identity  of  this  melaphyr  with  the  dike  and  also  with  the 
surface  flow.  To  this  end,  characteristic  specimens  of  mela- 
phyr fi'om — I.  the  flow  at  the  base  of  West  Porphyrlte  Hill, 
II.  the  flow  at  the  base  of  Great  Hill,  HI.  the  irregular  intru- 
sions in  the  basal  conglomerate,  and  lY.  the  great  dike,  have 
been  submitted  to  chemical  analysis,  through  the  kindness  of 
Dr.  T.  M.  Drown  of  the  Massachusetts  Institute  of  Technology. 
Four  closely  accordant  determinations  of  the  silica  and  of  the 
iron  oxide  (FegOg)  and  alumina  were  made  for  each  sample  by 
the  chemists  of  the  class  of  1892,  with  the  followino-  averao^e 
results  : — 


I 

II 

II 

IV 

SiO^ 

47.97 

51.05 

47.29 

54.47 

AiPg+FeA 

30.50 

30.16 

30.20 

25.25 

Although  these  results,  especially  for  the  silica,  do  not  agree 
so  well  as  could  be  desired,  the  differences  should  not  excite 
surprise,  when  we  consider  that,  since  these  rocks  are  all  highly 
altered,  and  filled  with  quartz,  epidote,  and  other  secondary  min- 
erals, it  is  well-nigh  impossible  to  select  average  or  normal 
specimens.  Probably  four  specimens  from  the  same  mass 
(dike  or  flow)  would  show  similar  differences.  Regarding, 
then,  these  different  outcrops  of  melaphyr,  of  whatever  form, 
as  essentially  identical  in  macroscopic,  microscopic,  and  chemi- 
cal characters,  we  may  reasonably  suppose  that  they  are  con- 
temporaneous, and  that  the  great  dike  is  one  of  the  vents  at 
least,  if  not  the  only  vent,  through  which  the  sheet  of  melaphyr 
overlying  the  basal  conglomerate  reached  the  surface.  The 
fissure  is  straight  and  regular  in  the  granite  ;  but  on  passing 
into  the  then  unconsolidated  gravel  of  the  basal  conglomerate 
it  naturally  became  very  irregular,  the  mela[)hyr  and  gravel  be- 
coming inextricably  mingled,  as  observed  on  the  eastern  edge 


93 

of  Granite  Plateau.  The  fact  that  wc  cannot  trace  tliis  dike 
above  the  basal  conglomerate  is  thus  readily  cxi)laincd  ;  for  this 
conglomerate  was  then  the  surface  deposit,  the  newest  sediment 
upon  the  ocean  floor.  Taking  all  these  features  into  consider- 
ation, it  is  doubtful  if  there  can  be  found  elsewhere  in  the  Bos- 
ton Basin  so  clear  an  exhibition  of  a  complete  eruption — com- 
plete in  both  its  intrusive  and  extrusive  phases. 

Correlation  of  the    Central    and    Western    Areas. — 

Up  to  this  point  the  stratigraphy  of  the  western  area  presents 
no  serious  or  insuperable  difficulty,  each  group  of  ledges  pre- 
senting certain  recognizable  features  common  to  some  of  the 
others.  But  on  attempting  to  extend  this  correlation  to  the 
central  area  the  case  is  very  different ;  for  the  break  between 
Great  and  Crescent  Hills,  the  only  point  where  the  two  areas  are 
in  contact,  is  so  profound  that  at  first  glance  the  two  sections 
seem  to  have  nothing  in  common.  The  peculiar  first  melaphyr 
and  the  porphyrite,  which  are  the  principal  keys  to  the  western 
area,  are  certainly  wholly  wanting  in  the  central  area.  I  have 
already  stated  that  the  heavy  bed  of  conglomerate  forming  Con- 
glomerate Plateau,  Round  Hill,  and,  probably,  the  ledges  to 
the  eastward  as  far  as  Conglomerate  Island  and  Green  Hill, 
should,  apparently,  be  correlated  with  the  conglomerate  under- 
lying the  lower  amygdaloidal  melaphyr  at  the  northern  end  of 
the  section  along  the  railroad.  If  this  is  done,  then,  while  the 
western  area  consists,  exclusive  of  the  fundamental  granite  and 
Melaphyr  Peninsula,  of  three  beds  of  conglomerate  and  two  in- 
tercalated flows  of  lava — one  basic  and  one  acid  ;  the  central  area 
is  made  up  of  two  beds  of  conglomerate,  each  of  which  is  over- 
lain by  a  flow  of  amygdaloidal  melaphyr.  The  base  of  the  lower 
and  principal  conglomerate  is  nowhere  exposed ;  but  I  see  no 
reason  to  doubt  that  it  rests  upon  the  porphji'ite,  and  is  the 
equivalent  of  the  third  conglomerate  in  the  western  area.  This 
interpretation  of  the  facts  has  the  merit  of  simplicity,  and  it 
leaves   no  observed    facts   unexplained.     The   summit    of   the 


94 

western  section  becomes  the  base  of  the  central  section ,  the  two 
together  including  four  beds  of  conglomerate  and  four  beds  of 
lava. 

Strictly  speaking,  the  basal  conglomerate  is  not  wanting  in 
the  central  area,  since  it  is  undoubtedly  represented  by  the 
patches  of  clastic  material  on  the  granite  south  of  Round  Hill ; 
and  this  hill  thus  admits  of  comparison  with  Great  Hill.  The 
relations  to  the  granite  and  the  boundary  fault  are  the 
same ;  but  the  Round  Hill  conglomerate  is  the  third  bed  and 
the  Great  Hill  conglomerate  is  the  second  bed. 

The  satisfactory  correlation  of  the  central  and  coastal  areas 
presents  still  greater  difficulties,  and  appeared  practically  im- 
possible, until,  fortunately,  a  partial  clue  was  affiDrded  by  the 
ledges  on  Rocky  Neck.  The  completion  of  the  Nantasket 
column  must,  therefore,  await  the  detailed  description  of  the 
Rocky  Neck  section  ;  and  it  will  suffice  here  to  state  that  all 
the  melaphyr  of  the  coastal  area  and  the  underlying  conglomer- 
ate of  Long  Beach  Rock  belong  above  the  highest  bed,  the 
third  melaphyr,  in  the  central  area. 

Melaphyr  Peninsula. — This  most  northerly  and  most  nearly 
isolated  part  of  the  western  area  is  the  only  part  that  does 
not  admit  of  ready  correlation  without  appealing  to  external 
evidence.  For  this  reason,  and  because  it  is  probably  the  new- 
est part  of  the  area,  its  description  has  been  reserved  to  the 
last.  Melaphyr  Peninsula  is  quite  certainly  made  up  of  two 
principal  flows  of  highly  basic  and  amygdaloidal  melaphyr. 
The  northern  and  older  flow  shows,  at  the  west  end,  three  fairly 
distinct  bands  of  coarse  amygdules,  averaging  about  three 
feet  thick.  The  lowest  is  near  the  water,  and  the 
second  is  separated  from  it  by  three  feet  and  from  the  third 
b}^  two  feet  of  compact  melaphyr.  The  exposed  thickness 
of  this  flow  is  about  15  feet,  and  it  is  clear  that  we 
see  only  the  superficial,  vesicular  part  of  it.  Immediately 
east  of  the  dike  (67),  the  second  and  third  bands  of  amygdules 


95 

are  united ;  but  farther  east  they  diverge  again  and  for  a 
hundred  feet  or  more  are  separated  by  from  one  to  four  feet  of 
compaet  melaphyr.  Beyond  that  they  seem  to  unite  ;  but  other 
amygdaloidal  bands  appear  below  them,  and  toward  the  east 
end  there  are  from  three  to  five  indistinet  bands.  These  bands 
may,  perhaps,  be  regarded  as  successive  waves  of  lava,  and 
essentially  parts  of  one  flow.  At  many  points  the  amygdules 
are  arranged  in  distinct  sheets  or  lines  parallel  with  the  main 
bands,  giving  rise  to  a  very  beautiful  flow-structure,  and  in 
none  of  the  Nantasket  ledges  is  the  flow-structure  more  per- 
fectly developed  than  here.  Both  the  lines  and  bands  dip  S. 
5°-10°,  the  strike  being  N.  80°  E. 

The  amygdules  range  from  pin-heads  to  one  inch  in  diameter, 
and  average  fully  one  fourth  inch.  They  are  chiefly  composed 
in  order  of  importance  of:  (1)  whitish,  compact  epidote  (mixed 
feldspar  and  epidote)  ;  (2)  quartz  ;  (3)  feldspar,  which  is  often 
reddish;  (4)  chlorite;  and  (5)  bright  green  epidote.  The 
compact,  whitish  epidote  also  forms  many  irregular  veinlets, 
which  conform  with  the  flow-structure,  or  cut  it  at  all  angles, 
and  pass  insensibly  into  the  large  amygdules.  In  all  these 
segregations,  the  quartz  was  the  last  mineral  to  be  deposited. 
Quartz  has  not  been  observed  in  the  same  amygdules  with 
chlorite,  but  feldspar,  epidote,  and  quartz  were  always  depos- 
ited in  that  order.  The  sequence  of  the  minerals  in  the  amyg- 
dules is  quite  varied  and  the  following,  stated  in  the  order  of 
abundance  and  beginning  on  the  outside  in  every  case,  have 
been  observed  :  (1)  mixed  feldspar  and  epidote,  and  quartz; 
(2)  feldspar  and  quartz;  (3)  feldspar,  epidote,  and  quartz; 
(4)  epidote  and  quartz  ;  and  (5)  mixed  epidote  and  feldspar, 
and  chlorite.  Occasionally  bright  red  jasper  occurs  with  the 
quartz.  The  amygdules  were  carefully  examined  to  see  if 
their  elongation  and  orientation  throw  any  light  upon  the  direc- 
tion of  the  flow.  The  smaller  amygdules  are  mainly  spherical ; 
and  the  larger  ones,  although  elongated,  are  usually  very  irreg- 
ular. So  far  as  the  evidence  goes,  it  favors  the  view  that  the 
lava  came  from  the  east. 


96 

The  second  flow  of  melaphyr  begins  with  about  fifteen  feet  of 
compact,  green  melaphyr  overlain  by  five  to  ten  feet,  in  one  to 
several  bands,  of  amygdaloidal  melaphyr.  It  is  similar  to  the 
first  flow,  except  that  the  amygdules  are  not  so  crowded  and 
the  bands  are  not  so  well  defined.  These  flows  are  strongly 
contrasted  with,  and  undoubtedly  much  newer  than,  the  flow 
forming  the  base  of  West  Porphyrite  Hill,  one  hundred  feet 
away.  The  isolation  of  this  outcrop  makes  an  absolutely  safe 
correlation  impossible.  It  is  reasonably  certain,  however,  that 
it  should  be  referred  to  either  the  upper  or  lower  amygdaloidal 
melaphyr  of  the  central  area.  It  looks  like  a  block  detached 
from  Melaphyr  Plateau  :  in  fact  the  lithological  resemblance 
is  almost  perfect.  And  this  view  is  strengthened  by  the  low- 
tide  ledges  of  precisely  the  same  melaphyr  which  stretch  away 
to  the  eastward,  in  the  direction  of  Melaphyr  Plateau,  and 
virtually  extend  Melaphyr  Peninsula  as  far  as  the  base  of  East 
Porphyrite  Hill.  It  is  a  foregone  conclusion,  then,  that  the 
short  beacli  connecting  Melaphyr  Peninsula  and  West  Porphj^- 
rite  Hill  conceals  an  important  fault  with  the  downthrow  to  the 
north,  an  extension,  apparently,  of  the  line  of  displacement 
dividing  the  coastal  and  central  areas.  The  slip  can  scarcely  be 
less  than  150  feet,  and  may  easily  be  100  feet  more. 

Rochy  JV^eck. 

The  part  of  Weir  Eiver  Bay  separating  Rocky  Neck  from 
Nantasket  is  not  a  geological  boundary  ;  for  tlie  Avestern  shore 
repeats,  with  slight  variations,  the  geological  conditions  with 
which  we  have  become  familiar  on  the  eastern  shore.  These 
variations  are,  however,  of  some  moment,  since  they  supply  miss- 
ing links  in  the  chain  of  evidence  essential  to  the  correlation  of  the 
Nantasket  strata.  But  although  Rocky  Neck  is  simply  a  de- 
tached part  of  Nantasket,  we  can  go  no  farther  west  in  the  study 
of  Nantasket  geology  ;  for  not  only  is  a  natural  boundary  pro- 
vided in  the  drumlins  of  World's  End,  Planter's  Hill,  and  Pine 


97 

Hill,  but  beyond  these  we  encounter,  in  Hingham   Harbor  sind 
northern   Hingham,    structural   features   of  a  radically   distinct 

type. 

The  eastern  base  of  Planter's  Hill  is  the  typical,  coarse,  and 
somewhat  reddish  granite,  which  extends  eastward  in  numerous 
ledges  to  Weir  River  Bay.  Advancing  north  along  the  west 
shore  of  Rocky  Neck,  we  find  the  granite,  near  dikes  22  and 
23,  overlain  by  the  basal  conglomerate.  It  is  largely  com- 
posed, as  on  the  Nantasket  side,  of  the  more  or  less  angular  de- 
bris of  granite  and  felsite,  including,  however,  few  large  frag- 
ments, and  it  has,  in  consequence,  a  prevailing  reddish  tint. 
Erosion  has  evidently  again  cut  just  to  the  base  of  the  con- 
glomerate over  a  considerable  area  ;  and,  as  on  Granite  Plateau, 
several  thin  patches  have  been  isolated  by  unequal  erosion. 
These  outliers  show  that  the  conglomerate  conforms  in  the 
usual  manner  to  the  fissures  and  other  inequalities  of  the  origi- 
nal granite  floor.  The  dip  is  S.  E.  0°-10°.  The  granite  rises 
from  beneath  the  conglomerate  on  the  northwest,  forming 
Granite  Head  ;  while  on  the  south  the  conglomerate  is  certainly 
cut  off  by  an  oblique  fault  with  a  decided  upthrow  on  the 
south.  Hence  the  conglomerate  is  naturally  thickest  toward  the 
east,  where  the  ledges  overlook  the  little  valley  running  north- 
west to  the  bay  north  of  Granite  Head.  In  this  natural  section 
the  conglomerate  encloses  considerable  layers  of  sandstone  and 
even  slate,  and  the  bedding  is  very  distinct.  The  fault  just 
referred  to  divides  the  basal  conglomerate  into  two  principal 
areas  :  and  in  going  eastward  the  conglomerate  again  appears, 
first  in  small  outliers  capping  the  granite  ledges,  and  then,  ap- 
parently, in  a  continuous  bed  reaching  to  the  shore.  This  is  a 
drift  slope,  however  :  and  the  outcrops  are,  perhaps,  insuflficient 
to  make  this  inference  entirely  safe. 

These  masses  of  granite  and  overlying  conglomerate  must 
be  bounded  on  the  north  by  a  ftiult  transverse  to  the  strike 
of  the  conglomerate   and  reachino;   from   Granite  Head  to  the 

OCCAS.   PAPERS   B.   S.    N.    H.    IV.    7. 


98 

eastern  shore,  the  continuation  of  the  boundary  fault  on  the 
Nantasket  side.  The  downthrow  is,  of  course,  to  the  north, 
and  evidently  greatest  toward  the  eastern  shore  ;  for  north  of 
this  fault,  on  the  western  shore,  we  again  find  the  granite  and 
basal  conglomerate.  The  granite  rises  somewhat  abruptly  to  a 
height  of  from  twenty  to  thirty  feet  on  the  west  end  of  the  hill, 
and  is  immediately  overlain  by  the  conglomerate,  matching  Gran- 
ite Point  on  the  Nantasket  side.  The  hill  is  precipitous  at  this 
end  on  both  the  north  and  south  sides,  being  bounded  by  fault- 
scarps  ;  and  these  natural  sections  show  that  the  granite  is  cut 
off  in  its  eastward  extension  beneath  the  conglomerate  by  a 
north-south  fault  having  an  easterly  throw  of  perhaps  10  or  20 
feet  (Fig.  12).  This  fault  appears  to  end  in  the  east-west 
faults,  and  accounts  for  the  form  of  the  conglomerate  outcrop,  as 
shown  on  the  map.  The  basal  conglomerate  on  this  hill  is 
similar  to  that  south  of  the  boundary  fault,  except  that  the 
pebbles  include  a  large  proportion  of  compact  forms  of  mela- 
phyr,  and  the  red  jasper  is  more  conspicuous.  The  dip  is  S.  E. 
.^o.iO'',  equivalent  to  a  thickness  of  thirty  or  forty  feet. 


Granite.  Conglomerate.  Melaphyr. 

Fig.  12. —  An  east-west  section  across  Rocky 

Neck,  north  of  the  boundary  fault. 

Scale,   i  inch  =  400  feet. 

The  conglomerate  is  certainly  separated  from  the  melaphyr 
on  the  southeast  by  a  fault,  which  is  probably  an  extension  of 
the  southwest- northeast  fault  in  the  granite,  already  referred  to, 
but  with  the  throw  reversed  (Fig.  12).  This  melaphyr  is  the 
compact,  green  and  purplish,  jaspery  variety — the  first  melaphyr 
—  which  normally  overlies  the   basal   conglomerate  ;    and  it   is 


99 

Indistiaguishable  from  that  flow  on  tlie  east  side  of  tlie  hay. 
It  is  never  amygdaloidal,  but  often  somewhat  brecciated.  '^I'h(; 
fault  just  referred  to  must,  therefore,  downthrow  to  the  south- 
east, but  probably  not  more  than  20  or  30  feet,  possibly  less.' 
Both  this  displacement  and  the  preceding  one,  parallel  with  it, 
are  evidently  added  to  the  boundary  fault,  so  that  this  now 
becomes  sufficient  to  conceal  entirely  the  basal  conglomerate  on 
the  downthrow^  side.  The  first  melaphyr  is  overlain  in  regular 
order  by  the  second  conglomerate  (Fig.  12),  which  is  thus, 
in  consequence  of  the  increased  displacement,  brought  vis  a  vis 
with  the  strongly  contrasting  basal  conglomerate  (Fig.  13). 
The  second  conglomerate  contains  so  much  melaphyr  debris 
derived  from  the  underlying  flow  that  it  is  not  always  easily 
distinguished  from  the  brecciated  portions  of  the  melaphyr.  It 
is  also  the  richest  in  red  jasper  of  all  the  Rocky  Neck  strata, 
agreeing  perfectly  in  this  respect  with  the  same  bed  in  the 
porphyrite  hills  of  Nantasket.  This  highly  jaspery  conglom- 
erate is  well  exposed  on  the  shore  for  nearly  two  hundred  feet, 
before  it  gives  way  to  the  underlying  melaphyr,  which  extends 
150  feet  farther,  but  bears  several  distinct  outliers  of  the  con- 
glomerate, as  mapped.  These  outliers  are  seen  to  rest  upon 
a  very  uneven  surface  of  the  melaphyr  ;  but  the  normal  dip 
is  plainly  S.  E.  10°  or  more.  At  the  northern  edge  of  the 
melaphyr  it  seems  to  end  quite  abruptly  ;  but  the  narrow  beach 
between  it  and  the  puddingstone  on  the  north  is  partly  occupied 
by  the  firm,  jaspery,  second  conglomerate, —  another  outlier 
let  down  by  faulting. 

We  have  now  reached  the  north  end  of  the  northeast-south- 
west fault  which  has  already  been  seen  to  bound  the  first  mela- 
phyr on  the  northwest.  It  appeal's  to  be  a  double  fault  at 
this  end  (Fig.  13)  ;  and  following  it  southwest,  we  find  that  it 
encloses  not  only  the  depressed  outlier  of  the  conglomerate, 
but  also  an  isolated  ledge  of  the  melaphyr  with  traces  of  the 
conglomerate  upon  it. 

The  hill  or  ridge  just    described,  composed  of  the  first  and 


100 

second  conglomerates  and  the  first  raelaphyr,  and  terminated 
by  the  granite  on  the  west  shore,  is  bounded  on  the  precip- 
itous northern  side,  as  previously  stated,  by  a  fault,  which,  as 
the  map  shows,  coincides  with  a  dike  (30).  The  dike  can  onlv 
be  traced  about  half-way  to  the  N.  E.  —  S.  W.  fault,  passing 
under  the  grass  ;  but  both  the  dike  and  its  displacement  un- 
doubtedly continue  until  the  faults  meet,  as  mapped.  This 
dike-fault  and  the  pai't  of  the  N.  E.-S.  W.  fault  northeast 
of  the  junction  may  be  regarded  as  one  continuous  fault, 
with  the  downthrow  on  the  north.  That  it  is  an  important 
displacement  is  evident  from  the  fact  that  the  rocks  north  of 
this  line  are  toAo/Z^/  different  from  those  south  of  it.  There  is 
first  a  broad  area  of  non-jaspery  conglomerate  —  a  normal 
puddingstone  —  sloping  gently  to  the  southeast  and  dipping 
in  the  same  direction  5°-15°.  It  forms  the  eastern  shore  for 
about  100  feet ;  but  the  true  thickness  of  the  bed  probably  does 
not  exceed  25  feet.  This  conglomerate  is  underlain  con- 
formably and  very  clearly  by  typical,  basic,  green,  amyg- 
daloidal  melaphyr,  which  is  identical  with  that  on  Melaphyr 
Peninsula  and  Melaphyr  Plateau  (Fig.  13).  The  melaphyr 
outcrops  broadly  around  the  conglomerate  and  the  exposures 
of  the  contact  are  all  that  could  be  desired.  The  contempo- 
raneous origin  of  the  melaphyr  is  sufficiently  demonstrated  by 
the  fact  that  near  the  base  of  the  conglomerate,  especially,  it 
encloses  many  water-worn  fragments,  from  three  to  eighteen 
inches  in  diameter,  of  exactly  the  same  kind  of  melaphyr.  The 
melaphyr  plainly  consists  of  two  flows,  each  of  which  is  amyg- 
daloidal  above  and  compact  below,  with  an  aggregate  thickness 
of  forty  or  possibly  fifty  feet ;  and  this  fact  seems  to  identify 
it  with  Melaphyr  Peninsula,  and  with  the  upper  flow  in  the 
Crescent  Hill  section  —  the  third  melaphyr.  If  this  correlation 
be  made,  the  displacement  along  the  dike-fault  can  scarcely 
fall  below  200  or  250  feet. 

The  east-west  fault  crossing  the  summit  of  the  hill,  with  a 
southerly  throw  of  ten  feet,  is  very  clearly  exposed  on  the  west 


101 


5. 


Granite.  .     Conglomerate.  Melaphyr. 


Fig.  13. —  A  north-south  section  across  Rocky  Neck. 
Scale,   i  inch  rzz  400  feet. 

side,  where  it  breaks  the  contact  of  the  conglomerate  and  mela- 
phyr (Fig.  13)  o  It  is  nearly  parallel  in  direction,  but  opposed 
in  throw,  to  the  fault  bounding  this  hill  on  the  north.  The  mela- 
phyr and  conglomei'ate  rise  here  in  a  sloping  cliff  of  20  to  40 
feet,  extending  the  entire  length  of  the  hill  and  overlooking 
the  small  plain  which  forms  the  extremity  of  the  Neck. 

The  melaphyr  north  of  this  fault-scarp  is  similar  to  that  south 
of  it ;   but  it  is  underlain  on  the  west  by  a  limited  bed  of  con- 
glomerate, which,  dipping  S.E.  2°— 5°,  must  abut  against  the  cliff 
of  melaphyr,  although  the  contact  is  not  exposed.     This  proves 
the  fault,  for  the  northern  conglomerate,  rising  toward  the  west, 
could   not   otherwise  fail  to  appear  in  the   western   half  of  the 
cliff.     This  conglomerate  is  of  normal  character,  with  an  average 
thickness  not  exceeding  ten  feet.     It  is  plainly  cut  on  the  north 
shore  by  a  north-south  fault,  sinking  the  conformable  contact 
with  the  overlying  melaphyr  about  eight  feet  on  the  east.     The 
conglomerate  is,  in  its  turn,   clearly  underlain  on  the  west  by  a 
non-amygdaloidal  melaphyr,  of  which  about  twenty  feet  are  ex- 
posed at  low  tide.     Superficially,  at  least,  this  melaphyr  is  highly 
brecciated,  so  that  it  is  not  quickly  distinguished  from  the  con- 
glomerate.    It  encloses  numerous  masses  of  compact  melaphyr 
similar  to  the  matrix,  more  or  less  rounded,  and  of  all  sizes  up 
to  eighteen  inches  in  diameter.     It  is  probably  not  a  conglom- 
erate or  tuff,  but  a  very  coarse  original  brecciation,   something 
similar  to  the  melaphyr  holding  the  pseudo-bombs  on  the  north 
side  of  Atlantic  and  Centre  Hills.      The  entire  rock  is  similar 
in  color  and  texture  to  the  melaphyr  of  Long  Beach  Rock  and 


102 

the  coastal  area  generally  ;  and  it  seems  impossible  to  correlate 
it  with  anything  else  in  the  Nantasket  region.  But  in  that  case 
the  conglomerate  and  the  highly  amygdaloidal  melaphyr  over- 
lying it  must  be  referred  to  the  very  summit  of  the  Nantasket 
section  ;  and  this  amygdaloidal  melaphyr  north  of  the  fault-scarp 
is  thus  widely  separated  in  time  from  the  very  similar  melaphyr 
south  of  the  fault.  This  view  also  implies  the  recurrence  at  the 
top  of  the  Nantasket  section  of  the  basic  flows  characterizing 
the  middle  portion  of  it. 

According  to  the  foregoing  interpretations,  the  Kocky  Neck 
section,  as  exposed  to  observation,  is  evidently  far  from  com- 
plete or  continuous  ;  but  the  following  list  of  the  Nantasket 
strata  will  show  more  clearly  than  either  the  map  or  sections 
just  what  the  deficiencies  are  :-^ 

Granite. 

First  conglomerate,  basal. 

First  melaphyr,  compact  and  jaspery. 

Second  conglomerate,  highly  jaspery. 

Porphyrite,  wanting. 

Third  conglomerate,  wanting. 

Second  melaphyr,  wanting. 

Fourth  conglomerate,  wanting. 

Third  melaphyr,  amygdaloidal. 

Fifth  conglomerate,  normal  puddingstone. 

Fourth  melaphyr,  compact  and  brecciated. 

Sixth  conglomerate,  ten  feet. 

Fifth  melaphyr,  amygdaloidal. 

On  referring  to  the  map  and  sections  (Figs.  12  and  13),  it  be- 
comes obvious  that  this  great  break  in  the  middle  of  the  scries 
is  due  wholly  to  the  east-west  dike-fault,  which,  as  previously 
explained,  divides  Rocky  Neck  into  two  areas  having  nothing 
in  common  so  far  as  the  stratigraphy  is  concerned.  This  im- 
portant displacement  is  quite  clearly  a  continuation  of  that  sep- 
arating Melaphyr  Peninsula  from  West  Porphyrite  Hill  and 
dividing  the  central  and  coastal  areas. 


103 

The  porphyritc  is,  possibly,  not  wholly  wanting  on  Kooky 
Neck  ;  for  the  small  point  jutting  into  Weir  River  Bay  south  of 
the  basal  conglomerate  and  mapped  as  granite,  consists  of  angu- 
lar masses,  probably  overlying  a  ledge,  of  a  semi-crystalline,  red 
rock,  apparently  quartzless,  and  resembling  the  Nantasket  por- 
phyritc. Its  rather  crystalline  texture  would  indicate  a  dike  of 
porphyrite  breaking  through  the  granite  ;  but  if  it  is  effusive,  a 
surface  flow,  important  bounding  faults  must  be  assumed  to  ac- 
count for  its  topographic  position  with  reference  to  the  granite. 

The    Nantasket    Peninsula    North    of  A.tlantic   Hill. 

As  previously  stated,  the  only  exposure  of  the  underlying 
rocks  north  of  Atlantic  Hill  is  the  ledge  of  slate  on  the  shore 
south  of  Thornbush  Hill ;  and  with  the  exception  of  the  ledge 
of  green  and  gray  contorted  slate  on  the  railroad  northwest  of 
Rockland  Hill,  which  has  been  doubtfully  referred  to  the  prin- 
cipal bed  of  tuff  in  the  Atlantic  Hill  section ,  this  is  the  only 
considerable  outcrop  of  slate  on  the  Nantasket  Peninsula.  It 
is  exposed  along  the  shore  for  about  900  feet  and  with  an  ex- 
treme breadth  across  the  strike  of  perhaps  400  feet.  It  is  a 
rather  coarse  or  arenaceous,  purplish  slate,  distinctly  and  finely 
banded  with  gray,  and  occasionally  with  greenish,  layers.  The 
strike  is  N.  65°— 70°  E.  ;  and  the  dip  is  variable,  but  always  low. 
In  the  western  ledges  it  is  S.  10°-15°  or  20°  at  the  most ;  but 
in  the  middle  ledges  the  beds  are  approximately  horizontal, 
broad  contortions  or  undulations  showing  gentle  dips  in  various 
directions  ;  while  the  eastern  ledges  show  a  northerly  dip  of 
10°— 20°.  This  is  evidently  an  oblique  section  of  a  flat  anti- 
cline. It  includes  no  pebbly  or  distinctly  arenaceous  beds ; 
and  the  exposed  thickness  of  the  slate,  although  difficult  to  es- 
timate, probably  does  not  exceed  40  or  50  feet.  The  slate  is' 
characterized  throughout  by  a  distinct  cleavage-structure,  which 
strikes  N.  80°  E.,  and  hades,  usually  to  the  north,  occasionally 
to  the  south,   0°-5°,    being   always   transverse  to  the  bedding 


104 

planes.  Somewhat  prominent  joint-planes  hade  S.  about  45°. 
By  taking  advantage  of  the  lowest  stage  of  the  tide,  it  is  pos- 
sible to  add  a  little  to  the  meagre  knowleds^e  of  the  o-eolo2:y  of 
northern  Hull  afforded  by  the  ledges  just  described.  The 
Toddy  Rocks,  one  fourth  mile  north  of  Telegraph  Hill,  were 
visited  with  a  boat  at  low  tide  and  found  to  be  ledges  of  a 
bright  green  slate,  with  very  perfect  cleavage,  which  strikes 
N.  75"  E.  and  hades  to  the  north  0°-5°.  The  true  bedding 
lines  dip  north  at  low  angles,  10°-20''.  Some  thin  and  very 
hard  compact  layers  of  a  gray  color  are  probably  intrusive 
(trap).  This  outcrop  is  evidently  essentially  similar  to  that 
south  of  Thornbush  Hill ;  and  slate  of  the  same  general  char- 
acter is  the  most  abundant  rock  in  the  drumlins  and  on  the 
stony  beaches  of  northern  Hull.  Bowlders  of  conglomerate 
and  sandstone  are  so  rare  as  to  indicate  that  they  have  prob- 
ably been  transported  by  ice  ;  and  we  hazard  little  in  regarding 
the  part  of  Hull  between  Point  Allerton  and  Windmill  Point 
as  having  a  continuous  foundation  or  axis  of  slate.  One  and 
one  half  miles  east  of  Point  Allerton,  and  fully  a  mile  south  of 
the  line  of  strike  of  the  slate  at  the  foot  of  Thornbush  Hill,  is 
Harding's  Ledge.  This  is  partly  bare  at  low  tide,  exposing  a 
hard  grayish  and  greenish  sandstone  varying  to  a  fine  conglom- 
erate. It  is  an  undoubted  ledge  ;  and  these  rocks  are  probably  on 
the  line  of  the  anticline  of  Hough's  Neck,  which  we  may  sup- 
pose to  extend  by  the  north  side  of  Grape  Island,  Bumpkin 
Island,  Strawberry  Hill,  and  Strawberry  Ledge  to  Harding's 
Ledge.  It  is  manifestly  impossible,  without  going  beyond  the 
limits  of  Hull,  to  reach  any  definite  conclusion  concerning  the 
relations  of  the  slate  beds  to  the  conglomerate  and  melaphyr 
series.  It  is  probably  safe  to  assume,  however,  that  here,  as 
elsewhere  in  the  Boston  Basin,  these  soft  and  arenaceous  slates 
■  normally  overlie  the  conglomerate,  the  latter  appearing,  as  a 
rule,  only  along  denuded  anticlines.  According  to  this  view, 
not  only  northern  Hull,  but  the  entire  peninsula  north  of 
Atlantic  Hill,  is  underlain  chiefly  by  slate;  and  to  account  for 


105 


the  broad  exposure  of  the  conglomerate  series  at  the  soiiIIkmii 
end  of  the  peninsuhi  we  have  only  to  add  one  more  to  the;  (;ast- 
west  displacements  with  the  downthrow  on  the  nortli  wliicli  we 
have  observed  in  the  Nantasket  ledges,  the  fault  crossing  tlic 
beach  near  the  base  of  Atlantic  Hill. 

THE    DIKES    OF    NANTASKET. 

The  dikes  of  this  district,  as  previously  stated,  consist  almost 
wholly  of  diabase  and  are  more  recent  than  tlie  bedded  rocks. 
The  existence  of  dikes  of  melaphyr  and  porphyrite,  at  least  in 
the  granite  and  the  older  strata,  is,  however,  probable,  a  prior-i ; 
and  two  dikes  of  melaphyr  have  actually  been  recognized  in  the 
Nantasket  ledges  ;  while  in  the  granite  ledges  along  the  Co- 
hasset  shore  there  is  a  fine  series  of  porphyrite  dikes.  The 
dikes  of  melaphyr  and  porphyrite  clearly  antedate  all  the  dia- 
base dikes,  dating,  probably,  from  the  effusive  eruptions  to 
which  they  correspond  in  composition  ;  and  they  will,  therefore, 
be  described  first,  in  each  topographic  division.  Since  the 
special  map  shows  so  clearly  the  location  and  approximate 
trend  of  each  dike,  and  the  descriptive  lists  give  in  addition  the 
exact  trend  and  the  hade,  width,  and  other  features,  it  is  only 
necessary  to  cite  here  such  additional  facts  as  may  be  of  particular 
interest. 

The  Melaphyr  Dikes. 


No. 

Trend. 

Hade. 

W^idth 

in 
feet. 

Remarks. 

1 
2 

E.-W. 

S.  0°-30^^  E. 

Veutical 
Vertical 

3-7 

32 

Irregular  in  form  and  liade. 
Kegular  in  form  and  uniform  in 
texture. 

The  two  dikes  belonging  to  this  class  have  been  referred  to 
in  connection  w^ith  the  associated  flows  of  melaphyr.  The  first 
one  (No.  1)  breaks  through  the  brecciated  and  compact  green- 
ish gray  melaphyr  on  the  nortli  side  of  Centre  Hill.     Except  in 


106 

being  more  compact  and  homogeneous,  it  is  very  similar,  litho- 
logically,  to  the  enclosing  melaphyr  ;  and  hence  is  quite  incon- 
spicuous and  not  easily  found.  The  outcrop  begins  on  the 
north  side  of  the  road  immediately  east  of  the  Waverly  House 
and  can  be  traced  due  east  several  hundred  feet  to  the  shore. 
It  is  somewhat  irregular  in  form,  and  varies  in  width  from 
three  feet  or  less  to  seven  feet,  narrowing  eastward.  The  hade 
is  also  variable,  but  averages  about  vertical. 

Its  compactness,  similarity  to  the  enclosing  melaphyr,  and 
irregular  outline  are  the  characters  that  especially  mark  this  as 
an  approximately  contemporaneous  dike.  The  melaphyr  of  the 
dike  is  firmly  welded  to  that  of  the  walls,  and  the  contacts  are 
clearly  neither  joint-planes  nor  fault-fractures.  Although  this 
is  the  only  undoubted  dike  of  melaphyr  that  I  have  observed  in 
the  Nantasket  flows,  the  eye  is  frequently  attracted  by  sharply 
defined,  vertical,  dike-like  contacts  between  the  ordinary  brec- 
ciated,  amygdaloidal  or  scoriaceous  melaphyr  and  more  com- 
pact forms  ;  but  as  a  rule  these  cannot  be  traced  far,  or  only 
one  wall  can  be  found,  and  it  is  quite  clear,  in  most  cases  at 
least,  that  they  are  not  dikes  in  the  ordinary  sense. 

The  second  dike  (2)  of  the  list  is  much  more  impressive  and 
interesting  than  the  first,  being  decidedly  the  largest  of  all  the 
Nantasket  dikes.  It  cuts  the  granite  south  of  the  west  marsh 
with  a  nearly  due  north-south  trend  and  a  width  of  32  feet.  Its 
interesting  relations  to  the  effusive  melaphyr  of  the  western 
area,  and  its  marked  change  in  form  on  passing  from  the  gran- 
ite to  the  basal  conglomerate  on  the  eastern  margin  of  Granite 
Plateau,  have  been  fully  described  (page  77)  ;  and  in  these 
respects,  that  is,  as  a  clearly  exposed  volcanic  vent  the  outflows 
from  which  can  be  fully  identified,  it  is,  perhaps,  unique  among 
the  dikes  of  the  Boston  Basin.  It  is  exposed,  as  the  map 
shows,  on  two  islands  in  the  marsh,  forming  the  western  edge 
of  one  and  the  eastern  edge  of  the  other.  It  commences  on  the 
north  with  a  due  north-south  trend,  but  on  passing  to  the 
southern  ledge  it  changes  to  S.  10°  E.,  and  then  to  S.   30°  E. 


107 

It  can  be  traced  almost  to  the  northern  shore  of  tlio  l)!iy  ;  hut  no 
trace  of  it  could  be  found  in  the  almost  continuous  ledi^cs  of 
granite  on  the  south  shore.  Its  last  observed  course,  however, 
would  carry  it  directly  up  the  mouth  of  Weir  River ;  and 
the  absence  of  additional  outcrops  is  not,  therefore,  evidence  of 
an  important  displacement.  That  this  great  dike  should  not, 
on  account  of  its  trend,  be  referred  to  the  third  or  newest 
system  of  diabase  dikes  is  obvious  not  only  from  its  lithological 
character,  but  also  and  especially  from  the  fact  that  it  is  clearly 
cut  by,  and  hence  is  older  than,  one  of  the  east- west  diabase 
dikes  (39),  the  outcrop  of  the  latter  continuing  directly 
through  the  melaphyr  dike. 

The  Diabase  Dikes. 

The  occurrence  of  the  diabase  dikes  in  three  systems  —  two 
older  systems  having  approximately  east-west  trends  and  a 
newer  north-south  system  —  has  been  noted  ;  and  also  the  fact 
that  the  east-west  systems  form  three  belts,  corresponding  in  a 
general  way  with  the  principal  dislocations  of  the  district.  All 
of  the  dikes  agree  in  direction  with  well-determined  faults, 
although  often  not  actually  coinciding  with  a  fault  of  sensible 
displacement.  The  two  east-west  systems  are  certainly  much 
more  closely  related  to  each  other  than  to  the  north-south 
system,  and  since,  in  the  absence  of  clearly  exposed  intersec- 
tions, they  are  not  always  readily  distinguished,  it  appears  best 
not  to  attempt  to  describe  them  separately ;  but  we  will, 
instead,  follow  the  classification  by  belts,  commencing  with  the 
northern  or  coastal  belt. 

Although  Mr.  Merrill  has  not  observed  any  marked  or 
constant  diiference  in  microscopic  characters  between  the  older 
or  east- west  dikes  and  the  newer  or  north- south  dikes,  they  are 
usually  somewhat  contrasted  in  external  aspect.  The  east-west 
dikes  are  commonly  of  a  dark  greenish  gray  color,  due  to  the 
abundance  of  secondary  chloritic  minerals  and  epidote.  They 
are,   relatively,   chemically  stable,   resisting  atmospheric  influ- 


108 


ences  and  preserving  glacial  striae  almost  as  well  as  the  mel- 
aphyr  and  granite;  and  they  rarely  weather  brownish,  or  show 
in  any  marked  degree  peroxidation  of  the  ferruginous 
compounds.  Finally,  they  are,  as  a  rule,  irregularly  jointed, 
exhibiting  only  exceptionally  a  regular  transverse  or  prismatic 
joint-structure.  The  north-south  dikes,  on  the  other  hand,  are 
usually  black  on  a  fresh  fracture  and  not  conspicuously  chloritic 
or  epidotic  ;  they  weather  readily  to  brownish  masses  of  rounded 
outline  and  later  to  a  bi'own  earth  ;  and  a  well-developed  cross 
or  columnar  jointing  is  often  a  conspicuous  feature  and  never 
wholly  wanting. 

East-West  Dikes  of  the  Northern  or   Coastal  Belt. 


W^idth 

/ 

No. 

Trend. 

Hade. 

in 
feet. 

Remarks. 

3 

N.  75"-80°  E. 

Vert.-N.ioo 

2.5-3.5 

Finely  porphyritic. 

4 

N.  75°  E. 

S.  15°-20° 

3 

Slightly  porphyritic. 

5 

N.  80°  E. 

S.  5° 

3-5 

Double  dike;  cut  by  11. 

6 

S.  80°  E. 

Vertical 

3-4 

Jogged  or  faulted  iu  crossing 
beach. 

■  7 

S.  80°  E. 

Vertical 

15-20 

Walls  irregular  and  faulted. 

8 

S.  82°  E. 

N.  25° 

2.75-3 

Cross-jointed. 

9 

S.  80°  E. 

Vertical 

10 

Probably  divided  or  composite 
toward  the  east. 

10 

S.  75°-80o  E. 

Vert. -N.  15° 

4-5 

Irregular  in  width  and  hade. 

11 

S.  75°-80°  E. 

N.  10° 

2.5 

Encloses  fragments  of  granite. 

12 

S.  75°  E. 

Vertical 

3.5-4.5 

Encloses  fragments  of  granite 
and  syenite. 

13 

S.  750-80°  E. 

Vert. -N.  10° 

.75-1.25 

Very  irregular,  at  east  end. 

14 

S.-75°-80°  E. 

N.  10°-20° 

4.5-7 

Very  irregular  and  brolcen. 

'  15 

S.  77°  E. 

N.  50-10° 

2-4? 

No  satisfactory  exposures. 

16 

S.  75°-80°  E. 

Vertical 

4 

Quite  irregular  in  trend  and 
hade. 

17 

S.  80°  E. 

N.  10°-15° 

4-5 

Eegular ;  probably  a  branch  of 

^18. 
Eegular ;    unusually   strong 

18 

S.  750-8O"  E. 

N.  45° 

8 

hade. 

19 

S. 75°-80°  E. 

N.  30° 

8-9 

Very  irregular  and  branching. 

The  dikes  of  the  coastal  belt  present  ninny  interesting  and  in- 
structive features.  As  the  map  plainly  shows,  they  form  a 
well-defined  belt  coinciding  with  the  immediate  shore-line ;  and 


109 


it  is  very  certain  that  tlieir  non-occurrence  over  the  hrond  area 
of  nielaphyr  formini^  Atlantic  and  Centre  Hills  cannot  he 
attributed  to  the  absence  of  exposures.  Only  three  dikes  in  this 
belt  (3—5)  belong  to  the  first  or  oldest  system  ;  while  no  fewer 
than  fourteen  (G-19)  find  a  place  in  the  second  system.  The 
older  dikes  will  be  described  first  and,  so  far  as  possible,  in  the 
topographic  order  of  the  outcrops. 

On  the  northeast  corner  of  Atlantic  Hill,  the  melaphyr  south 
of  the  great  dike  (7)  is,  for  a  breadth  of  nearly  twenty  feet, 
very  shaly,  being  divided  into  thin  vertical  layers  running  N. 
80°  E.  Careful  examination  discovers  in  the  middle  of  this 
shaly  zone  a  dike  (3)  having  the  same  trend.  It  must  meet 
the  great  dike  near  the  east  side  of  Valley  Beach  ;  and  is  prob- 
ably cut  and  slipped  by  it,  although  the  intersection  is  not 
exposed;  while  to  the  westward  it  passes  under  the  shingle 
beach.  It  is  finely  porphyritic,  and  reappears  with  the  same 
lithological  character,  width,  and  hade  on  Gun  Rock  and  the 
half-tide  ledge  to  the  eastward. 

From  25  to  75  feet  south  of  the  melaphyr  dike  (1)  on 
Centre  Hill,  and  extending  from  near  the  end  of  Centre  Hill 
Avenue  across  the  ledges  to  the  shore,  is  the  narrow  chasm 
holding  the  very  obscure  outcrops  of  the  next  dike  (4),  the  atti- 
tude and  dimensions  of  which  are  taken  from  the  fissure  ratlier 
than  from  the  dike  itself.  It  trends  toward  the  middle  of  Gun 
Rock,  but  no  traces  of  it  could  be  found  on  that  promontory. 
Nearly  150  feet  south  of  this  dike  is  a  broader  chasm  or  de- 
pression, in  which  a  small  dike  occurs,  or  possibly  in  the  lower 
end  of  the  chasm  two  small  parallel  dikes  with  several  feet  of 
melaphyr  between  them.  Looking  eastward  across  the  bay  we 
see  in  line  with  this  depression  a  more  sharply  defined  chasm  in 
the  first  ledge  south  of  Gun  Rock.  Here  the  double  dike  is 
well  developed  in  a  fissure  five  feet  wide,  the  south  dike  meas- 
uring 2.5  feet  and  the  north  dike  1  foot.  Keeping  this  course, 
we  find  the  same  double  dike  cutting  the  ledge  east  of  Gun 
Rock.       The  two  members  here  measure  3  and  1.5  feet  respec- 


no 

tively,  with  3  feet  of  separating  melaphyr.  The  dike  is  thus 
widening  eastward,  and  it  undoubtedly  begins  on  Centre  Hill 
near  where  it  is  first  observed.  In  the  third  or  eastern  outcrop 
it  is  very  plainly  cut  by  a  dike  of  the  second  system  (11),  this 
intersection  (Fig.  14)  being  the  one  chiefly  relied  upon  to 
determine  the  relative  ages  of  the  two  systems 

We  pass  now  to  the  dikes  of  the  second  system,  commencing 
with  the  three  typical  examples  crossing  Long  Beach  Rock. 
The  most  northerly  of  these  (10)  is  about  50  feet  from  the 
north  end  of  the  ledge  at  low  tide.  It  crosses  the  strike  of  the 
conglomerate,  but  the  intersection  is  under  water.  About  60 
feet  of  the  melaphyr  separate  this  from  the  large  dike  (9), 
which  crosses  the  eastern  end  of  the  conglomerate,  and  is  there 
seen  to  be  attended  by  slight  faulting,  the  contact  between  the 
conglomerate  and  melaphyr  being  displaced  in  sucli  a  way  as 
to  show  a  downthrow  to  the  north  of  about  5  feet.  The  third 
dike  (8)  is  120  feet  south  of  this  and  crosses  the  conglomerate 
contact  without  apparent  faulting.  It  is,  perhaps,  the  most 
distinctly  cross-jointed  of  all  the  east-west  dikes.  These  three 
dikes  appear  to  cross  the  low-tide  ledge  north  of  Valley  Beach  ; 
and  10  appears  again  on  the  south  side  of  Gun  Rock,  but  with 
a  northerly  hade  of  15°.  In  the  place  of  9  we  find  on  the  shore 
south  of  Gun  Rock  a  series  of  six  small  parallel  dikes,  with  an 
aggregate  thickness  of  9  feet,  which  appear  to  be  the  equiva- 
lent and  continuation  of  it.  But  8,  if  it  extends  this  far, 
must  pass  under  the  beach. 

The  shore  along  the  northern  base  of  Atlantic  and  Centre 
Hills  appears  to  be  determined  immediately  by  the  great  dike 
(7),  which  must  also  pass  under  the  beach  south  of  Gun 
Rock.  It  shows  several  small  faults  or  lateral  shifts  of  from  3 
to  8  feet  at  the  base  of  Atlantic  Hill,  and  suffers  a  displace- 
ment of  perhaps  10  feet  in  crossing  Valley  Beach,  passing 
beneath  the  foot-bridge  in  front  of  the  Waverly  House.  The 
smaller  dike  (6)  parallel  with  this  is  well  exposed  in  front  of 
tlie  Pacific  House,  but  cannot  be  traced  more  than   half-way  to 


Ill 

the  Waverly  House,  although  there  are  plenty  of  ledges  on  its 
course.  It  is  quite  possible,  however,  that  it  passes  under  the; 
street,  in  front  of  the  Waverly  House,  as  far  as  Valley  Reacli. 
It  is  perhaps  possible,  also,  that  this  dike  cuts  4  and  5,  l)ut 
more  probable  that  they  end  before  reaching  it.  On  crossing 
the  beach  east  of  the  Pacific  House,  this  dike  is  seen  cuttino- 
through  the  first  ledge,  near  the  road.  Its  characters  are 
unchanged  ;  but  in  crossing  the  beach  it  is  evidently  jogged  or 
shifted  to  the  south  about  25  feet,  as  if  a  transverse  fault  inter- 
vened (see  map). 

Gun  Rock,  with  its  broad  surface  of  bare  melaphyr,  is  an 
uncommonly  favorable  point  for  the  study  of  dikes.  As 
already  pointed  out,  it  embraces  two  dikes  (3  and  5)  of  the 
first  system  and  five  dikes  (9-13)  of  the  second  system. 
Beginning  on  the  west  shore  we  have  first  the  double  dike  (5) 
in  a  fissure  about  five  feet  wide.  About  23  feet  north  of  this 
begins  the  series  of  small  dikes  supposed  to  be  the  extension  of 
the  large  dike  (9)  on  Long  Beach  Rock.  The  following  suc- 
cession is  readily  made  out : — 


Separating  melaphyr, 


(1) 

Diabase, 

0.5  feet, 

(2) 

2.0  " 

(3) 

3.0  " 

(4) 

1.0  " 

(5) 

1.5   " 

(6) 

1.0   " 

2 

feet. 

1 

" 

2 

" 

3 

" 

3 

'  ' 

11 

feet. 

9   feet. 

Some  75  feet  farther  north,  at  the  base  of  the  steep  slope,  is 
the  extension  of  10  ;  and  about  100  feet  of  melaphyr  separates 
this  from  11,  which  hades  N.  10°  and,  passing  under  the  house 
at  the  top  of  the  cliff,  reappears  on  the  east  side  of  the  ledge, 
and  again  in  the  same  direct  line  on  the  ledge  east  of  Gun 
Rock,  where  it  cuts  the  double  dike  of  the  first  system  (Fig. 
14).  The  intersection  is  clear  and  well  exposed;  and,  so  far 
as  one  intersection  can,  it  establishes  the  relations  of  these  two 


112 

systems.  It  is  a  curious  fact,  however,  that  of  the  ten  possible 
intersections  of  the  dikes  of  these  two  systems  on  or  near  Gun 
Rock,  only  this  one  is  actually  exposed  above  the  water. 


Fig.  14. —  Plan  showing  the  intersection  of  dikes  on 
the  shore,  east  of  gun  rock,  and  the  enclosed 
fragments  in  dikes  ii  and  12.  scale,  i  inch  1=25  feet. 

The  next  dike  in  regular  order  is  the  continuation  of  3  of  the 
first  system,  and  the  exposure  is  perfect  from  one  side  of  the 
promontory  to  the  other.  About  40  feet  north  of  this  dike, 
and  parallel  with  it  in  strike  and  hade,  is  an  interesting  quartz 
vein  one  inch  or  a  little  more  in  width.  It  is  perfectly  straight 
and  continuous,  its  outcrop  dividing  the  entire  mass  of  Gun 
Rock  by  one  sharply-defined  white  line.  Dike  12  of  the  second 
system  is  about  120  feet  north  of  11  ;  and  nearly  20  feet 
farther  north  is  13,  a  parallel  and  much  smaller  dike.  Dike  13 
is  closely  parallel  to  11,  appearing  just  to  clear  the  extremity  of 
the  ledge  east  of  Gun  Rock  and  to  be  continued  in  a  similar  dike 
(13)  on  Green  Hill  Ledge.  But  dike  12  takes  a  more  diagonal 
course,  being,  on  the  eastern  ledge,  100  feet  from  13  and  only 
42  feet  from   11.      The   large  dike   (12)  is,  perhaps,  the  most 


113 

interesting  of  the  series.  It  crosses  Gun  Rock  ;ind  all  the 
ledges  to  the  east,  increasing  in  width  from  3  to  4.5  feet.  On 
the  summit  of  Gun  Rock  it  makes  a  clear  intersection  of  th(! 
quartz  vein  just  described,  without  faulting'  it.  Throuo-hoiit 
its  entire  extent  this  dike  has  the  appearance  of  being  very 
coarsely  porphyritic  along  a  zone  from  six  inches  to  a  foot  wide, 
between  the  middle  of  the  dike  and  the  south  wall.  Careful 
examination  shows,  however,  that  it  is  not  truly  porphyritic, 
but  what  appear  to  be  indigenous  feldspars  are  really  angular 
fragments  of  a  coarsely  crystalline  feldspathic  rock  which  is , 
apparently,  pai-tly  granite  but  mainly  syenite.  The  fragments 
range  from  the  smallest  size  up  to  six  inches  or  more  in  lengtli  ; 
and  their  true  character  is  most  obvious  on  the  eastern  ledge 
(Fig.  14).  Dike  11  presents  the  same  feature  for  a  part  of  its 
course.  The  enclosed  fragments  in  this  dike  are  chiefly  coarse 
granite  ;  and  they  begin  very  abruptly  at  a  point  about  20  feet 
from  the  intersection  of  the  double  dike  and  the  same  distance 
from  the  shore.  They  are  of  all  sizes  up  to  six  inches  and,  as 
in  12,  are  crowded  in  a  layer  from  6  to  12  inches  wide  between  the 
middle  and  south  wall  of  the  dike.  Although  no  granitic  rocks 
arc  developed  on  the  surface  nearer  to  Gun  Rock  than  Jerusa- 
lem Road,  the  inference  is  irresistible  that  they  actually  under- 
lie Gun  Rock  ;  and  the  restricted  distribution  of  the  frag- 
ments in  the  dikes  is  a  very  plain  indication  that  the  upward 
movement  of  the  melted  rock  in  these  fissures  had  nearlj^ 
ceased  before  the  underlying  granitic  rocks  were  sufficiently 
disintegrated  to  yield  this  debris. 

The  intersection  of  the  quartz  vein  on  Gun  Rock  by  a  dike 
of  the  second  system,  and  irs  perfect  parallelism  with  the  dikes 
of  the  first  system,  show  that  it  probably  dates  from  the  form- 
ation of  the  first  system,  or  else  from  some  period  between 
the  two  systems.  South  of  this  vein  is  another,  smaller  and 
parallel  with  it,  but  not  observed  to  meet  any  of  the  dikes. 
Under  the  Park  House  on  the  summit  of  Centre  Hill  are  two 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  8. 


114 

conspicuous  quartz  veins  from  2  to  3  inches  wide,  parallel,  and 
75  feet  apart.  Curiously  enough,  however,  they  conform  in 
direction  with  the  second  system  of  dikes.  This  is  only  one 
of  several  indications  that  the  dikes  of  the  two  systems  are  not 
widely  separated  in  time. 

The  outermost  dike  on  Gun  Rock  (13)  is  undoubtedly  con- 
tinued in  the  Green  Hill  conglomerate,  as  mapped.  In  the 
eastern  half  of  the  ledge,  both  this  dike  and  14  are  especially 
characterized  by  extreme  irregularity  and  lack  of  continuity, 
the  conglomerate,  apparently,  having  been  repeatedly  faulted 
after  they  were  formed.  One  result  of  this  disturbance  is  that 
they  converge  eastward,  so  that,  although  50  feet  apart  along 
the  north-south  dike  (62)  they  are  almost  in  contact  at  the  east 
end  of  the  ledge.  About  50  feet  from  the  extreme  northern 
edo^e  of  the  cono-lomerate  and  22  feet  north  of  14  on  62,  but 
diverging  from  it  eastward,  is  dike  15.  It  is  not  Avell  exposed, 
but  it  is  clearly  a  dike  of  the  second  system.  The  chief  interest 
at  this  point  centers  in  the  clear  intersections  of  all  these  dikes 
(13—15)  by  the  single  small  representative  (62),  in  all  the 
coastal  area,  of  the  third  system  of  dikes. 

Dike  16,  on  the  north  side  of  Green  Hill  Rock,  is  quite 
irregular  in  trend,  but  doubtless  belongs  to  the  second  system. 
If  so,  it  is  probably  parallel  with,  and  passes  to  the  south  of, 
all  the  dikes  (17-19)  on  Black  Rock.  Dikes  17  and  18 
converge  eastward,  their  outcrops  being  50  feet  apart  at  the 
west  end  of  the  island  and  almost  in  contact  at  the  east  end. 
Dike  19  on  the  northeast  corner  of  the  island  ends  abruptly  on 
the  west  and  is  much  given  to  irregular  branching,  one  of  the 
easterly  branches  splitting  a  prominent  vein  of  quartz. 

By  far  the  most  prominent  set  of  joints  in  the  rocks  of  the 
coastal  area  trends  due  north  and  south.  They  form  or  bound 
the  valley  between  Atlantic  and  Centre  Hills  and  are  very 
marked  along  the  shore  of  Centre  Hill  and  Gun  Rock,  and,  in 
fact,  along  the  entire  Nantasket  coast,  producing  the  many 
fissures   and   chasms    that   indent    the   shore.      It  is  certainly, 


115 


therefore,  a  very  interesting-  fact  that  tlie  only  dike  of  the 
coast-al  area  conforming'  in  direction  with  tlicsc  joints  is  the  one 
small  example  of  the  third  system.  East-west  joints  are  also 
well  developed  in  the  coastal  melaphyr  ;  but  they  have,  with  i'cw 
exceptions,  a  decided  southerly  hade  of  20°  to  30°  ;  while  the 
east-west  dikes,  almost  without  exception,  arc  vertical  or  have 
a  northerly  hade,  showing  a  tendency  to  be,  as  a  rule,  a[)proxi 
mately  normal  to  the  bedding  planes  of  the  melaphyr,  tuff,  and 
conglomerate. 

East-  West  D.ikes  of  the   Central  Belt. 


Width 

No. 

Trend. 

Hade. 

in 
feet. 

Remarks, 

20 

N.  78^  E. 

Vert.-N.15° 

4-6 

Nortli  end  of  Rocl<j^  Neck. 

21 

N.  77"  E. 

N.  150-20° 

3-3.5 

Probably  a  branch  of  20. 

22 

N.  78"  E. 

Vert.-N.  3° 

8-12 

Probably  faulted  six  times. 

23 

N.  750-80°  E. 

Vertical 

10-15 

Parallel  with  22on  Rocky  Neck. 

24 

N.  75°  E. 

S.  10° 

5-6 

Porphyritic.    Parallel  with  22. 

25 

N.  80°  E. 

N.  200-25° 

4-5 

Porphyritic;  a  fault-dilce. 

-26 

S.  75°  E. 

N.  5° 

2.5-3 

Cuts  25  on  Melapliyr  Plateau. 

27 

S.  75°  E. 

N.  15° 

3 

Porphyritic ;  a  fault-dike. 

28 

S.  80°  E. 

Vertical 

6 

Regular;    and   no   apparent 
faults. 

29 

S.  80°  E. 

Vert.-N. 10° 

6-8,5 

Possibly   not   one    continuous 
dike. 

30 

S.  75°  E. 

N.  10° 

6 

An  important  fault-dike. 

31 

S.  80°  E. 

N.  20°-30° 

8-9 

Possibly  a  continuation  of  34. 

32 

S.  80°  E. 

N.  15°-20° 

3-4 

BranclVof  31. 

33 

S.  75°  E. 

N.  150-20° 

2 

Probably  a  branch  of  31. 

34 

S.  75°-80o  E. 

N.  40°-50o 

8-10 

Double  dike. 

35 

S.  75°-77°  E. 

N.  10°-15° 

5.5-7 

Unrecognized  west  of  the  rail- 
road. 

36 

E.-W. 

N.  5°-10° 

1-1.5 

Branch  of  35. 

The  central  belt  extends  from  the  shore  of  Strait's  Pond,  near 
Folsom's  Island,  across  Conglomerate  and-  Melaphyr  Plateaus, 
and  thence  along  the  south  shore  of  Nantasket  Harbor  to  Rocky 
Neck.  Both  of  the  east-west  systems  are  represented  in  this 
belt ;  but,  as  before,  the  second  system  largely  predominates. 
Although  the  exposures  are,  as  a  rule,  less  satisfactory  than  in 
the  coastal  belt,  individual  dikes  can,  in  several  instances,  be 


116 

traced  much  farther,  the  extreme  distance  being  one  and  a  half 
miles.  It  is  a  singular  fact  that  the  numljer  of  observed  dikes 
is  exactly  the  same  as  in  the  coastal  belt. 

The  most  northerly  dikes  of  the  first  system  (20  and  21)  on 
the  northern  extremity  of  Rocky  Neck  are  of  similar  character, 
and,  converging  eastward,  may  be  regarded  as,  possibly, 
branches  of  the  same  dike.  The  next  dike  of  this  system  (22) 
first  appears  on  the  western  shore  of  Eocky  Neck  with  a  Avidth  of 
from  8  to  10  feet ;  and  only  20  feet  south  of,  and  parallel  with,  it 
is  the  very  similar  companion  dike  (23),  enclosing  a  large 
wedsre  of  o^ranite.  These  two  dikes  can  be  traced  in  the  same 
direct  line  only  400  or  500  feet  at  the  farthest ;  but  it  is  quite 
certain  that  they  are  shifted  to  the  eastward  by  the  fault  between 
the  o^ranite  and  conorlomerate  and  continue  to  the'  east  shore. 
This  is  clearly  the  case  with  the  northern  one,  w^hich  may  be 
safely  correlated  with  the  dike  of  the  same  number  in  the 
western  area,  although  it  is  100  feet  too  far  north  to  be  just  in 
line  with  it.  Crossing  the  bay,  we  find  this  dike,  with  a  width 
of  12  feet,  dividing  the  half-tide  ledge  of  melaphyr  off  Granite 
Point;  and  striking  the  shore  on  the  north  side  of  the  point, 
with  a  trend  N.  80°  E.,  it  takes  a  perfectly  straight  course  to 
the  southwest  corner  of  East  Porphyrite  Hill,  where  it  is  jogged 
to  the  south  its  own  width  and,  skirting  the  edge  of  the  con- 
glomerate, is  lost  beneath  the  grass.  It  is  probably  either 
broken  here  by  a  fault  not  marked  on  the  map,  or  dies  out  and 
is  resumed  along  a  parallel  line  ;  for  what  appears  to  be  essen- 
tially the  same  dike  begins  a  little  to  the  northward  and,  though 
cut  and  shifted  by  three  dikes  of  the  younger  system  (27,  29, 
31),  can  be  traced  across  the  hill  to  the  shore,  where  it  is  well 
exposed  with  a  breadth  of  9  feet.  Closely  parallel  with  this 
dike,  on  East  Porphyrite  Hill,  and  sharing  the  same  accidents, 
but  with  the  intersections  less  clearly  exposed,  is  24 ;  and  this 
dike,  which  is  distinctly  porphyritic,  might  safely  becorrelated 
with  the  dike  of  the  same  general  character  (25)  east  of  the 
bay  and  the  railroad,  but  for  the  fact  that  it  lies  so  far  to  the 


117 

iioftli  as  to  involve  a  displacement  of  iinprohahh;  nia^iiitiidf!. 
The  latter  (25)  is  a  good  example  of  a  fault-dike  (Fig.  5)  and 
its  northerly  hade  agrees  w^ell  with  the  throw.  It  is  cut  on  the 
nortliern  edge  of  Melaphyr  Plateau  by  the  most  noi'therly  dike 
of  the  second  system  (26)  ;  and  near  the  railroad  both  of  these 
dikes  are  dislocated  by  the  same  transverse  fracture,  the  hori- 
zontal displacement  being  7  feet  for  dike  26  and,  on  account  of 
its  greater  hade,  15  feet  for  dike  25. 

The  fault-dike   (27)  on  East  Porphyrite  Hill  is  one  of  the 
most  important  in  the  district,  marking  a  vertical  slip  of  about 
50  feet.     It  is  porphyritic  and  strikingly  similar  to  25,  east  of 
the  bay  ;  but  since  they  belong,  one  to  the  first  and  the  other  to 
the  second  system,  it  seems  best  not  to  connect  them.     This  is, 
however,  one  of  many  indications  that  the  two  systems  are  not 
very  distinct,  chronologically.      The    next  dike   (28),  though 
presenting  precisely  the  same  characters  on  Rocky  Neck   and 
Melaphyr  Peninsula,  cannot  be  identified  on  East  Porphyrite 
Hill ;  for  it  seems  best  to  connect  the  next  dike  to  the  south  on 
the  hill  (29)  with  the  dike  south  of  28  on  Rocky  Neck.     The 
last-mentioned  dike  (29),  like  27,  breaks  the  dikes  of  the  first 
system  on  East  Porphyrite  Hill ;  and  it  is  certainly  continued 
on  the  east  side  of  the  bay.      In  fact  it  can  be  identified  with 
reasonable  certainty,  as  shown  on  the  map,  from  east  of  Hull 
Street  to  the  western  shore  of  Rocky  Neck.     The   outcrop  on 
the  north  side  of  West  Porphyrite  Hill,  however,  is  quite  un- 
certain ;    and  it   may   be    that  what   is    regarded   here  as   one 
continuous  dike   may  include  two  or  three  independent  dikes. 
In  contrast  with  this  dike  of  exceptional  length,  the  next  dike 
(30)  has  been  recognized  only  on  the  west  side  of  Rocky  Neck. 
It  is   interesting  chiefly  on  account  of  the  important  displace- 
ment which  accompanies  it,  being  the  only  clear  fault-dike  on 
Rocky  Neck. 

The  next  dike  (31)  is  especially  characterized  by  its  strong 
northerly  hade.  It  probably  ends  westward  against  the  fault 
between  Cliff  Plateau  and  West  Porphyrite  Hill.     At  any  rate, 


118 


it  begins  abruptly  and  Avith  its  full  width  on  the  northern 
escarpment  of  the  plateau,  not  a  trace  of  it  or  of  33,  which  is 
probably  a  branch  of  31,  appearing  on  the  bare  ledges  of  the 
hill.  It  crosses  the  composite  N.— S.  dike  (66)  without  per- 
ceptible displacement,  slips  the  dikes  of  the  first  system  on  East 
Porphyrite  Hill  and  is  itself  thrown  abruptly  to  the  south  about 
25  feet  at  the  eastern  base  of  the  hill.  It  crosses  the  bay  with- 
out further  important  displacement  and  is  readily  recognized  on 
both  sides  of  the  railroad  by  its  width  and  hade.  It  can  be 
followed  by  frequent  outcrops  across  Melaphyr  Plateau,  gradu- 
ally approaching  and  appearing  to  unite  with  29.  But  they 
are  widely  separated  in  the  conglomerate  ledges  beyond,  31,  on 
account  of  its  ofreater  hade,  sufFerins:  a  ijreater  lateral  displace- 
ment  in  crossing  the  fault  between  Melaphyr  and  Conglomerate 
Plateaus.  In  the  ledges  west  of  Hull  Street  some  interesting- 
inclusions  of  conglomerate  are  clearly  exposed.      The  last  ap- 


FiG.  15. —  Natural  section  of  tiik  inclined  double 

DIKE   (34)    ON    THE  WESTERN  SHORE  OF    RoCKY  NeCK. 

Scale,  1  inch  =  6  feet. 


119 

pearance  of  this  dike  to  the  eastward  is  on  the  shore  of  Strait's 
Pond,  more  than  a  mile  from  its  starting  point.  On  Melaphyr 
Plateau  it  appears  to  give  off  a  branch  (32),  whioli  can  be 
traced  for  a  considerable  distance  parallel  with  the  main  dike. 
Granite  Head  on  Rocky  Neck  is  crossed  near  the  northern 
edge  by  a  double  dike  (34)  having  a  northerly  hade  of  50°  and 
the  following  section  from  south  to  north  :  main  dike,  7  to  8 
feet ;  granite  parting,  6  to  18  inches  ;  small  dike,  15  inches 
(Fig.  15).  It  can  be  traced  entirely  across  the  head,  and 
presents  the  same  section  and  hade  in  each  escarpment.  This 
dike  crosses  the  valley  occupied  by  the  boundary  fault  without 
sensible  displacement  and  reappears  on  the  opposite  slope  at  the 
point  where  it  crosses  the  great  N.  E.— S.  W.  fault,  which  here 
downthrows  to  the  southeast.  There  are  indications,  however, 
that  the  dike  is  subsequent  to  the  fault  and  has  simply  been 
jogged  in  crossing  it ;  for  it  is  shifted  or  jogged  to  the  north 
some  50  feet  in  crossing  the  fault.  If  the  fault  Avere  subse- 
quent to  the  dike,  the  displacement  should  be  in  just  the  oppo- 
site direction.  The  character  of  the  dike  is  unchanged,  except 
that  it  is  simple  instead  of  composite,  no  trace  of  the  smaller 
member  being  observed.  On  account  of  the  hade,  the  outcrop 
curves  in  passing  down  the  hill  to  the  shore  ;  but  it  is  not  clear 
enough  to  determine  with  certainty  its  relations  to  22.  Crossing 
Weir  River  Bay  in  the  same  direct  line,  and  apparently  without 
displacement,  it  reappears  as  a  double  dike  with  a  strong 
northerly  hade  (40°-50°)  on  the  southwest  corner  of  West  Por- 
phyrite  Hill.  The  section  here  from  south  to  north  is  :  diabase, 
3  feet ;  conglomerate,  8  feet ;  and  diabase,  7  feet.  The  hade  is 
most  marked  in  the  southern  member,  which  is  possibly  a 
branch  of  the  other,  but  more  probably  an  independent  dike  ; 
for  on  Cliff  Plateau  they  are  reversed  in  position,  the  small 
dike,  which  is  now  on  the  north,  retaining  its  stronger  hade. 
Near  the  eastern  edge  of  the  plateau  it  suddenly  disappears, 
apparently  joining  22  ;  possibly,  however,  it  is  continued  along 
a  more  northerly  line  by  the  other  inclined  dike   (31).      The 


120 


last  of  the  large  dikes  of  this  belt  (35)  first  appears  immedi- 
ately east  of  the  railroad,  at  the  junction  of  the  conglomerate 
and  underlying  melaphyr,  and  can  be  traced  thence  directly 
across  Melaphyr  Plateau,  where,  passing  beneath  the  private 
road,  it  reappears  in  the  eastern  part  of  Conglomerate  Plateau- 
The  small  dike  (36)  south  of  35  near  the  railroad  is,  doubtless, 
a  branch  of  the  latter. 


East— West  Dikes  of  the  Sotithern  Belt. 


Width 

No. 

Trend. 

Hade. 

in 
feet. 

Remarks. 

37 

N.  75°  E. 

N.  10°? 

5? 

Cuts  44  and  45? 

38 

N.  70°  E. 

Vert.-N.  5° 

6-7 

Porphyritic  and  branched. 

39 

N.  80°  E. 

N.  ? 

6-10?    . 

Cuts  the  great  melaphyr  dike. 

40 

N.  88°  E. 

Vert.-N.  10° 

3.5-4.5 

Irrejiular. 

41 

N.  85°  E. 

S.  10° 

4.5 

Possibly  unites  with  40. 

42 

N.  75°-80°  E. 

N.  10°    ■ 

6-7 

In  granite ;  regular. 

43 

S.  80°  E. 

N.  10° 

.25-. 5 

Very  regular  and  continuous. 

44 

S.  73°  E. 

N.  10° 

2.5-4 

Ends  abruptly  toward  the  east. 

45 

S.  75°-80°  E. 

N.  ? 

2? 

Imperfectly  exposed. 

46 

S.  80°  E. 

N.  5°-10° 

10-12 

Regular  and  continuous. 

47 

S.  80°  E. 

N.  10° 

6 

Brancli  of  44. 

48 

S.  85°  E. 

N.  10° 

7 

"        "     " 

49 

S.  75°  E. 

N.  10° 

.5-1 

11       It     11 

50 

S.  72°  E. 

N.  15°-20° 

.5-.75 

11     .1 

51 

S.  80°  E. 

N.  ? 

7 

Porphyritic. 

52 

S.  75°-80°  E. 

N.  5°-10° 

6-8 

11 

On  account  of  the  marshes,  the  dikes  of  the  southern  belt 
appear  much  less  continuous  than  those  of  the  central  belt ; 
and  they  present,  on  the  whole,  fewer  points  of  interest. 
Although  six  dikes  in  this  belt  are  referred  in  the  table  to  the 
first  system,  it  is  doubtful  in  several  cases  whether  this  refer- 
ence should  stand.  Thus  37,  on  Great  Hill,  has  the  normal 
trend  of  the  first  system,  but  it  appears  to  cut  two  small  dikes 
(44  and  45)  of  the  second  system.  The  two  systems  are  thus 
no  longer  distinct  in  both  age  and  trend,  as  in  the  coastal  area. 
Dike  38  has  the  extreme  trend  of  the  first  system  (N.  70°  E.). 
It  appears  first  on  the  west  shore  of  Weir  River  Bay  and  can 
b>!  traced  about    half-way  across  Granite  Plateau,  or  until  it 


121 

meets  the  great  dike  of  the  second  system  (4()).  Ft  is  some- 
what branchinp;  ns  well  as  poi-[)hyi-itic,  and  cross(;s  the  channel 
without  sensible  displacement,  although  it  is  somewhat  jogged 
in  the  plateau.  The  E.— W.  dike  (oD)  in  the  granite  south  of" 
the  west  marsh  is  imperfectly  exposed  ;  and  its  chief  point  of 
interest  is  that  it  cuts  the  great  melaphyr  dike  (2). 

The  dikes  beyond  the  east  marsh  (40  and  41)  have  rather 
noncommittal  trends,  not  belonging  distinctly  to  either  system. 
These  are,  possibly,  extensions  of  the  Great  Hill  dikes  ;  but  it  is 
impossible  to  prove  it.  They  converge  eastward  and  upward. 
The  dike  in  the  granite  southeast  of  Round  Hill  (42),  on  the 
other  hand,  is,  so  far  as  can  be  seen,  a  very  typical  example 
of  the  first  system. 

The  most  northerly  dike  of  the  second  system  in  this  belt  is  the 
smallest  one,  the  solitary  dike  on  Crescent  Hill  (43).  It  can 
be  traced  continuously  across  the  conglomerate  and  iuelaphyr 
from  the  western  to  the  eastern  face  of  the  hill.  If  it  were  con- 
tinued across  the  railroad  in  the  same  direct  line  it  would  strike 
Great  Hill  about  40  feet  from  the  northern  end  ;  but  no  trace  of 
it  could  be  found  in  the  bare  ledo-es  of  conij-lomerate  and  sand- 
stone.  Advancing  south  across  Great  Hill,  we  come  first  to 
dikes  44  and  45,  which,  as  abeady  stated,  appear  to  be  cut  by 
37  of  the  first  system.  These  two  dikes  are  somewhat  con- 
verging eastward,  and  44  ends  very  abruptly  in  the  conglom- 
erate before  reaching  the  face  of  the  hill.  The  principal  dike 
of  this  entire  belt  is  46,  which  begins  on  the  western  shore  of 
Granite  Plateau  with  a  breadth  of  about  12  feet.  It  is  jogged 
to  the  north  8  or  10  feet  in  ascending  the  slope,  and  cannot  be 
clearly  traced  more  than  half-way  across  the  plateau,  passing 
under  the  grass  ;  but  there  is  a  natural  path  for  it  under  the 
narrow  meadow  along  the  north  side  of  the  high,  precipitous 
ledge  of  granite  and  conglomerate  to  the  western  marsh. 
Here  it  is  directly  in  line  with  what  is  clearly  the  same  dike  in 
Great  Hill.  On  the  precipitous  western  face  of  Great  Hill, 
just  north  of  the  junction  of  the  conglomerate  and  granite,  it 


122 


measures  11.5  feet ;  and  about  half-way  across  the  hill  it  appears 
to  divide  into  two  nearly  equal  dikes,  which  are  separately 
numbered  C47  and  48) .  Near  the  point  of  bifurcation,  which  is, 
unfortunately,  concealed,  a  much  smaller  branch  (49)  starts 
from  the  north  side  and  can  be  traced  across  to  the  western  cliff. 
The  similar  small  dike  (50)  just  south  of  46,  on  the  western 
shore  of  Granite  Plateau  is  probably  also  a  branch  of  this  main, 
parent  dike.  Parallel  with  46  on  the  south  is  51,  which  can 
be  traced  for  about  the  same  distance  across  Great  Hill,  crossing 
the  boundary  fault  between  the  conglomerate  and  the  granite 
without  sensible  displacement.  The  most  southerly  dike  on 
Granite  Plateau  (52)  is,  perhaps,  a  continuation  of  51.  It  is 
first  seen  about  half-way  across  the  plateau  and  can  be  followed 
to  the  western  shore  of  Weir  River  Bay,  crossing  and  probably 
cuttins:  38  under  the  water. 


East-  West  Dikes  South   of    Weir  River 


Width 

No. 

Trend. 

Hade. 

in 
feet. 

Remarks. 

5:^ 

N.  80°  E. 

N.  5°-10° 

1 

In  iirauite  ^waat  of 

railroad. 

54 

N.  80"^  E. 

N.  5°-10° 

3 

" 

55 

N.  80°  E. 

N.  10°-15° 

3.5 

" 

56 

N.  80°  E. 

N.  10° 

4  ? 

"      " 

57 

N.  80°  E. 

east   " 

58 

N.  80°  E. 

N.  30°-40° 

10-15? 

"      " 

59 

S.  80°   E. 

N.  35° 

4 

west   " 

60 

S.  85°   E. 

N.  10° 

7 

"      " 

61 

E.-W. 

S.  15° 

2.5 

"       east   " 

No  special  or  systematic  searcli  has  been  made  for  dikes 
among  the  granite  ledges  south  of  Weir  River  Bay  ;  and  the 
few  that  were  casually  observed  have  been  noted  chiefly  to 
render  the  special  map  more  uniform  and  complete,  and  not 
on  account  of  their  intrinsic  interest.  It  sln^uld  l)e  understood, 
however,  that  these  outlying  non-sedimentary  areas  of  the  map 
have  not  been  uniformly  treated  in  this  respect,  and  un- 
doubtedly many  dikes  have  been  overlooked,  especially  west  of 


123 


the  biiy  and  :ilon<>;  Rockland  Street  and  Jerusalem  Road.  Tlie 
dikes  of  this  section  may  be  veg-arded  as  forming*  a  fourth  east- 
west  belt,  and  it  is  noteworthy  that,  for  the  first  time,  the 
older  system  predominates.  The  outcrops  are  so  imperfect 
that  no  attempt  will  be  made  to  describe  these  dikes  in  detail. 

North-South  Dikes. 


Width 

No. 

Trend. 

Hade. 

ill 
feet. 

Remarks. 

62 

N.-S. 

W.  3'J-5" 

.75-1.75 

Cats  13,  14,  and  15. 

63 

N.-S. 

Imperfect  exposure. 

64 

N.-S. 

W.  lO'-' 

1.5 

Irreiiular  and  interrupted. 

65 

N.-S. 

Imperfect  exposure. 

66 

N.-S. 

Vertical 

40 

Composite,  six  parallel  dikes. 

67 

N.-S. 

Vertical 

3 

Probably  ends  against  fault  on 
the  soutli. 

68 

N.-S. 

E.3° 

2.-2.25 

69 

N.-S. 

Vertical 

1-1.5 

70 

N.  15°  E. 

W.  5^^-10° 

2.5 

In  iiranite  south  of  Weir  River 
Bay. 

The  dikes  of  the  third  or  newest  system,  with  the  normal 
trend  N.-S.,  have  their  best  development  in  the  central  and 
western  ai'eas,  only  one  having  been  observed  in  the  coastal  area 
and  none  on  Rocky  Neck.  Unfortunately,  the  only  clear  and 
satisfactory  intersections  with  the  east-west  dikes  are  those 
afforded  by  the  small  dike  in  the  Grreen  Hill  ledge  ;  but  that 
the  dikes  of  this  system  are  all  of  about  the  same  age,  and  newer 
than  the  east- west  dikes,  there  can,  I  think,  be  no  reasonable 
doubt.  Tliey  are,  as  previously  explained,  darker  colored 
(less  chloritic  and  epidotic)  than  the  older  dikes,  more  prone 
to  weather  brownish  and  to  disintegrate  on  exposure,  and 
more  generally  characterized  b}^  transverse  or  columnar  jointing. 

The  single  small  example  of  this  system  in  the  coastal  area 
(62)  breaks  through  the  Green  Hill  conglomerate  and  cuts  all 
the  east-west  dikes  (13,  14,  15)  in  this  ledge,  without  sensibly 
displacing  them.  But  it  is  itself  slipped  or  jogged  to  the  west 
4  feet  near  its    intersection   with    14 ;   and  north  of  15   it  is 


124 

double,  giving  off  a  branch  parallel  with  itself.  It  runs 
directly  toward  the  middle  of  Green  Hill  Rock,  but  no  trace  of 
it  can  be  found  in  this  bare  mass  of  porphyrite. 

The  fault  bounding  Melaphyr  Plateau  on  the  west  is 
accompanied  by  several  small  dikes,  one  of  which  (64)  can  be 
traced,  with  some  interruptions  and  dislocations,  the  entire 
breadth  of  the  plateau,  crossing  and  probably  cutting  all  the 
east- west  dikes.  The  most  important  and  decidedly  the  most 
interesting  of  all  the  dikes  of  this  system  is  the  parallel  series 
(66)  separating  East  Porphyrite  Hill  and  Cliff  Plateau.  Near 
the  shore  these  afford  the  followinaf  section  from  east  to  west : — 


Separating  melaphyr,  3.     feet. 

"  couaiomerate.      5.75.  " 


50. 


74.75  feet. 

This  composite  dike  or  series  of  dikes  is  thus  equivalent  to 
about  40  feet  of  diabase  and  75  feet  of  separating  conglomerate 
and  melaphyr,  or  115  feet  for  the  entire  fractured  zone.  The 
series  is  actually  exposed  for  only  a  few  rods  ;  but  there  is  a 
clear  path  for  it  between  Conglomerate  Hill  and  Cliff  Plateau 
to  the  northern  end  of  the  western  marsh.  Almost  in  the 
same  direct  line  at  the  southern  end  of  the  marsh  is  the  largest 
of  all  the  Nantasket  dikes  (2),  which  might  be  regarded  as 
due  to  the  union  and  continuation  of  the  composite  dike.  This 
view  is  precluded,  however,  by  the  lithological  contrast,  the 
southern  dike  being  a  true  melaphyr  and  undoubtedly  contem- 
poraneous with  the  first  flow  of  that  rock,  while  the  northern 
or  composite  dike,  representing  the  latest  period  of  igneous 
activity  at  Nantasket,  is  very  much  newer.  Probably  both  the 
melaphyr  dike  and  the  composite  dike  are  intercepted  by  the 
boundary  fault  under  the  western^marsh. 


(1) 

Diabase, 

9.75   feet. 

(2) 

4.25      " 

(3) 

7.5 

(4) 

6.5 

(5) 

11.25      " 

(6) 

.5    foot. 
39.75  feet. 

125 

The  three-foot  dike  (f)7)  so  clearly  exposed  on  the  north 
shore  of  Mclaphyr  Peninsula  and  traceable  across  West 
Porphyrite  Hill  certainly  does  not  extend  so  far  in  this  line  as 
Granite  Plateau,  probably  ending  under  the  water  against  the 
N.  E.-S.  W.  fault.  Nearly  200  feet  west  of  this  line,  on  the 
north  shore  of  the  plateau,  is  68,  which  can  be  traced  about 
150  feet  back  from  the  water  ;  and  about  500  feet  farther  west, 
following  the  shore,  is  69,  of  which  only  about  50  feet  in 
length  are  exposed. 

DIKES    ON    THE    COH ASSET    AND     SCITOATE    SPIORE. 

Dikes  are  abundant  in  the  granitic  rocks  south  of  Nantasket 
and  the  boundary  faults  ;  in  fact,  almost  every  large  ledge  or 
considerable  exposure  of  the  granite  and  diorite  shows  upon 
careful  examination  one  or  more  dikes  of  diabase.  As  a  rule, 
however,  except  along  the  shore,  where  the  almost  continuous 
ledges  are  clean  and  bare,  the  outcrops  of  the  dikes  are  obscure 
andean  be  correlated  only  to  a  limited  extent;  /.  e.,  the  same 
dike  can  rarely  be  traced  with  certainty  from  one  outcrop  to 
another.  And  when  we  further  consider  that  the  dikes 
naturally  tend,  through  their  more  rapid  erosion,  to  follow  the 
depressions  and  the  drift-covered  portions  of  the  surface  ;  and 
also  that  the  drift-deposits  and  marshes  are  continuous  over 
large  areas,  especially  toward  the  south  and  south-east,  the 
futility  of  attempting  to  trace  out  or  map  the  dikes  of  Cohasset 
and  Scituate  becomes  apparent.  In  the  more  limited  Nantasket 
area  the  conditions  are  comparatively  favorable  for  such 
thorough  work  ;  but  it  is  probable  that  even  here  many  dikes  are 
wholly  concealed,  and  the  map  claims  to  represent  with  only 
approximate  accuracy  and  completeness  the  dikes  which  are 
actually  exposed.  Particular  attention  was  given  to  the  Nan- 
tasket dikes,  because  it  was  seen  that  the  outcrops  are  sufficient 
to  permit  of  their  correlation  with  the  faults  of  the  district  and 
in  systems  of  diffisrent  ages;  and  about  all  that  it  has  seemed 
wise  to  attempt  in    the    study    of  the   dikes    of   Cohasset  and 


^     126 

Scitiiate  is  to  determine  to  what  extent  they  can  be  referred  to 
the  same  systems  as  the  Nantasket  dikes.  Systematic  observa- 
tions for  this  purpose  were  confined  to  the  immediate  vicinity  of 
the  shore  ;  and  the  following  paragraphs  and  accompanying  lists 
give  the  results  of  a  complete  canvass  of  the  littoi-al  dikes  from 
Green  Hill  to  the  shore  beyond  the  Glades.  No  attempt  has 
been  made  to  trace  any  of  the  dikes  inland  or  beyond  the  clean 
exposures  afforded  by  the  waves  ;  but  such  casual  observations 
as  have  been  made  away  from  the  shore  indicate  that  the  littoral 
zone  is,  in  the  number  and  trends  of  the  dikes,  representative  of 
the  entire  area.  No  dikes  have  been  represented  on  the  general 
map,  partly  on  account  of  the  inconveniently  small  scale,  but 
chiefly  to  avoid  giving  the  impression  of  great  inequality  in  the 
distribution  of  the  dikes. 

It  is  readily  apparent  that  the  dikes  of  Cohasset  and  Scitnate 
exhibit  a  general  agreement  in  character  and  trend  with  those 
of  Nantasket.  No  dikes  of  melaphyr  have  been  certainly 
identified  ;  but  in  their  place,  as  previously  explained  (page 
105),  there  is  a  well-defined  series  of  porphyrite  dikes.  These 
are  clearly  older  than  all  of  the  diabase  dikes  and  probably  date 
from  the  Nantasket  flows  of  porphyrite.  Each  of  the  tlu'ee 
systems  of  diabase  dikes  is  clearly  represented,  and  the  ex- 
posed intersections  show  that  their  relative  ages  are  essentially 
unchanged.  But  the  oldest  system,  having  the  normal  trend 
N.  75°  to  80°  E.,  largely  predominates,  though,  as  at  Nantas- 
ket, it  cannot  always  be  clearly  distinguished  from  the  second 
system,  with  the  normal  trend  S.  75°  to  80°  E.  Only  a  few 
dikes  can  be  referred  to  the  second  system.  The  dikes  of  the 
newest  or  N.-S.  system  also  are  relatively  less  numerous  than 
in  the  Nantasket  area ;  and  it  is  especially  noteworth)'  that  they 
become  less  numerous  eastward,  not  a  single  clear  example 
having  been  observed  beyond  the  Cohasset  Rocks  or  moi-e  than 
three  fourths  of  a  mile  east  of  Green  Hill  J)ea<'h.  This 
apparent  limitation  of  the  newer  dikes  to  the  Nantasket  area 
and  its  inunediate  vicinity  is  an  inlcrcstiiig  and  suggestive  fact. 


127 


but  a  more  extended  study  would  be  required  to  discover  its 
full  significance.  An  occasional  dike  only,  such  as  92,  109, 
114,  136,  etc.,  is  distinctly  aberrant  or  ambiguous  in  trend; 
and  some  of  these  afford  intersections  determining  the  relative 
age  and  hence  the  system.  Although  faults  are,  perha[)s,  as 
numerous  in  Cohasset  as  in  Nantasket,  none  have  been 
definitely  located  ;  and  we  can  only  conjecture  that  their 
relations  to  the  dikes  are  unchanged.  The  dikes  are  often 
observed  to  coincide  with  prominent  joint-planes  in  the  granite  ; 
but  as  at  Nantasket,  the  joint-structure  is  evidently  mainly  of 
more  recent  origin  than  the  dikes. 

The   Porphyrite   Dikes. 


71 

72 
73 

74 

75 

76 

77 
78 
79 
80 

81 

82 
83 

84 
85 
86 

87 


N. 
N. 
N. 

N. 

N. 

N. 

N. 
N. 
N. 
N. 


5°E. 
30°  E. 
15°  E. 

5°E. 

,  18°  E. 

,  50E. 

,  40°  E. 

-S. 
-S. 
20°  E. 


N.-S. 


N. 
N. 

N. 
N. 
N. 


20°  E. 
20°  E. 

15°  E. 
15°  E. 
15°  E. 


N.  10°  E. 


Hade. 

Width 
in 
feet. 

E.  15°-20° 
E.  10° 
Vertical 

4 
3 

18 

Vertical 

2 

E.  5° 

8 

Vei'tical 

13-14 

N.W.10°-15° 
Vertical 
Vertical 
Vertical 

4-5 
4-6 
10-15 
8-10 

Vertical 

8-10 

Vertical 
Vertical 

1-1.5 
2-3 

Vertical 
Vertical 
Vertical 

6 

25-30 

3-4 

E.  10° 

8-8.5 

Remarks. 


In  granite  on  Green  Hill  Beach. 

75'  E.  of  71 ;  irregnlar. 

15'  E.  of  72;  imperfectlj'  ex- 
posed. 

250'  E.  of  73;  near  Black  Rock 
House. 

Near  Black  Rock  House ;  cut 
by  88. 

Behind  Black  Rock  House; 
cut  by  88  and  89. 

Branch  of  76 ;  cut  by  88  and  89. 

About  450'  E.  of  77  ;  cut  by  89. 

About  2550'  E.  of  Forest  Ave. ; 
cut  by  150. 

About  3000'  E.  of  Forest  Ave. ; 
cut  by  90  and  91. 

About  450' E.  of  81. 

6'  east  of  82 ;  probably  con- 
nected with  81. 

Near  83  ;  cut  bv  92  and  93. 

About  100'  E.  of  84:  cut  by  94. 

About  200'  E.  of  85;  also  cut 
by  94. 

Wesi}  end  of  Pleasant  Beach ; 
cut  by  97  and  98. 


The  dikes  of  this  series  have  not  been  observed  along  the 
entire  shore,  but  only  for  about  one  mile,  between  Green  Hill 
Beach  and  Pleasant  Beach  or  Walnut  Angle.  This  is  the 
comparatively  straight   and  eminently  rock-bound  part  of  the 


128 

shore  known  ns  Cohasset  Rocks.  Although  it  is  convenient  to 
class  all  the  dikes  of  the  above  list,  provisionally,  as  porpliy 
rite,  it  is  well  to  bear  in  mind  that  a  few  at  least  are  more 
basic  and  that  the  list  quite  possibly  includes  some  melaphyr 
dikes.  The  chief  points  of  interest  presented  by  the  porphy- 
rite  dikes  have  been  noted  in  connection  with  the  Black  Rock 
flow  ;  while  most  of  the  details  of  any  importance,  including 
especially  those  required  for  the  identification  of  the  individual 
dikes,  are  presented  in  the  table.  We  learn  from  the  table 
that  nine  of  the  seventeen  dikes  are  visibly  cut  by  the  oldest 
system  of  diabase  dikes.  On  account  of  the  approximate 
agreement  in  trend,  intersections  by  the  north-south  diabase 
dikes  Avere  scarcely  to  be  expected  ;  but,  fortunately,  one  such 
is  presented  ;  and  it  is,  perhaps,  the  prettiest  and  most  unique 
phenomenon  to  be  observed  among  all  the  dikes  of  this  district. 
The  relations  of  the  two  dikes  are  shown  in  Fig.  1(3.  The 
diabase  dike  (150)  advances  obliquely  from  the  south  until  it 
strikes  the  east  wall  of  the  porphyrite  dike  (80),  follows  this 
wall  for  25  feet,  amputating  a  branch  of  the  porphyrite  dike, 
and  then  passes  in  a  graceful  double  curve  diagonally  through 
the  latter  and  follows  the  west  wall  as  far  as  cither  can  be 
traced, — 24  feet.  The  intersecting  dike  is  a  typical  example  of 
the  third  system  of  diabase  dikes — black,  brownish-weathering, 
and  beautifully  cross-jointed.  This  porphyrite  dike  (80)  is  the 
one  aflfbrding  the   analysis   given    on   page   59.      On  the  north 


■rrvTTTTTTUX^ ,  "/%!/.+  +  V*  +*  +Yt+  +  +-^  +  »■  *■  +  »-*  +  +  +  *-- 
P"iG.   i6. — Plan  showinc  the  intersection  of  a  porimiy- 

RITE      dike      (So)       by      a      DlABASfC      DMvl':    (150) 
ON    THE    COIIASSET  RoCKS.      SCALE, 
I    INCH  =:  25     FEET. 


129 


side  of  Beach  Island,  near  Sandy  Beach,  is  a  small  exposure  of 
a  porphyrite  (?)  dike — N.  5°  E.,  4  feet,  vertical — which  is 
cut  off  by  a  fault. 

The  Diabase  Dikes. 
No  attempt  is  made  in  the  list  of  E.-W.  dikes  to  distinguish 
or  separate  the  first  and  second  systems  ;  but  in  both  this  and 
the  N.— S.  list  the  dikes  are  described  in  topographic  order, 
beginning  in  each  case  at  the  western  end  of  the  Cohasset  Rocks, 
near  Green  Hill  Beach  and  Forest  Avenue.  The  data  of  the 
lists  are  sufficient  for  the  identification  of  the  individual  dikes, 
if  taken  in  order;  but  in  the  brief  notes  which  follow  the  lists, 
some  additional  landmarks  and  bearings  are  oiven. 


East-West  Dikes. 


Width 

No. 

Trend. 

Hade. 

in 
feet. 

Remarks. 

88 

N. 

80°  E. 

S.  5° 

3.5 

Coarsely  porpliyritic;  cuts  75, 
76,  and  77. 

89 

N. 

80°  E. 

Vertical 

6.5-7 

Exposed  aljout  750' ;  cuts  76, 
77,  and  78. 

90 

N. 

72°  E. 

N.  5° 

25-30 

Branches  toward  east  end  :  cuts 
81;  cut  by  152-3-4. 

90a 

N. 

85°  E. 

N.  5° 

2-3 

A  northern  brancl;  of  90. 

91 

N. 

85°  E. 

S.5° 

1.5-2 

A  south,  brand)  of  90  ;  cuts  81. 

92 

E. 

-W. 

Vertical 

2-3 

Exposed  over  300';  probably 
unites  with  93. 

93 

N. 

85°  E. 

S.  5° 

2-3 

8'  S.  of  92;  92  and  93  cut  84. 

94 

N. 

80°  E. 

Vertical 

9 

Exposed  about  300' ;  cuts  85 
and  86 ;  mai>netic. 

95 

N. 

80°  E. 

S.5° 

3-3.5 

60'  S.  of  94;  cut  by  156. 

96 

N. 

80°  E. 

Vert.-S.  5° 

8 

75'  S.  of  95  :  parallel  with  flow- 
structure  of  granite. 

97 

N. 

80°  E. 

N. 15°-20° 

4-5 

125'  S.  of  96;  cuts  87. 

98 

N. 

80°  E. 

Vertical 

1-2 

50'  S.  of  97;  irreiiuhir;  cuts  87. 

99 

N. 

65°-70°  E. 

N.  20° 

2-4 

E.  end  Pleasant  Beach ;  cut  by 
100;  irregular. 

100 

N. 

80°  E. 

Vertical 

5 

E.  end  Pleasant  Beach ;  ex- 
posed long  distance. 

101 

N. 

75°  E. 

N.  5° 

1 

150'  S.  of  100. 

102 

N. 

80°  E. 

S.  5°-10° 

3-4 

About  250'  S.  of  101. 

103 

N. 

85°  E. 

25'  S.  of  102;  W.  end  of  Sandy 
Beach ;  irregular. 

104 

N. 

75°-80°  E. 

Vertical 

3-4 

N.  side  of  Beach  I. ;  probably 
cut  by  105  and  107. 

105 

N. 

85°  E. 

Vertical 

10 

N.  side  of  Beach  I. ;  exposed 
long  distance. 

OCCAS.  PAPEKS  B.  S.  N.  H.  IV.  9. 


130 


N.  70°-80°  E. 
N.  85°  E. 

N.  80°  E. 

E.-W. 

N.  80°  E. 
E.-W. 

N.  82°-90°  E. 
S.  75°  E. 

E.-W. 

N.  85°  E. 
N.  80°  E. 
N.  80°  E. 
N.  80°  E. 
N.  80°  E. 

N.  80°  E. 

N.  75°  E. 
S.  75°  E. 
N.  80°  E 
N.  70°  E. 
Irregular. 
N.  70°  E. 
N.  70°  E. 
N.  70°  E. 
N.  77°  E. 

N.  700-80°  E. 

N,  80°  E. 
N.  80°  E. 
N.  80°  E. 
N.  82°  E. 
N.  80°  E. 
E.-W. 

N.  70°  E. 

N.  75°  E. 

N.  82°  E. 
N.  80°  E. 
N.  80°  E. 

E.-W. 

N.  75°  E. 

N.  70°  E. 


Vertical 
Vertical 

4-5 
3.5 

S.  5° 

15 

N.  15°-20° 

Irreg'lar 

N.  10° 

1 

Vertical 

2.75 

Vertical 

5-6 

N.  15° 

5 

S.  5° 

N.  5° 
N.  10° 
N.  10° 
N.  10° 

N.  5° 

2.5 

5 

2 

1 
3 
1-1.25 

N.  5° 

1 

Vertical 
Vertical 
Vertical 
N.  15° 
Vertical 
Vertical 
Vertical 
N.  15° 
N.  10° 

8 

6-7 

6 

3.5 

10 

3-4 

9 

1.5 

6 

Vert.-N.  10° 

2-5 

N.  5° 
N.  5° 
Vertical 
Vert.-N.  3° 
N.  10° 
Irregular 

2.75 

11 

3-4 

4 

.5-.75 

2.5 

N.  3° 

3 

N.  10° 

.5-.75 

N.  15° 

N.5° 

Vertical 

3 

2.75-3 
9 

Vertical 
Vertical 

5.5-6 
2 

Vertical 

.5-3 

1 

70'  S.  of  105 ;  very  irregular. 

50'  S.  of  106  and  75'  N.  of  arti- 
flcial  harbor ;  faulted. 

S.    side   of    artiticial    harbor: 
branching. 

E.  side  of  Beach  I. ;  compos- 
ite and  branching. 

Cuts  111  obliquely,  both  longi- 
tudinally and  vertically. 

30'   N.  of  112;  interesting  in- 
tersection by  110. 

First    dike    N.    of    outlet    of 
Little  Harbor;  branching. 

First  dikeE.  of  outlet  of  Little 
Harbor. 

40'  N.  of  113.    Dikes  113  to  118 

18'N.  of  114.     are  all  on  shore 

150'  N.  of  116.  E.  of  the  outlet 

5'  N.  of  116.    of  Little  Harbor. 

25'N.  of  117. 

60'  N.  of  118;  150'  from  point; 
exposed  300'. 

Point  nearest  Quamino  Rock ; 
exposed  200'. 

N.  W.  shore  of  Sandy  Cove. 

Extreme  end  of  Hominy  Point. 

South  side  of  White  Head. 

]  In  coarse  gran- 

130'  S.  of  124.     ite  on   E.  side 

75' S.  of  125.     [-of      Deacon 

115'  S.  of  126.  I  Bourne's       Is- 

12'  S.  of  127.  J  land. 

On     Hog    Rock    in    Cohasset 
Harbor 

On  shore  W.  and  E.  of  Glades 
Hotel ;  coarselv  porphyritic. 

6'  N.  of  130  ;  near  Glades  Hotel. 

About  200'  N.  of  131 ;  in  diorite. 

About  90'  N.  of  132  ;  in  diorite. 

30'  N.  of  133  ;  in  coarse  granite. 

20'  N.  of  134  ;  in  coarse  granite. 

15'  S.    of    130;    E.    of'"Glades 
Hotel. 

30'    S.    of    131;    E.    of   Glades 
Hotel. 

70'  S.  of  135;  on   Strawberry 
Point. 

About  100' S.  of  138. 

About  25'  S.  of  139. 

About  175'  S.  of  140;  irregular, 
branching. 

About  75'  S.  of  141 ;  branching. 

Headland    E.    of     Strawberry 
Point. 

About   300'    S.  of  143;  in  old 
quarry. 


131 


145 

N.  70°  E. 

Vertical 

1-1.25 

About  100'  S.  of  144,  in  quarry  ; 
porpliyritic. 

146 

N.  75°  E. 

N.  5° 

1-1.5 

Separated  by  short  beach  from 
145. 

147 

N.  75°  E. 

N.  5° 

2-3 

Near  140. 

148 

N.  55°  E. 

N.  30° 

8 

150'  S.  of  147;  coarsely  por- 
phyritic  and  branching. 

As  a  rule,  the  east-west  dikes  cut  the  shore  ledges  very 
obliquely  ;  and  in  going  eastward  we  cross  the  first  system 
from  north  to  south,  and  the  second  system  from  south  to 
north.  The  outcrops  begin  in  the  rear  of  the  Black  Rock 
House,  where  88  is  readily  recognized  by  its  coarsely  and 
distinctly  porpliyritic    character.      About  50  feet  south  of  88  is 

89,  a  splendid  dike,  which  can  be  traced  almost  continuously 
for  fully  750  feet.  About  midway  of  the  outcrop  it  is  some- 
what irregular  and  split  up  ;  and  the  well-preserved  glacial 
striae  upon  it,  and  especially  the  freshness  of  an  inscription  bear- 
ing the  date  1852,  testify  to  the  stable  and  resistant  character 
of  those  diabase  dikes  which  have,  by  deep-seated  alteration, 
been  largely  changed  to  such  almost  indestructible  species  as 
epidote,  chlorite,  magnetite,  and  quartz.  A  little  more  than 
half   a    mile    (2860    feet)    from    Forest  Avenue  brings  us  to 

90,  the  largest  of  all  the  east-west  dikes  ;  and  its  outcrop  is 
quite  a  center  of  eruption  (Fig.  17).  It  cuts  one  of  the 
porphyrite  dikes  (81)  and  is  cut  by  no  less  than  four  of  the 
north-south  diabase  dikes.  The  small  dikes  (90a  and  91)  are 
probably  branches  of  90.  The  magnetic  dike  (94)  is  so 
highly  charged  with  magnetite  as  to  make  the  compass  observa- 
tions quite  unreliable.  It  not  only  cuts  85  and  86,  but  just 
before  entering  the  sea  is  cut  by  156  of  the  third  system,  which 
also  cuts  95.  The  next  three  dikes  of  this  series  (96,  97,  98) 
follow  in  reo-ular  order  and  brino-  us  to  Walnut  Angle  or  the 
eastern  end  of  Cohasset  Rocks.  No  dikes  have  been  observed 
in  the  ledges  on  Pleasant  Beach  ;  but  near  its  eastern  end  we 
strike  100,  which  can  be  traced  the  entire  length  of  the  head- 
land, trending  just  north  of  the  extreme  north  end   of  Brush 


132 

Island.  In  like  manner,  103,  on  the  east  side  of  this  headland, 
is  exactly  in  line  with  a  large  dike  near  the  southern  end  of 
Brush  Island. 


ISi 


I5i 


Fig.     17. — Plan  showing  a  group  of   intersecting  dikes 
on  the   cohasset  rocks-     scale,   i  inch  =50  feet. 

Crossing  Sandy  Beach,  we  come  at  once,  on  the  north  side 
of  Beach  Island,  to  104  and  105,  which  can  be  traced  for  long- 
distances,  the  latter  enclosing,  near  the  eastern  shore,  a  long 
narrow  mass  of  granite.  The  second  dike  beyond  (107)  is 
slipped  about  twice  its  width.  The  large  dike  south  of  the 
artificial  harbor  divides  eastward  into  two  unequal  branches 
separated  by  from  8  to  10  feet  of  granite  ;  and  15  feet  north  of 
it  is  a  one-foot  branch.  The  composite  dike  (109)  on  the  east 
side  of  Beach  Island  is  near  the  contact  of  the  coarse  typical 
granite  and  the  mixed  OTanite  and  diorite.  The  intersection  of 
111  by  110  (Fig.  18)  is  a  clear  case  of  the  confusion  of  the 
first  and  second  systems.  The  intersection  is  very  oblique, 
extending  over  20  feet  along  111.  Immediately  north  of  110 
is  an  unenumerated  six-inch  dike.  The  next  dike  (112)  is  a 
fine  and  instructive  example  of  a  branching  fissure,  and  shows 
flow-structure  parallel  with  the  walls. 

The  dike  on  Hog  Rock  (129)  is  exposed,  again  in  the 
re-entrant  angle  of  the  shore  south  of  Strawberry  Point.  The 
next  dike  (130)  is  beautifully  porphyritic,  with  large  clustered 


133 


Fig.  iS. —  Plan  showing  the   intersection    of   two 

DIKES    on    the    east    SIDE    OF    BeACH    IsLAND. 

Scale,  i  inch  =  8  feet. 

crystals.  It  outcrops  west  of  the  hotel,  east  of  the  hotel,  and 
then,  after  some  faulting  and  branching,  on  the  east  side  of 
Strawberry  Point.  The  remaining  dikes  on  Strawberry  Point 
(131-142)  are  easily  found  and  identified  by  taking  the  ledges 
in  order  from  west  to  east,  one  of  them  (135)  cropping  on  three 
distinct  ledges. 

North- South    Dikes. 


149 

149« 

150 

151 

152 
153 
154 
155 

156 

157 


Trend. 


N.  10°  W. 


,-S. 

50-10°  E. 

.-S. 

.-S. 
.-S. 
,-S. 
8°  W. 

5°-10°  E. 

15°-45°  E. 


Hade. 

Width 

in 

feet. 

Vertical 

3-4 

Vertical 

Vertical 

2-3 

Vertical 

3-4 

Vertical 
Vertical 
Vertical 
Vertical 

3-6 
6-7 
7.5 
3-3.5 

Vertical 

2 

S.  E.30°-45o 

2-8 

Remarks. 


On  Cohasset  Rocks,  400'  E.  of 

Blaclv  Eock  House. 
In  tlie  valley  immediately  E. 

of  the  Black  Rock  House ; 

imperfectly  exposed. 
On   Coliasset   Rocks ;    cuts   80 

obliquely. 
Interrupted   and  faulted;  cuts 

90. 
Cuts  90 ;  irregular. 
Cuts  90 ;  unites  witli  152. 
Cuts  90;  brandling  northward. 
About  200'  E.  of  154,  on  Cohas- 
set Rocks. 
On   Cohasset  Rocks;    cuts  94 

and  95. 
On  Hominy  Point,  near  White 

Head. 


On  the  short,  stony  beach  immediately  east  of  the  Black  Rock 
House  there  are  traces  of  several  —  at  least  three  and  possibly 
more — dikes  (149a)  of  this  system,  recalling  the  composite  dike 
of  the  Nantasket  area  (66).     The  last  of  the  north-south  dikes 


134 


(157)  is  the  only  really  doubtful  one.  This  is  the  dike  shown 
in  Fig.  19.  In  tracing  it  across  the  ledge  to  the  mainland,  its 
easterly  trend,  hade,  and  thickness  are  all  greatly  increased  ; 
and  it  would,  perhaps,  be  best  to  class  it  as  a  highly  aberrant 
member  of  the  first  system. 


Fig.  19. — Dike  157  cutting  a  ledge  of   granite,  near  White 
Head,  on  Hominy  Point,  Cohasset. 

SEQUENCE       OF       EVENTS       EECORDED       IN       THE       NANTASKET 

LEDGES. 

It  is  unnecessary  to  review  or  summarize  farther  the  evidence 
supporting  the  conclusions  that  have  been  reached  in  tlie  preced- 
ing pages  concerning  the  succession  of  the  beds  of  conglomerate 
and  the  flows  of  lava ;  but  we  may  fitly  close  this  section  with 
a  general  statement  of  the  geological  history  of  Nantasket  so 
far  as  it  is  recorded  in  the  hard  rocks.  The  granite,  Avitli  the 
associated  diorite  and  felsite,  is  the  fundamental  rock  in  tliis 
part  of  the  Boston  Basin  ;  and  it  had,  a[)parently,  been 
exposed  to  erosion  for  a  long  time  before  the  basal  conglom- 
erate was  deposited  over  it,     The  character  of  this  conglomerate 


135 

is,  however,  a  sufficient  indication  that  outflows  of  lava  and, 
very  probably,  the  deposition  of  conglomerate  had  already 
begun  in  the  deeper  parts  of  the  basin  ;  and  we  may  fairly 
suppose  that  both  conglomerates  and  lavas  older  than  any 
exposed  in  southern  Nantasket  underlie  at  a  great  depth  the 
northern  part  of  the  peninsula.  Eventually  the  slow  subsidence 
of  the  land  then  in  progress  carried  the  shore-line  over  and 
beyond  the  Nantasket  area,  and  the  so-called  basal  conglom- 
erate was  formed  upon  the  uneven  and  fissured  surface  of  the 
granite.  This  conglomerate  had  attained  a  maximum  thickness 
of  perhaps  50  feet,  when  the  first  flow  of  melaphyr  was  spread 
over  it.  This  was  probably  originally  an  ordinary  compact, 
black  lava  ;  but  through  subsequent  alterations  it  has  become 
greenish  and  purplish,  compact  and  jaspery.  The  lava-flow 
being  submarine,  the  sedimentary  process  was  uninterrupted  ; 
and  the  uneven  and  scoriaceous  upper  surface  of  the  melaphyr 
was  slowly  covered  by  the  second  conglomerate,  which  is 
largely  composed  of  debris  derived  from  the  melaphyr,  and  like 
this  melaphyr  is  especially  characterized  by  the  segregations  of 
red  jasper.  When  the  second  conglomerate  had  attained 
approximately  the  same  thickness  as  the  basal  conglomerate, 
a  flood  of  more  acid  lava  (porphyrite)  was  spread  over  the  sea- 
bottom  to  a  depth  of  from  50  to  possibly  more  than  100  feet ;  and 
all  the  porphyrite  in  the  Nantasket  area  probably  belongs  to 
what  was  once  one  continuous  flow,  increasing  in  thickness 
eastward,  or  toward  the  vent  from  which  it  issued.  Over 
the  porphyrite  was  gradually  accumulated  the  third  conglom- 
erate to  a  thickness  of  from  50  to  100  feet  or  more,  but  termi- 
nated at  last  by  a  comparatively  thin  flow  of  highly  basic  and 
vesicular  basaltic  lava,  which  is  recognized  now  as  a  typical 
green  and  amygdaloidal  melaphyr.  The  fourth  conglomerate, 
with  a  thickness  of  from  20  to  30  feet,  separates  this  second 
melaphyr  from  the  very  similar  third  melaphyr.  The  latter 
consists,  however,  of  two  flows,  having  an  aggregate  thickness  of 
from  40  to  50  feet.     Once  more  the  beach  conditions  prevailed, 


136 

and  the  fifth  conglomerate  was  formed.  When  the  volcanic 
activity  was  again  renewed  over  this  area,  the  eruptions  were, 
for  the  first  time,  partly  of  an  explosive  character,  forming,  as 
the  Atlantic  Hill  section  shows,  beds  of  tuff  alternating  Avith 
beds  or  flows  of  compact  and  brecciated  melaphyr.  This  marks 
the  culmination  of  the  volcanic  energy,  flow  succeeding  flow, 
until  they  attained  an  aggregate  thickness  of  several  hundred, 
possibly  five  hundred,  feet.  This  series  of  eruptions  completes 
the  conglomerate  and  melaphyr  series  as  now  developed  in  the 
Nantasket  ledges  ;  but  on  the  extremity  of  Rocky  Neck  we  find 
evidence  that  the  great  melaphyr  was  finally  covered  by  a 
sixth  bed  of  conglomerate,  and  that  by  a  flow  of  green,  amyg- 
daloidal  melaphyr  resembling  the  second  and  third  sheets  of 
that  rock. 

The  volcanic  energy  finally  died  out ;  and  these  alternating 
sheets  of  conglomerate,  melaphyr,  and  porphyrite  were  probably 
covered  by  a  great  thickness  of  conglomerate  and  sandstone 
without  interbedded  lavas.  But  of  this  upper  conglomerate 
series  there  are  now  no  visible  traces  in  the  Nantasket  Penin- 
sula, although  Harding's  Ledge,  as  already  explained,  aftbrds 
some  evidence  that  it  underlies  the  middle  part  of  the  beach,  in 
the  vicinity  of  Strawberry  Hill.  As  the  subsidence  progressed 
and  the  water  became  deeper  and  the  shore  more  remote,  the 
deposition  of  the  coarser  fragmental  rocks  over  this  area 
gradually  ceased,  the  conglomerate  changing  through  sandstone 
to  slate,  which,  we  may  fairly  suppose,  underlies  almost  the 
entire  peninsula  north  of  Atlantic  Hill,  and  has  a  thickness  of 
several  hundred,  possibly  a  thousand,  feet.  The  deposition  of 
the  slate  probably  occupied  a  much  longer  time  than  that  of 
the  conglomerate  series  ;  but  it  was  finally  terminated  by  the 
period  of  disturbance  during  which  the  sediments  of  the  Boston 
Basin  were  strongly  folded,  faulted,  and  elevated  to  form  dry 
land.  It  is  to  this  geological  revolution  that  we  owe  nearly  all 
those  structural  complexities  which  make  the  stratigraphy  of 
the  Boston    Basin  such   a  difficult  problem.       This  was   also 


137 

probably  a  period  of  intense  igneous  activity,  tlie  nunieroiis 
dikes  of  diabase  traversing  the  ledges  of  Nantasket  and  other 
parts  of  the  Boston  Basin  appearing  to  date  from  this  time. 
The  dikes  were  probably,  in  many  cases,  the  feeders  or  channels 
of  supply  of  effusive  eruptions  ;  but  these  surface  lavas  of  every 
form,  not  being  protected  by  later  sediments,  have,  at  least  in 
in  the  Nantasket  district,  been  long  since  completely  swept 
away  by  the  agents  of  erosion,  which  daring  all  subsequent 
time  have  worked  unceasingly  and  so  efficiently  that  not  only 
have  these  ancient  volcanoes  been  destroyed  but  the  very 
foundations  on  which  they  stood.  In  southern  Nantasket  the 
entire  thickness  of  the  slate  series  and  probably,  on  the  average, 
half  of  the  conglomerate  series  have  been  removed  ;  and  over 
broad  areas  erosion  has  exposed  the  original  granite  floor  or  cut 
deeply  into  it. 

AGE  OF  THE  NANTASKET  ROCK8. 

Up  to  the  present  time  no  fossils  have  been  found  in  the  Nan- 
tasket ledges.  It  is,  of  course,  improbable  that  any  ever  will  be 
found  in  the  conglomerate  and  volcanic  series  ;  but  we  may 
reasonably  entertain  the  hope  that  the  slate  ledges  will  yet  afford 
us  some  clue  to  their  geological  age.  Fragments  of  a  some- 
what calcareous  slate  have  been  observed  in  the  drift  of  Straw- 
berry Hill,  and  suggest  the  possibility  that  beds  of  impure 
limestone  (of  organic  origin,  if  not  actually  fossiliferous)  underlie 
the  northern  end  of  the  peninsula.  It  seems  impossible  to  feel 
quite  sure  that  the  hard  massive  slate  on  the  railroad,  northwest 
of  Rockland  Hill,  which  has  been  referred  provisionally  to  the 
bed  of  tuff  outcropping  at  the  base  of  Atlantic  Hill,  is  not  really 
an  isolated  exposure  of  the  Cambrian  slate  of  Braintree  and 
Weymouth.  The  Weymouth  beds  are  now  referred  by 
Walcott,  provisionally  at  least,  to  the  Lower  Cambrian  ;  and 
the  Braintree  beds  to  the  Middle  Cambrian^  The  granite  and 
diorite  are  undoubtedly  here,  as  elsewhere  in  the  Boston 
'  Tenth  Ann.  Keport  U.S.  Geol.  Survey,  p.  567. 


138 

Basin,  younger  than  these  Cambrian  strata  ;  and  it  is  perfectly 
clear  that  the  conglomerate  and  melaphyr  series  is  newer  than 
the  granite,  and  therefore  distinctly  more  recent  than  the 
Middle  Cambrian.  They  are  quite  certainly  Paleozoic,  but 
whether  as  late  as  the  Carboniferous  strata  in  the  Narrasfansett 
Basin  is  at  least  doubtful.  The  conglomerate  and  melaphyr 
and  newer  slates  of  the  Nantasket  area  may  be  safely  correlated 
with  the  same  rocks  in  other  parts  of  the  Boston  Basin  ;  and  it 
is  hoped  that,  before  the  revision  of  the  geolog}'^  of  the  basin 
is  completed,  evidence  will  be  forthcoming  which  will  finally 
and  definitely  settle  the  question  as  to  the  age  of  these  strata  ; 
but  for  the  present  the  problem  must  be  regarded  as  unsolved. 
In  January  of  this  year  (1892),  Mr.  T.  A.  Watson  found 
on  Pleasant  Beach,  Cohasset,  a  smoothly  rounded  and  evi- 
dently water-worn  bowlder,  between  five  and  six  inches  in 
diameter,  of  a  highly  fossiliferous,  compact,  red  (ferruginous) 
limestone.  The  fossils  appear  to  belong  wholl}^  to  two  species, 
which  have  been  identified  by  Mr.  C.  D.  Walcott^  as  the  two 
Lower  Cambrian  types  StraparolUna  remota  Billings  and 
Hyolithes  communis  Billings.  Although  Mr.  Watson  was 
unable,  after  the  most  thorough  search,  to  find  a  second  speci- 
men of  this  limestone  on  the  Cohasset  shore,  its  Cambrian  age 
made  it  seem  highly  probable  that  it  had  been  derived  from 
some  point  within  the  Boston  Basin,  and  possibly  within  the 
Nantasket  and  Cohasset  district,  thus  encouraging  the  hope 
that  it  would  yet  aiFord  us  the  desired  clue  to  the  geological  age 
of  the  Nantasket  strata.  But  this  hope  has  been  dispelled  ;  for 
Mr.  Watson  has  recentl}^  found  several  water-worn  fragments 
of  precisely  the  same  kind  of  limestone,  holding  the  same  fossils, 
but  not  quite  so  abundantly,  on  the  beach  at  Bass  Point, 
Nahant.  We  can  no  longer  doubt  that  tliis  rock  is  a  part  of 
the  Cambrian  limestone  of  Nahant,  although  representing  a 
more  ferruginous  and  more  conspicuously  fossiliferous  bed  than 
any  now  exposed  on  that  peninsula  ;  and  the  Cohasset  speci- 
men must  be  regarded  as  a  solitary  glacial  erratic. 

1  Proc.  Biological  Society  of  Wasliiiigton,  VII.,  155. 


THE  SURFACE  GEOLOGY  OF  NANTASKET  AND 
COHASSET. 

As  a  starting  point  for  the  surface  geology  of  this  district, 
including  both  its  glacial  and  post-glacial  history,  we  must 
accept  the  preglacial  peneplain,  the  evidence  for  which  has 
been  presented  in  sufficient  detail  in  the  general  description  of 
the  topography  (pages  5  to  7).  The  Mesozoic  and  Cenozoic 
eras  are,  apparently,  unrecorded  here,  except  in  the  erosion  of 
the  hard  rocks.  But,  while  it  is  to  these  lono;  ireoloo-ical 
cycles  that  we  must  refer  the  removal  of  a  great  thickness  of 
stratified  rocks  from  this  area,  and  the  wearing  down  to  the 
base-level  of  tlie  hard  granitic  rocks,  lavas,  and  conglomerates, 
the  rugged  contours  presented  today  by  the  preglacial  pene- 
plain undoubtedly  date  chiefly  from  the  marked  elevation  of 
the  land  which,  it  is  probable,  ushered  in  the  great  ice-age. 
On  account  of  the  intense  hardness  of  the  rocks,  these  deeply 
incised  lines  were  only  partially  effaced  by  the  powerful  abra- 
sive action  of  the  ice-sheet :  but,  as  we  have  already  seen,  they 
are  to  a  large  extent  obscured  or  concealed  by  the  non-lithified 
deposits  of  glacial  and  post-glacial  origin. 

The  principal  events  recorded  in  the  surface  geology,  since 
the  development  of  the  preglacial  peneplain,  are  :  (1)  A  con- 
siderable elevation  of  the  land  at  the  beoinnins;  of  the  o^lacial 
epoch.  The  elevation  was,  probably,  a  principal  cause  of  the 
subsequent  glaciation,  and  is  proved  by  the  deeply  eroded 
character  of  the  peneplain.  (2)  Long-continued  glaciation, 
during  which  the  divided  peneplain  was  strongly  eroded  and 
the  ground-moraine  or  bowlder-clay  accumulated  irregularly 
upon  it,  chiefly,  at  the  last,  in  the  form  of  drumlins.  (3) 
A  marked  depression  of  the  land,  accompanied  by  the  final 
melting  and  retreat  of  the  ice-sheet,  and  the  accumulation, 
either  in  the  sea  or  in  temporary  lakes  and  streams,  of  the  ex- 


140 

tensive  deposits  of  modified  drift.  (4)  A  slight  re-elevation 
of  the  land  immediately  after  the  disappearance  of  the  ice 
and,  probably,  during  the  formation  of  the  modified  drift. 
The  elevation  is  proved  in  other  parts  of  the  Boston  Basin  by 
the  occurrence  of  clay  beds  above  sea-level,  and  generally,  it  is 
believed,  by  cut  terraces  on  the  drumlins  and  marine  deltas  of 
sand  and  gravel.  (5)  The  development  of  the  modern  shore, 
including  the  growth  of  the  beaches  and  marshes,  accompanied 
by  a  slow  movement  of  subsidence. 

There  remains  now,  in  order  to  complete  this  paper,  simply 
the  task  of  setting  forth  more  fully  the  tangible  and  material 
facts  upon  which  the  foregoing  statements  rest,  taking  up  the 
topics,  so  far  as  possible,  in  the  chronological  order. 

DEUMLINS,    GLACIAL    STRIAE,    AND    BOWLDEES. 

The  unmodified  drift  or  till  of  this  district,  so  far  as  it  now 
exists  above  the  sea  and  is  not  covered  by  the  modified  drift, 
occurs  almost  wholly  in  more  or  less  typical  drumlins.  The 
positions,  outlines,  heights,  and  names  of  these  are  indicated  on 
the  general  map,  so  far  as  they  have  been  determined. 

Rockland  Hill,  however,  is  only  an  incipient  drumlin,  and 
might  be  more  properly  classed  as  a  lenticular  slope  of  till. 
Telegraph  Hill,  World's  End,  Planter's  and  Pine  Hills,  Tur- 
key Hill,  Bear  Plill,  and  others  are  examples  of  composite 
drumlins  ;  what  were  originally  separate  accumulations  of  till 
becoming  more  or  less  perfectly  united  by  their  continued 
growth. 

As  the  map  shows,  the  longer  diameters  of  the  drumlins 
have  without  exception  a  strong  easterly  trend,  varying 
between  south-southeast  in  the  southern  and  east-southeast  in 
the  northern  part  of  the  district,  and  agreeing  closely  at  all 
points  with  the  glacial  striae  on  the  ledges. 

The  following  observatious  on  the  directions  of  the  striae  em- 
brace the  extreme  range  and  are  sufficiently  numerous   to  show 


141 

the  normal  courses.  It  is  very  obvious  tli;it  in  cro.ssint^  what 
is  now  the  Nnntasket  Peninsula  the  ice  was  stroni>lv  infiucnccd 
by  the  eastward  trend  of  the  main  valley  of  the  Boston  Basin, 
being  held  to  this  course  partly,  perhaps,  by  the  eastward  slo[)e 
of  the  ground  and  the  proximity  of  the  sea  in  that  direction, 
and  partly  by  the  continuous  barrier  of  granite  200  feet  or 
more  in  height  presented  by  the  south  shore  of  the  harbor. 
But  when  the  ice  had  escaped  from  this  influence  by  flowino- 
out  of  the  harbor  into  the  bay,  or  by  scaling  the  ledges  to  the 
peneplain  of  Cohasset,  it  resumed  more  nearly  its  normal 
south-southeast  trend,  as  shown  by  the  drumlins  and  the  glacial 
striae  along  the  Cohasset  shore. 

Directions  of  Glacial  Str-iae. 

Slate  ledge  south  of  Thonibusli  Hill S.  720-75°  E. 

Northeast  base  of  Atlantic  Hill S.  43°  E. 

Double  dike  on  Little  Gun  Rock S.  37°-47°  E. 

Dike  13  in  Green  Hill  Ledge S.  80°'E  . 

Slate  on  the  railroad  south  of  Nantasket  Station.     .     .  S.  3o°-40°  E 

East  side  of  the  bay,  near  the  railroad S.  35°-40°E. 

North  base  of  East  Porphyrite  Hill S.  47°-50°E. 

West  end  of  Granite  Plateau S.  530-57°  E. 

East  side  of  Rocky  Neck S.  43°  E. 

West  side  of  Rocky  Neck S.  41°  E. 

South  side  of  Weir  River  Bay S.  40°  E. 

Jerusalem  Road,  south  of  Straits'  Pond S.  36°  E. 

"      near  Green  Hill  Beach S.  80°  E. 

Cedar  Street,  Cohasset. S.  30°-35"E. 

West  side  of  Little  Harbor S.  22°-25°E. 

Cohasset  Rocks,  east  of  GreenHill  Beach S.  70°  E. 

"  "       one  fourth  mile  east  of  beach.        .     .  S.  46°  E. 

"  "       near  Pleasant  Beach S.  40°  E. 

Beach  Island,  near  the  artificial  harbor S.  32°-S5°  E. 

"      near  Pleasant  Beach S.  25°-30°E. 

Strawberry  Point,  Scituate  shore S.  2.5°E. 

Scituate  shore,  south  of  Strawberry  Point S.  25°E. 

Doubtless  the  most  interesting  feature  presented  by  the 
composition  of  the  till  is  the  occurrence  of  fossil  shells  in  some  of 
the  drumlins.  Mr.  Warren  Upham  has  noted^  the  finding  of 
fragments  of  shells  in  the  sea-cliffs  at  several  points  about 
Boston    Harbor,     including    the    northern     member     of   Tele- 

'  Proc.  B.  S.  N.  H.,  XXIV.,  127-14L 


142 


graph'  Hill  and  Sagamore  Head  in  the  Nantasket  area.  He 
has  also  called  attention  to  the  fact  that,  so  long  ago  as  the 
Revolutionary  War,  a  fort  was  built  on  the  top  of  Telegraph 
Hill,  and  a  well  was  dug  inside  the  fort  of  which  the  commander. 
Gen.  Benjamin  Lincoln,  wrote  as  follows^ :  "There  is  a  large  fort 
on  the  E.  Hill,  in  which  there  is  a  well  sunk  90  feet,  which 
commonly  contains  80  odd  feet  of  water.  In  digging  the  well, 
the  workmen  found  many  shells,  smooth  stones  and  different 
stratas  of  sand  and  clay  similar  to  those  on  the  beach  adjoining 
to  the  hill.  These  shells  and  appearances  were  discovered 
from  near  the  top  of  the  ground  to  the  bottom  of  the  well." 
Of  more  than  thirty  species  of  shells  collected  by  different 
observers  from  the  till  of  the  Boston  Basin,  Mr.  Upham  has, 
in  a  hasty  examination,  found  but  three —  V^enus  mercenai'ia 
(the  round  clam  or  quahaug) ,  Cyclocardia  borealis,  a.ndClioua 
sulpJmrea  (a  boring  sponge) — in  the  section  at  the  north  end  of 
Telegraph  Hill;  and  traces  of  the  round  clam,  which  is  by  far 
the  most  abundant  species  in  all  the  sections,  in  Sagamore 
Head.  More  recently,  by  careful  searching,  with  the  assistance 
of  Mr.  H.  D.  Card,  I  have  found  the  shells  in  the  eastern 
scarps  of  Point  Allerton  Great  Hill,  and  Strawberry  Hill ;  and 
have  extended  the  list  of  fossils  known  to  occur  in  the  till  of  the 
Nantasket  Peninsula  to  eleven  species,   as  follows  : — 

List  of  Species  in  the  Nantasket  Drimilins. 


Species. 

Telegraph 

Point 

Strawberry 

Hill. 

Allerton. 

Hill. 

Balanus  {sp.  f). 

* 

* 

Tritia  trivittata,  Adams. 

* 

Ilyanassa  obsoleta,  Stimp. 

* 

Crucihtilum  striatum.  Say. 

* 

Buccinum  undatum,  Linnfi. 

* 

Mya  arenaria,  Linng. 

* 

Venus  mercenaria,  Linne. 

* 

* 

* 

Cyclocardia  borealis,  Conrad. 

* 

* 

* 

Astarte  undata,  Gould. 

* 

* 

* 

Scapharca  transversa,  Say. 

* 

* 

Cliona  sxdphurea,  Verrill. 

* 

* 

* 

1  Geographical  Gazetteer  of  the  Towns  in  the  Commonwealth  of  Massachusetts,  1875, 
p.  56.     (Only  a  small  part  of  this  work  was  published.) 


143 

I  have  failed  to  find  even  the  slightest  truce  of  shells  in  :iiiv 
of  the  druinlins  south  of  Nantasket  Beach.  This  is  an  instance, 
however,  where  negative  evidence  is  of  very  little  value,  the 
apparent  presence  or  absence  of  shells  depending  very  largely 
upon  the  character  of  the  exposure.  A  deep  section  is  usually 
essential ;  for,  as  Mr.  Upham  has  explained,  the  shells  are  very 
likely  to  have  been  dissolved  out  of  the  upper,  oxidized  or 
weathered  layer  of  the  till.  It  is  probably  for  this  reason  that 
they  are  not  found  in  the  shallow  sections  afforded  by  Point 
AUerton  Little  Hill,  Little  Hog  Island,  White  Head,  World's 
End,  and  Planter's  Hill,  as  well  as  all  the  inland  drumlins. 
We  may  reasonably  hope,  however,  that  shells  will  yet  be  found 
in  Green  Hill  and  on  Bumkin  Island.  That  shells  actually 
occur  in  the  drumlins  of  Cohasset  we  have  satisfactory  evidence 
in  the  fact,  to  which  my  attention  has  been  called  by  Mr.  H. 
W.  Nichols,  that  fragments  of  the  round  clam  were  found  by 
Mr.  Titus  Burbank  some  years  ago  in  digging  a  well  near  the 
summit  of  James  Hill,  northeast  of  Scituate  Pond.  The  well 
is  45  feet  deep  :  and  the  shells  were  observed  only  near  the 
bottom.  They  cannot,  therefore,  be  referi'ed  to  any  post- 
glacial source. 

Although  shells  are  now  so  generally  wanting  in  the  buff  or 
oxidized  till,  evidence  of  their  former  existence  is  afforded  by 
the  calcareous  material  which  may,  in  certain  cases,  be  detected 
in  the  till.  This  evidence  is  especially  clear  in  the  buff  till  of 
Telegraph  Hill.  The  carbonate  of  lime  has,  in  part  at  least, 
been  dissolved  and  segregated,  locally  but  firmly  cementing 
the  till,  and  forming  in  the  finer  parts  regularly  rounded  concre- 
tions, from  a  fraction  of  an  inch  to  several  inches  in  diameter, 
which  are  sometimes  attached  to  the  flat  surfaces  of  the  large 
stones  or  bowlders.  These  nodules  effervesce  freely  with  acid, 
and  no  other  source  of  the  carbonate  than  the  shells  is  apparent. 

Mr.  Upham  has  correctly  explained  the  shells  in  the  till, 
which  are  always  in  a  fragmentary  form,  as  having  inhabited 
the  bottom  of  Boston  Harbor  before  the  coming  of  the  ice-sheet, 


144 

by  which  they  were  gathered  up  and  incorporated  in  the  till ; 
and  he  has  shown  that  some  of  the  species,  at  least,  like  the 
round  clam,  are  now  found  only  in  more  southern  waters, 
indicating  that  the  sea  in  this  vicinity  just  before  the  glacial 
epoch  was  warmer  than  at  the  present  time.  The  shells  are 
also  of  special  interest  as  proving  not  only  that  Boston  Harbor 
was  in  existence  in  preglacial  times,  for  otherwise  we  could  not 
know  but  that  this  basin  was  above  sea-level  before  as  well  as 
during  lis  occupation  by  the  ice-sheet ;  but  they  also  show  that 
the  harbor  has  now  approximately  its  preglacial  outlines  and 
hence  that  the  sea  has  regained,  on  this  coast,  very  nearly  its 
preglacial  level. 

The  peneplain,  as  already  stated,  and  especially  when  we 
make  an  allowance  for  the  glacial  erosion  upon  its  surface, 
proves  that  this  coast  is  now  one  hundred  feet  or  more  above 
its  preglacial  level.  But  this  greater  elevation  is  off-set,  so 
far  as  the  harbor  is  concerned,  by  the  excessive  erosion  which, 
being  a  valley  in  comparatively  soft  rocks,  it  suffered  in  glacial 
and  especially  in  immediately  preglacial  times. 

Although  the  fragments  of  shells  are  unquestionably  the  most 
interesting  feature  of  the  till  observed  in  Nantasket  and  Cohasset, 
other  components  of  more  normal  character — the  ordinary 
stones  and  bowlders — demand  brief  mention.  No  particularly 
striking  instances  of  glacial  transportation  have  been  noted. 
The  till  of  the  Nantasket  Peninsula,  as  may  be  most  readily 
seen  in  the  marine  sections  of  the  drumlins,  consists  very 
largely  of  fragments  of  slate,  which  is  undoubtedly  the  under- 
lying rock  not  only  of  this  })eninsula  but  of  the  main  part  of 
the  harbor  to  the  northward.  Bowlders  of  conglomerate  are 
not  wholly  wanting  nortli  of  Atlantic  Hill,  but  they  are  so  few 
and  small  as  to  prove  tiiat  it  can  occur  only  very  s[)aringly,  if 
at  all,  in  the  harbor  area  or  beneath  the  extensive  drift-deposits 
north  of  the  harbor.  In  fact,  pretty  much  all  of  the  congloni- 
ei-ate  in  the  till  north  of  the  Nantasket  ledges  might  be  referred 
with  considci-able   probability   to   the    known  exposures  of   that 


M5 

rock  in  Mcdford,  on  the  extreme  northern  niar<rin  of  the  l>oston 
Basin,  or  at  least  to  the  probabk;  eastward  extension  of  those 
beds  beneatli  the  drift ;  although  the  strong  easterly  trend  of 
the  glacial  movement  in  the  harbor  allows  us  to  regard  the 
])roniinent  conglomerate  ledges  in  the  central  part  of  the  basin 
as  a  possible  source  of  some  of  the  conglomerate  erratics  in  the 
Nantasket  Peninsula  and  Cohasset.  In  the  Nantasket  drunilins, 
granitic  rocks,  felsites,  quartzite,  etc.,  froiu  the  high  land 
north  of  the  Boston  Basin,  form  a  small  but  very  obvious  or 
noticeable  fraction  of  the  till,  and  in  some  cases  exhibit  lith- 
ological  peculiarities  enabling  us  to  refer  them  to  the  parent 
ledges,  as  in  the  case  of  the  bright  red  felsite  from  Saugus 
Centre.  A  portion  of  what  miglit  be  mistaken  for  conglom- 
erate is  brecciated  felsite,  and  another  portion  is  brecciated  or 
pebbly  slate.  Over  the  granitic  area  of  Cohasset  and  Scituate, 
as  a  matter  of  course,  the  crystalline  rocks  predominate  in  the 
till,  and  the  slate,  which  does  not  bear  glacial  transportation 
well,  is  subordinate,  except,  perhaps,  in  the  finest  part  of  the 
till.  There  is,  however,  among  the  larger  masses  or  fragments 
in  the  till,  a  quite  liberal  sprinkling  of  both  melaphyr  and  con- 
glomerate from  the  Nantasket  ledges.  These  are  especially 
noticeable  along  the  shore,  the  number  and  average  size,  with 
occasional  exceptions,  diminishing  rapidly  as  the  distance 
increases. 

The  surface  distribution  of  the  larger  bowlders  indicates  that 
they  are  not  equally  abundant  in  all  parts  of  the  till ;  for 
while  the  majority  of  the  drumlins  are  remarkably  smooth,  pre- 
senting on  the  sui^face  but  few  bowlders  larger  than  those  to  be 
seen  in  the  stone  walls,  others,  like  Booth  Hill  in  Scituate,  are, 
at  least  on  certain  slopes,  thickly  strewn  with  masses  from 
three  to  eight  feet  or  more  in  diameter.  The  bowlders  are,  in 
some  cases  at  least,  clearly  most  abundant  along  those  lines 
where  the  till  has  been  eroded  by  standing  or  running  water  and 
which  may  thus  be  regarded  as  in  some  sense  ancient  shores  or 

OCCAS     PAl'EKS    B.    S.    N.    H.    IV.    10. 


146 

channels,  resembling  the  modern  shore  in  this  respect ;  or  over 
areas  where  there  is  at  least  evidence  that  the  till  has  been  sub- 
jected to  the  sorting  action  of  water  and  the  finer  materials 
removed.  It  is  reasonable  to  suppose  that  the  bowlders  are 
most  abundant  in  the  lower  parts  of  the  drumlins,  and  in  gen- 
eral where  the  till  is  thinnest,  as  may  be  observed  at  many 
points  over  the  rocky  peneplain  of  Cohasset ;  and  it  is  certainly 
not  improbable  that,  as  suggested  to  me  by  Mr.  Upham,  this 
is  owing,  not  wholly  to  the  fact  that  the  source  of  the  bowlders 
is  near  at  hand,  but  partly  to  an  actual  combing  of  bowlders 
out  of  the  till  or  ground  moraine  as  it  was  swept  past  the 
ledges  by  the  movement  of  the  ice,  the  ledges  thus  serving  ;is 
gathering  points  for  bowlders. 

An  occasional  bowlder  merits  special  attention  on  account  of 
its  size  ;  and  some  of  these  are  rather  striking  also  in  their  sit- 
uations. As  might  be  expected,  the  largest  erratics  consist 
almost  exclusively  of  granite,  the  most  massive  and  resistant 
rock  of  the  region.  One  of  the  most  impressive  examples 
observed  in  the  Nantasket-Scituate  area  is  the  block  of  granite 
on  Booth  Hill,  in  Scituate,  known  as  Hatch's  Rock  (Fig.  23). 
It  is  very  regular  in  form,  approximately  twenty  feet  square 
with  an  average  height  above  the  ground  of  at  least  ten  feet. 
It  stands  on  the  summit  of  the  hill  and  some  fifteen  feet  above 
a  broad  level  terrace  or  platform.  Near  the  eastern  base  of 
Booth  Hill,  on  the  grounds  of  Mr.  Silas  Peirce,  is  Toad  Rock, 
a  block  of  diorite  about  twenty  feet  long  resting  on  a  local 
deposit  of  modified  drift. 

A  granite  bowlder  about  twenty  feet  long,  twelve  feet  wide, 
and  ten  feet  high  lies  in  the  woods  close  on  the  south  side  of 
Beechwood  Street,  about  one  mile  northwest  of  Beech  wood 
Village,  with  several  others  nearly  as  large  near  it ;  and  east 
of  the  Village  a  block  of  granite  which  is  approximately  a  ten- 
foot  cube  lies  on  the  west  side  of  Bound  Brook,  a  few  rods  south 
of  the  street ;  while  in  the  rocky  woods  south  of  Scituate  Hill 
may  be  found  a  striking  example  of  a  large  bowlder  which  has 
been  disrupted  by  the  action  of  the  frost. 


147 

In  a  walk  along  the  shores  ofNantasket,  Cohasset,  and  Scit- 
iiate,  one  observes  occasional  large  bowlders,  sometimes  local 
and  sometimes  traveled  or  truly  erratic,  but  the  only  one  that 
need  be  specially  mentioned  is  an  angular  block  of  coarse, 
massive,  pinkish  granite,  one  among  a  large  ninnl)er  of  smaller 
masses,  on  the  eastern  shore  of  the  Glades,  in  Scituate,  and 
nearly  opposite  the  Osher  Rocks.  It  is  about  twenty-five  feet 
long  ;  but  being  of  precisely  the  same  lithological  character  as 
the  neighboring  ledges,  it  would,  perhaps,  escape  particular 
notice,  except  for  its  position,  being  supported  by  smaller 
bowlders  and  the  ledge  in  such  a  manner  that  when  the  tide  is 
out  one  can  pass  under  it  from  one  side  to  the  other.  The  fact 
that  it  maintains  this  insecure  position,  although  exposed  to 
the  full  force  of  the  breakers,  testifies  to  its  great  weight. 

The  best  example,  however,  of  a  perched  block,  and  one  of 
the  largest  bowlders  in  Cohasset,  is  the  mass  of  granite  on  the 
estate  of  Mr.  Edward  Wheelwright  west  of  Little  Harbor, 
which  has  long  been  known  as  Tittling  Rock  (Fig.  20). 
Going  north  from  Cohasset  Village  on  Jerusalem  Road,  we 
come  in  about  half  a  mile  to  the  residence  of  Mr.  Wheel- 
wright, whence  a  private  road  runs  nearly  due  west  for  a 
considerable  distance  through  beautiful  rocky  woodland, 
bringing  us,  when  about  two  thirds  of  a  mile  from  the  highway, 
to  Tittlino-  Rock.  The  bowlder,  which  lies  close  to  the  road  on 
the  south,  is  a  rude  and  somewhat  oblique  parallelopipedon 
measuring  approximately  20  feet  in  extreme  length  (north- 
south),  12  feet  in  breadth,  and  12  feet  in  greatest  height.  Its 
location  is  almost  the  highest  point  in  this  part  of  the  peneplain, 
probably  100  feet  above  the  sea.  It  rests  upon  a  low, 
glaciated  ledge  of  granite  sloping  gently  to  the  north,  seeming 
to  lie  in  a  shallow  depression  or  glacial  trough  ;  and  the  actual 
base  or  supporting  surface  is  not  more  than  six  feet  long  and 
from  two  to  three  feet  wide.  But  although  seemingly  so 
nicely  poised,  considerable  force  would  probably  be  required  to 
disturb  it ;  and  it  is   not  properly  a  rocking  stone. 


148 


Fig.  20 — TiTTLixG  Rock,  Cohasset. 

On  the  northwest  slope  of  Telegraph  Hill,  just  above  the 
junction  of  the  main  hill  with  the  lower,  flat-topped  part,  is  a 
bloclv  of  coarse  granite  about  fifteen  feet  in  diameter  and  from  six 
to  seven  feet  hioh  above  the  ground.  A  verv  similar  block  rests 
in  a  similar  position  on  the  north  side  of  Strawberry  Hill.  It 
is  about  fifteen  feet  square  and  eight  if  not  ten  feet  thick.  Two 
bowlders  of  granite,  essentially  similar  to  these,  are  similarly 
situated  on  the  northern  slope  of  Otis  Hill,  in  Hingham.  The 
occurrence  of  these  isolated  blocks  on  the  northern  aspects  of 
the  hills,  and  at  approximately  the  height  of  one  of  the  principal 
sand  plains,  is  certainly  suggestive  of  their  transportation  by 
floating  ice  rather  than  the  ice-sheet ;  and  it  appears  necessary 
to  reg^ard  them  as  havino-  been  derived  from  the  o-ranite  ledo-es 
north  of  the  Boston  Basin,  the  nearest  possible  source  of  granite 
of  this  chnracter,  in  the  direction  of  glacial  movement,  being- 
twelve  to  fifteen  miles  distant. 


GLACIAL    POTHOLES. 


Four  years  ago,  my  attention  was  first  called  by  Mr.  T.  T. 
Bouve  to  the  existence  of  a  fine  series  of  glacial  potholes 
on  Cooper's   Island   in  Little  Harbor,    Cohasset,  and  I  had  the 


149 


pleasure  a  little  later  of  examining  the  locality  in  company 
with  him  and  Mr.  Warren  Uphani.  In  the  followinf  year  Mr. 
Bouve  published  a  full  account  of  the  potholes/  from  which 
the  greater  part  of  the  following  description  is  taken,  together 
with  the  illustrations  (Figs.  21  and  22). 


V^^^^ 


J"'  ,„7>' 


Fig.   21. —  Pothole,  Cooper's  Island. 

Cooper's  Island,  so-called,  is  the  peninsula  consisting  of 
Sfranite  ledsres  and  salt-marsh  which  extends  into  Little  Harbor 
from  the  west  side.  The  eastern  end,  which  reaches  nearly  to 
the  middle  of  the  harbor,  is  an  approximately  north-south 
rid^e  of  o-ranite  one  fourth  of  a  mile  lona^  and  from  15  to  25  feet 
high.  This  ridge  is  divided  transversely  near  the  southern 
extremity  by  a  short  stretch  of  grass  land  ;    and  it   is   near  the 

1  Proc.  B.  S.  ISr.  H.,  XXIV,  219-228. 


150 

southern  end  of  the  main  ridge,  on  the  east  side  and  quite  close 
to  the  water  that  the  potholes  occur.  The  most  southerly  and 
most  perfect  holes  are  on  the  north  side  of  a  slight  indentation 
of  the  rocky  shore  and  may  be  readily  found  when  the  tide  is 
out  (Fig.  21).  Of  the  lowest  and  best  preserved  of  these 
(No.  1)  the  bottom  is  still  intact  and  perfect,  holding  water  to 
a  depth  of  21  inches  and  having  a  well-defined  rim  just  at  the 
surface  of  the  water.  The  diameter  at  the  rim  is  25^  inches  ; 
below  the  rim,  30  inches.  Above  this  rim  the  whole  southern 
side  of  the  hole  is  wanting  ;  but  on  the  northern  side  the  rock 
is  smoothly  concave  and  the  characteristics  of  the  pothole  are 
plainly  discernible  for  a  height  of  four  feet,  with  a  breadth  of  from 
three  to  four  feet,  making  the  total  depth  about  six  feet.  The 
whole  has  the  appearance  of  a  large,  imperfect  or  one-sided 
pothole  with  a  smaller  and  perfect  pothole  in  the  bottom  of  it. 
Exterior  to  this  pothole,  the  tide  sinks  below  the  level  of  its 
bottom,  but  at  high  tide  all  is  covered. 

The  second  pothole  has  its  bottom  three  and  one  half  feet 
above  that  of  No.  1  ;  and  their  centers  are  three  feet  apart  hori- 
zontally. The  whole  southern  side  of  this  hole  (No.  2)  is 
wanting  and  water  can  now  stand  in  it  only  to  the  depth  of 
about  two  inches.  Above  the  bottom,  the  granite  is  smoothly 
concaved  for  a  breadth  of  three  feet  and  a  height  of  five  feet ; 
and  this  hole,  when  entire,  was  evidently  about  as  large  as  the 
first.  The  slope  of  what  remains  of  the  walls  of  these  holes 
shows,  apparently,  that  the  flow  of  water  over  the  rocks  was 
from  the  west  and  northwest.  Of  the  third  pothole  there  is 
but  little  to  be  said,  except  that  it  is  small  and  shallow.  It  is 
about  five  feet  above  the  bottom  of  No.  2,  still  in  a  northwest 
direction  ;  and  there  may  be  traced  from  it  in  the  same  general 
direction  a  narrow  water-worn  channel  about  six  feet  in  length. 
The  hio-h-tide  mark  is  about  midway  between  the  second  and 
third  potholes. 

It  is  quite  obvious  thtit  these  potholes  were  once  entire. 
Their  sharply-defined  vertical  edges  are   seen  to  coincide  with 


151 


one  well-marked  joint-plane  sloping  steeply  to  the  south  ;  and 
the  southern  sides  of  all  three  of  the  holes  have,  apparently, 
been  carried  away,  by  the  removal  of  a  single  large  joint-block 
of  the  granite.  Since  this  block  appears  to  have  been  torn 
away  bodily,  leaving  an  angular,  unglaciated  surface,  and  no 
trace  of  the  block  itself  can  be  seen  in  the  vicinity,  we  may 
fairly  regard  its  removal  as  the  last  work  accomplished  by  the 
ice-sheet  on  the  lee  side  of  this  ledffe. 


Fig.  22 — Pothole,  Cooper's  Island. 


The  fourth  pothole  is  or  was  the  largest  of  all,  and  hence 
has  been  commonly  called  the  "Well."  Passing  over  the  rocky 
elevation  in   a  northerly  direction,    it   may   be  found   about  a 


152 

hundred  feet  distant  from  the  others  ;  and  it  is  a  little  farther 
than  the  others  from  the  shore,  its  bottom  being  about  four 
feet  above  high  tide  (Fig.  22).  The  bottom  of  this  hole, 
which  is  still  entire,  so  as  to  hold  water  to  a  depth  of  about  one 
foot,  is  cut  in  the  solid  granite  at  the  base  of  a  small  scarp. 
It  is  somewhat  oval  in  form,  the  horizontal  diameters  varying 
from  two  feet  ten  inches  to  four  feet.  The  ledge  rises  abruptly 
nine  feet  from  the  margin  of  the  Well  and  ten  feet  from  its 
bottom  ;  and  the  Well  itself  was  probably  as  deep  at  least  as 
ten  feet,  the  curvature  and  wearing  of  the  rock  clearly  showing 
this.  The  beautifully  rounded  and  water- worn  form  of  the 
granite  on  the  southeast  side  of  the  Well,  above  the  water,  is  a 
plain  indication,  however,  that  it  has  not  been  entire  on  this 
side  since  the  water  ceased  to  swirl  through  it.  On  the  north 
side  the  concave  wall  is  vertical  or  slightly  overhanging  ;  but 
on  the  west  side  the  wall  is  widely  flaring,  receding  upwards  in 
such  a  way  as  to  indicate  that  the  water  entered  at  this  point, 
the  Well  agreeing  in  this  respect  with  the  group  of  potholes 
( 1  to  3  j .  It  seems  probable  that  the  pothole  was  entire  in  its 
early  stages  ;  but  as  it  increased  in  size  the  outer  or  lee  wall 
was  either  worn  through  or  carried  away  bodily  by  the  ice, 
after  which  the  rush  of  water  continued  long  enough  to  develop 
the  graceful  contours  of  its  natural  exit,  as  shown  in  the 
figure. 

Besides  these  four  potholes,  there  are  other  depressions 
which  are  evidently  incipient  or  embryo  potholes.  A  linear 
group  of  these  may  be  observed  about  twenty  feet  north  of, 
and  parallel  with,  the  southern  series  (1  to  3).  The  upper 
one  is  shallow,  like  the  bowl  of  a  spoon,  about  a  foot  across, 
showing,  extending  from  it,  a  water- worn  channel  sloping 
easterly  about  ten  feet  to  the  brink  of  the  ledge,  connecting  the 
first  depression  in  its  course,  with  two  others  of  similar  form  ; 
while  on  a  lower  surface  there  is  a  larger  depression  just  where 
the  water  from  the  first  might  descend. 

Considering  the  shallowness  of  the  entire  portions  of  all  the 


153 

potholes,  and  tlie  probabilitv  ot"  tlieii-  visitation  hv  fenerations 
of  both  the  Indian  and  the  white  man,  it  is  not  snr|)risino-  that 
nothing  of  their  contents  is  left  in  or  al)out  them.  There  is, 
however,  one  rounded  stone  in  the  possession  of  Mr.  Charles 
S.  Bates,  the  owner  of  Cooper's  Island,  which  tradition  states 
to  have  been  taken  from  the  deepest  pothole  (No.  1).  It  is  a 
smoothly  rounded  and  nearly  spherical  ball  of  granite  about 
four  inches  in  diameter, — a  typical  pothole  bowlder  ;  and  there 
seems  to  be  no  reason  to  question  the  truth  of  the  tradition. 

On  one  of  the  higher  ledges  south  of  Little  Harbor  and 
Beach  Street,  and  near  the  village,  there  is  a  smooth  hollow  in 
the  granite,  about  a  foot  across  and  nine  inches  deep,  which, 
from  its  form,  has  been  called  the  "Devil's  Armchair." 
Although  not  well-defined,  it  is  clearly  a  small  pothole  ;  and  in 
this  instance,  as  in  others,  an  apparently  water-worn  channel 
extends  east -southeast  on  the  rather  abrupt  slope  of  the  ledge 
for  several  feet,  ending  in  another  and  smaller  hole.  Durino- 
the  past  year  I  have  discovered  still  another  pothole.  This  is 
on  Cohasset  Rocks,  immediately  behind  the  Black  Rock 
House,  on  a  bare  surface  of  granite  sloping  down  into  the  sea, 
and  only  two  or  three  feet  above  the  high-tide  level.  It  is  a 
smoothly- worn  and  sharply-defined  basin,  somewhat  pear- 
shaped  in  outline,  and  measuring  41  by  33  inches  in  maximum 
length  and  breadth,  the  major  axis  trending  about  S.  15°  W. 
It  holds  about  six  inches  of  water,  but  the  depth  below  the 
well-defined  rim  varies  from  (3  to  18  inches,  being  shallowest 
on  the  side  toward  the  sea. 

The  potholes  of  Cooper's  Island,  although  on  the  shore,  do 
not  directly  face  the  sea,  and  the  Well,  especially,  is  quite  shut 
in  by  the  granite  ledge  on  the  seaward  side,  as  the  cut  shows. 
They  are  also  partly  above  the  high-tide  level ;  and  the  lower 
ones  are  bathed  only  by  the  quiet  waters  of  Little  Harbor.  If 
the  sea  were  high  enough  to  sweep  over  Beach  Island  and  the 
other  barriers  between  Little  Harbor  and  the  Atlantic,  the  pot- 
holes would  be  completely  submerged  and   so  still  beyond   the 


154 

reach  of  the  surf.  In  short,  both  the  situation  and  the  forms 
of  the  potholes  make  it  impossible  to  regard  them  as  in  any 
sense  the  product  of  marine  erosion.  It  is  equally  clear 
that  no  ordinary  or  surface  river  ever  flowed  over  these  ledges. 
We  are  forced,  therefore,  to  ascribe  their  origin  to  the  action 
of  glacial  streams.  But  here  the  question  arises  as  to  whether 
they  have  probably  been  formed  by  subglaciai  rivers  rushing 
along  over  the  ledges,  or  by  streams  which,  flowing  over 
the  surffice  of  the  ice-sheet,  plunge  through  crevasses  to 
the  solid  rocks  below.  Such  a  waterfall  in  the  ice  is  called  a 
moulin  or  glacial  mill  ;  and  since  the  crevasses  must  often  be 
of  great  depth — hundreds  or  even  thousands  of  feet — these 
o-lacial  mills  are  generally  recognized  as  the  most  efficient  of  all 
agencies  in  the  formation  of  potholes.  Subglaciai  streams, 
it  is  believed,  must  usually,  like  ordinary  rivers,  follow  the 
depressions  of  the  rocky  surface  ;  while  moulins — the  chief 
source  of  the  subglaciai  stream — may  strike  the  ledges  with 
resistless  force  at  almost  any  point,  and  especially  on  the  crests 
and  southeastern  or  lee  slopes  of  prominent  ledges  and  ridges, 
where  the  conditions  are  most  favorable  for  the  formation  of 
profound  crevasses.  A  strong  presumption  is  thus  created  in 
favor  of  the  glacial  mills  as  a  cause  of  the  potholes  of  Cohasset. 
One  difficulty,  however,  still  remains.  As  the  ice-sheet  moves 
continuously  forward,  carrying  the  crevasses  and  moulins  with 
it,  how  is  it  that  the  potholes  escape  elongation  in  the  direction  of 
the  movement?  Of  course  a  mo2ilt7i  cannot  move  forward 
indefinitely,  but  only  until  a  new  crevasse  is  formed  behind 
the  one  through  which  the  water  first  fell,  a  few  feet  or  yards  at 
the  most ;  and  the  moulin  then  returns  to  its  starting  point.  But 
why  do  the  potholes  not  show  even  this  small  amount  of  elonga- 
tion ? 

Mr.  Bouve  has  discussed  this  point  in  the  following  words^ : 
"  It  has,  indeed,  been  thought  strange  that,  as  the  ice  moved 
continuously  on,  the  holes  were  not  found  generally    elongated 

1  Proc.  B.  S.  N.  H.,  XXIV,  224. 


155 

in  the  direction  of  the  movement  of  the  ghicier  rather  than 
circular.  Such  thought,  however,  is  only  consistent  with  the 
presumption  that  the  holes  were  made  just  where  the  water 
first  fell  upon  the  rock  surface  below.  Far  more  reasonable  is 
it  to  suppose  that  the  holes  were  formed  somewhat  distant  from 
this  place,  where  the  masses  of  rocks  borne  by  the  waters 
found  a  lodging  in  some  depi'ession  and  there  by  rotation 
worked  out  the  potholes.  The  ice  might  move  on  and  the 
waters  descend  through  the  moulin  far  from  where  they  first 
fell,  yet  continue  their  flow  in  the  same  direction  as  at  first  and 
go  on  with  the  work  of  rotating  the  contents  of  the  hole  through 
a  whole  season.  In  such  case  there  could  be,  of  course,  no 
reason  to  expect  the  elongation."  In  explanation  of  potholes 
in  close  proximity  to  others  and  yet  seemingly  independent,  as 
in  the  case  of  the  Well,  Mr.  Bouve  says  :  "  Observation  upon 
Alpine  glaciers  shows  that  as  a  crevasse  is  carried  forward  by 
the  general  movement  of  the  ice  it  closes.  Subsequently  a  new 
one  is  formed  just  where  in  relation  to  the  land  at  the  margin 
of  the  glacier  the  former  one  existed  ;  and  the  waters  again 
descend  upon  the  rock  surface  near  where  they  before  fell,  but 
not  often,  probably,  in  exactly  the  same  place  ;  and  thus  pot- 
holes are  formed  contio;uous  to  each  other  and  vet  far  enouo-h 
distant  to  make  it  evident  that  they  were  not  ])roduced  by  the 
same  flow  of  water."  This  is  undoubtedly  a  true  explanation 
of  many  glacial  potholes  ;  and  its  weakest  point,  as  applied  to 
most  of  the  potholes  of  Cohasset  is  that  it  requires  the  water 
to  flow  up  over  instead  of  around  the  prominent  ledges. 

Mr.  Upham  has  suggested  a  diflTerent  explanation.  He 
says^ :  "  The  time  of  the  excavation  of  these  glacial  potholes 
was  probably  the  early  part  of  the  epoch  of  glaciation,  when  the 
ice-sheet  was  being  formed  upon  the  land  by  snow-fall.  Upon 
any  hilly  country  the  ice  must  have  attained  an  average  depth 
somewhat  exceeding  the  altitude  of  the  hills  above  the  adjoin- 
ing lowlands  before  any  general   motion  of  the  ice-sheet  could 

[iProc.  B.  S.  N.  H.,  XXIV,  226-228, 


166 

begin.  During  this  process  of  slow  accumulation  of  the  ice- 
sheet,  the  summer  melting  upon  its  surface  would  produce 
multitudes  of  rills,  rivulets,  and  brooks,  which  might  unite 
into  a  large  stream,  and  this,  pouring  through  a  crevasse  and 
melting  out  a  cylindric  moitlin,  might  fall  a  considerable  depth 
to  the  bed  rock,  perhaps  one  or  two  hundred  feet  or  more  upon 
an  area  so  moderately  uneven  as  Cohasset,  while  yet  the  ice- 
motion,  though  sufficient  to  permit  the  formation  of  the  crevasse, 
might  not  have  gained  a  definite  current  to  carry  the  crevasse, 
nioulin,  and  water-fall  away  from  the  spot  where  they  were 
first  formed.  We  may  thus  explain  the  continuation  of  a  glacial 
water-fall  in  one  place  while  it  was  excavating  one  of  these 
"  Giant's  Kettles  "  or  potholes.  After  the  ice-sheet  acquired  a 
current  because  of  the  greater  thickness  and  pressure  of  its  mass, 
such  deep  cylindric  excavations  in  the  bed-rock  could  not  be 
made  ;  and  during  the  recession  and  final  dissolution  of  the  ice- 
sheet,  it  seems  probable  that  its  receding  border  had  steeper 
gradients  and  consequently  even  more  rapid  motion  than  in  the 
culmination  of  the  glacial  epoch." 

In  criticism  of  this  view  it  may  be  pointed  out,  first,  that 
while  the  movement  of  the  ice-sheet  was  still  practically  nil,  it 
would  probably  have  accommodated  itself  to  the  irregularities  of 
its  rocky  floor  without  cracking  ;  second,  that  the  face  of  the 
country,  before  it  had  been  swept  by  the  long-continued  advance 
of  the  ice-sheet,  was  probably  buried  beneath  a  considerable 
thickness  of  soil  and  half-decomposed  rock,  the  product  of 
chemical  and  mechanical  decay  during  long  preglacial  ages  ; 
and  third,  that  it  makes  no  adequate  allowance  for  glacial  erosion 
of  the  hard  rocks  during  the  long  periods  of  the  maximum 
development  and  waning  of  the  ice-sheet,  which  would  inevitably 
have  obliterated  the  potholes.  In  short,  Mr.  Upham's  view 
carries  as  its  logical  consequence  the  reduction  of  glacial  erosion 
not  only  to  a  minimum,  but  almost  to  nothing. 

While  seeking  for  additional  light  upon  this  problem  it  has 
occurred  to  me,  first,  that  a  moulin  may  remain  approximately 


nr 


157 

stationary,  while  the  ice  moves  on,  through  the  bafkvvard  ero- 
sion and  melting  of"  its  up-stream  side;  and,  second,  that  when 
a  pothole  is  formed  at  the  bottom  of  a  rnonlin  it  is  not  the 
direct  impact  of  the  water  upon  the  face  of  the  ledge  that  does 
the  work,  nor  do  the  stones  carried  down  by  tlie  water  wear 
the  ledges  appreciably  by  their  direct  fall,  but  the  pothole  is  due 
to  their  subsequent  movement  and  especially  their  rotation  by 
the  water.  This  rotation  implies  an  antecedent  depression  or 
hollow  to  hold  the  stones,  and  thus  the  conditions  are  seen  to 
be  essentially  the  same  as  for  ordinary  river  potholes.  Since 
the  rotation  of  stones  in  a  pre-existing  hollow  is  an  essential 
condition  of  the  glacial  as  of  other  potholes,  and  the  moulin 
simply  supplies  the  power,  it  would  seem  to  make  little  or  no 
difference  whether  the  water  plunges  into  the  up-stream  side, 
the  middle,  or  the  down-stream  side  of  the  hollow.  The  pot- 
hole will  be  made  where  and  only  where  the  hollow  is  ;  and 
during  the  progress  of  a  ojioidioi  across  the  hollow  there 
wOuld  not,  apparently,  be  any  mai-ked  tendency  to  elongate  it. 
In  the  case  of  a  linear  group  of  potholes  on  the  lee  slope  of  a 
ledge,  it  is  reasonable  to  suppose  that  the  upper  one,  which,  on 
Cooper's  Island,  is  always  the  smallest  and  most  indefinite,  marks 
the  shifting  position  of  the  moulin,  and  that  the  others  were 
formed  by  the  subglacial  flow  of  water  from  the  bottom  of  the 
moulin. 

Reverting  once  more  to  the  question  as  to  what  phase  of  the 
continental  glaciation — the  beginning,  the  maximum,  or  the 
waning — was  most  favorable  for  the  formation  and  preservation 
of  glacial  potholes,  it  may  be  noted  :  First,  that  during  the  de- 
velopment of  the  ice-sheet  precipitation  was  mainly  in  the  form 
of  snow,  and  greatly  exceeded  the  Avaste  by  melting  ;  the  true 
glacier  ice  was  covered  by  a  great  thickness  of  neve  and  snow  ; 
the  water  escaped  largely  by  seepage  ;  and  the  superficial  streams 
were  small.  Second,  that  while  the  growing  sheet  of  ice,  neve, 
and  snow  must  have  been  virtually  stationary  for  a  long  time, 
it  must  also  have  attained  quite  a  high  velocity  (for  an  ice-sheet) 


158 

before  crevasses  extending  up  through  the  thick  layers  of 
yielding  neve  and  snow  to  the  surface  could  be  formed  ;  and  the 
latter  condition  must,  of  course,  be  realized  before  glacial 
moulins  are  a  possibility.  Tiiird,  that  it  is  in  tlie  highest  degree 
improbable  that  during  the  period  of  maximum  glaciation  such 
comparatively  slight  inequalities  as  the  Cohasset  ledges  could 
have  produced  crevasses  traversing  the  entire  thickness  of  the  ice- 
sheet ;  just  as  in  the  case  of  inequalities  in  the  bed  of  a  river, 
the  surface  disturbance  diminishes  with  the  depth  of  the  water. 
Fourth,  that  since  the  decay  and  final  disappearance  of  the 
ice-sheet  were  due  to  a  general  amelioration  of  the  climate, 
resulting  in  part  it  is  })robable  from  a  depression  of  the  land, 
it  was  probably  accomplished  by  such  a  general  ablation  of  its 
upper  surface  as  would  have  caused  only  a  slight  increase  in 
the  gradient,  an  increase  that  may  have  been  neutralized,  so 
far  as  its  effect  upon  the  rate  of  movement  of  the  ice  was  con- 
cerned, by  the  greater  depression  of  the  land  toward  the  north, 
and  the  steadily  diminishing  thickness,  weight,  and  power  of 
the  ice.  It  certainly  seems  more  reasonable  to  suppose  that 
the  rate  of  movement  of  the  ice-sheet  diminished  as  its  weight 
and  power  declined,  than  that  it  continued  to  increase  until  the 
final  disappearance  of  the  ice.  Fifth,  that  when  the  decay  of 
the  ice-sheet  over  any  area  was  well  advanced,  it  must  have 
presented,  at  least  during  a  considerable  part  of  the  year,  a  sur- 
face of  hard,  brittle  ice  ;  the  conditions  being  thus  more  favor- 
able than  at  any  previous  period  for  the  existence  of  superglacial 
streams  and  of  crevasses  traversing  the  entire  thickness  of  the 
ice  ;  and  it  is  obvious  that  such  crevasses  are  now  consistent 
with  a  very  slow  movement  of  the  ice  over  very  slight  inequal- 
ities of  the  ground,  for  the  ice  has  lost,  in  its  loss  of  weight, 
not  only  the  power  to  flow  rapidly  forward,  but  also  the  power 
to  flow  around  the  ledges  and  mold  itself  accurately  against 
them  without  rupture  of  its  mass.  Sixth,  that  with  the  dimin- 
ishino-  thickness  of  the  ice-sheet  there  must  have  come  a  time 
when  it  could  no  longer  overcome  the  friction    of  so  rouo^h   a 


159 

surface  as  the  Boston  Basin  presents,  and  it  became  essentially 
stationary,  or  was  subject  to  slight  local  movements  only  ;  and 
presenting  still  a  fairly  continuous  sheet  of  solid  ice,  with 
surface  streams  and  crevasses,  the  conditions  were  especially 
favorable  for  the  formation  of  normal  glaci-al  potholes  by 
stationary  moulins.  Seventh,  the  contours  and  distribution  of 
the  modified  drift  in  this  region  indicate  that  the  ice-sheet 
became,  later,  very  ragged,  being  frayed  out  at  the  margin 
and  divided  into  numerous  detached  masses,  so  that  important 
surface  streams  and  ynoulins  ceased  to  exist.  In  conclusion, 
attention  may  be  called  once  more  to  the  fact  that  the  glacial 
potholes  prove  by  their  very  existence  that  there  has  been  no 
appreciable  glaciation  of  the  rocks  in  which  they  occur  since 
they  were  formed.  In  other  words,  they  are  a  record  of  a 
time  when  the  ice-sheet  still  covered  the  land,  but  had  become 
nearly  or  quite  stationary. 

It  may  be  remarked  in  this  connection,  also,  that  while  the 
till  in  this  region  is  found  to  lie  almost  universally  upon  strongly 
glaciated  rock  surfaces,  the  deposits  of  modified  drift  repose  in 
part  upon  glaciated,  but  very  largely  upon  unglaciated  and 
distinctly  water-worn  sui-faces.  It  is  a  common  circumstance 
also  to  find  the  protruding  and  stoss  portions  of  ledges  from 
which  the  sand  and  gravel  have  been  removed  more  or  less 
distinctly  glaciated,  while  all  the  depi'essed  and  lee  surfaces  are 
beautifully  sculptui'ed  by  water,  showing  many  incipient  pot- 
holes connected  with  winding  channels  and  separated  by 
smoothly  hummocky  and  undulating  surfaces.  The  glacial 
striae  often  end  abruptly  at  the  margins  of  these  depressed  areas, 
showing  very  distinctly  the  co-operation  or  simultaneous  action 
of  water  and  ice  at  a  time  when  the  ice  had  but  little  erosive 
power,  and  proving  very  clearly,  also,  that  the  water  was 
subglacial. 

MODIFIED    DRIFT    AND    TERRACES. 

It  is  generally  conceded  that  the  recession  and  final  dis- 
appearance   of  the   ice-sheet  was    attended  by,    and    probably 


1(30 

consequent  upon,  a  marked  depression  of  the  land.  This 
depression  was,  apparently,  more  than  sufficient  to  restore  the 
preglacial  rehations  of  the  hmd  and  sea  ;  or  at  least  evidence  is 
not  wanting  that  the  shore-line  was,  for  a  brief  [)eriod,  somewhat 
above  its  present  level.  The  re-elevation  of  the  land  here 
referred  to  amounted  to  520  feet  in  the  vicinity  of  Montreal, 
225  feet  on  the  coast  of  Maine,  but  only  to  20  or  possibly  30 
feet  in  the  Boston  Basin  ;  and  the  evidences  for  both  the  eleva- 
tion and  the  slow  subsidence  which  has  since  been  in  progress 
are  much  less  perfectly  developed  in  the  Nantasket  area  than  in 
other  parts  of  the  Basin. 

Partly  by  subglacial  and  superglacial  streams,  but  mainly 
by  the  great  torrents  and  tiie  temporary  lakes  resulting  from  the 
final  melting  of  the  ice-sheet,  the  till  or  bowlder  clay  was  very 
largely  modified,  — that  is,  washed,  assorted,  and  stratified  in 
the  sand  plains  or  deltas,  gravel  ridges  or  eskers  and  kames,  and 
clay  beds.  The  explanation  of  modified  drift  in  general  requires 
a  constant  blending  or  intermingling  of  glacial,  fluvial,  and 
lacustrine  conditions;  and  probably,  also,  in  the  Boston  Basin, 
for  the  clay  beds  if  not  the  lower  deposits  of  sand,  we  may  pos- 
tulate marine  conditions. 

Among  the  various  forms  of  modified  drift  the  eskers  or 
gravel  ridges  appear  usually  to  be  the  oldest  ;  and  being  the 
product,  chiefly,  of  superglacial  and  subglacial  streams,  they 
evidently  record  the  actual  presence  of  the  ice-sheet  as  a  fairly 
continuous  body  of  ice  and  not  as  the  detached  masses  of  ice 
indicated  by  the  kettles  and  other  features  of  the  sand  plains. 
If  anv  important  or  esker-formino-  o-lacial  streams  crossed  the 
Nantasket  and  Cohasset  area,  their  accumulations  have  been 
effaced  by  subsequent  erosion  or  deposition  ;  for  no  distinct 
eskers  have  been  observed.  The  sand  plains,  also,  are  rather 
scantily  and  imperfectly  developed. 

Sand  plains,  in  general,  we  must  regard  in  part  as  repre- 
senting the  flood-plains  of  great  rivers  flowing  from  the  melting 
ice-sheet  and  in  part  as  deltas  formed  by  these  rivers  in  temporary 


161 

lakes  or,  possibly,  in  the  case  of  the  lowest  plains,  in  the  sea. 
Most  of  these  lakes  were  iindoubtedly  truly  glacial, — that  is, 
enclosed  partially,  sometimes  wholly,  by  solid  walls  of  ice. 
The  kettles,  dimpling  the  sand  plains,  it  is  well  understood,  are 
due  to  the  subsequent  melting  of  masses  of  ice  which  were 
buried  in  the  stratified  deposits  ;  and  the  steep  and  sharply- 
defined  marginal  slopes  which  the  sand  plains  often  present  are 
doubtless  in  part  the  natural,  free,  growing  edges  or  fronts  of 
the  deltas,  and  in  part  due  to  the  subsequent  melting  of  walls 
of  ice  against  which  either  flood-plain  or  delta  deposits  had 
been  accumulated,  the  sand  and  gravel,  as  it  was  thus  grad- 
ually let  down,  naturally  assuming  the  maximum  declivity. 

The  Nantasket  Peninsula,  being  then  surrounded  by  deeper 
water  than  at  present  and  even  more  remote  than  now  from  the 
main  land  and  the  mouths  of  rivers,  was  almost  exempt  from 
these  delta  and  flood-plain  deposits.  The  higher  plains  are 
entirely  wanting  here,  and  the  lowest,  which  varies  usually 
from  fifteen  to  twenty-five  or  thirty  feet  in  height,  is  developed 
in  only  a  very  scanty  and  fragmentary  manner,  as  on  the  south 
side  of  Point  Allerton.  In  Cohasset  and  Scituate  the  plain, 
having  usually  a  height  of  from  forty  to  sixty  feet,  is  well  devel- 
oped at  intervals.  It  forms  the  comparatively  level  land  along 
the  railroad  between  Hingham  and  Cohasset  Village  ;  and  in  the 
northern  part  of  the  village,  and  especially  between  Little 
Harbor  and  the  railroad,  it  is  a  very  typical  plain,  this  part 
of  the  village  being  designated  as  the  "Plain"  on  the  published 
maps  of  the  town.  It  rises  quite  abruptly  45  feet  from  the 
marshes  of  Little  Harbor  and,  deeply  dimpled  with  beautiful 
kettles,  slopes  gently  down  on  the  south  to  the  marshes  and 
James  River.  One  of  the  kettles,  on  the  north  side  of  Beach  Street 
near  the  Plain,  has  long  been  known  as  the  "Punch  Bowl"; 
and  several  of  them  hold  permanent  ponds,  the  pond  near  the 
south  end  of  the  Common  being  an  example.  When  this  plain 
was  formed,  the  ice  must  have  completely  filled  the  basin  of 
Little  Harbor  ;  and  the  stream  supplying  the  detritus  probably 

OCCAS.  FAPEKS,  B.  S.  N.  H.    IV.      11. 


162 

came  from  the  west  or  northwest.  It  is  interesting  to  observe 
that  this  deposit  of  sand  is  probably  the  only  barrier  between 
Little  Harbor  and  Cohasset  Harbor  at  this  point. 

The  considerable  deposits  of  modified  drift  in  the  Beechwood 
district,  south  and  southeast  of  Scituate  Pond,  may  be  regarded 
as  a  fragmentary  development  of  this  plain.  A  much  lower 
plain,  which,  quite  probably,  is  of  marine  origin,  has  a  very 
perfect  development  in  the  district  south  of  the  Glades.  It 
terminates  on  the  east  in  Mitchell's  Beach  and  presents  a  very 
flat,  unbroken  surface,  from  10  to  15  feet  above  the  sea,  over  an 
area  nearly  half  a  mile  square.  It  is  also  scantily  developed 
along  the  northern  edge  of  the  peneplain,  in  the  vicinity  of 
Rockland  Street,  and  at  other  points  ;  and  we  may  reasonably 
suppose  that  farther  inland  it  underlies  the  broad,  level,  and 
swampy  valley  of  Bound  Brook  and  its  tributaries. 

Beds  of  clay,  or  the  finer  portion  of  the  modified  drift,  appear 
to  be  wholly  wanting  above  sea-level  in  the  Nantasket  and  Co- 
hasset area.  There  can  be  but  little  doubt,  however,  that  they 
are  extensively  developed  under  the  salt  marshes  and  in  the 
drowned  or  marine  portions  of  the  valleys  generally.  The  slow 
development  of  the  salt  marshes  during  post-glacial  time 
probably  commenced  in  every  important  instance  with  a  broad, 
level  expanse  of  the  finest  glacial  silt  or  clay  covered  by  a  mod- 
erate depth  of  water  and  supporting  a  luxurious  growth  of  eel 
grass.  In  the  meshes  of  this  dense  jungle  or  mat  of  vegetation 
the  detritus  brought  by  the  tides  is  entrapped  and  slowly  sinks 
to  the  bottom.  The  upper  surface  of  the  deposit  is  thus  slowly 
raised  to  the  low-tide  level,  when  the  conditions  become  unfa- 
vorable for  the  growth  of  eel  grass  and  the  area  enters  upon  the 
mud-flat  stage.  Although  the  main  sweep  of  the  flowing  and 
ebbing  tides  is  now  through  more  or  less  definite  channels  or 
creeks,  but  little  additional  silt  lodges  on  the  unprotected  flat 
until  it  has  been  overrun  from  the  shore  outward  by  higher 
forms  of  vegetation,  when  the  silting-up  process  proceeds  un- 
checked until  the  high-tide  level  is    reached.     A    dense,   firm 


163 

mat  of  peat-forming  vegetation  then  covers  the  surface,  and  the 
marsh  is  complete.  In  other  parts  of  the  Boston  Basin,  how- 
ever, and  generally  wherever  excavations  have  been  made  in 
the  salt  marshes,  the  evidence  is  conclusive  that  a  slow  sub- 
sidence has  permitted  the  formation  of  several  successive  beds  of 
peat,  separated  by  beds  of  clay. 

Although,  as  the  beaches,  marshes,  and  marine  cliffs  so 
plainly  indicate,  the  sea  has  undoubtedly  maintained  its  present 
level  for  a  very  long  time,  the  rocks  of  our  coast  are  so  intensely 
hard  and  resistant  that  there  are  very  few  points  where  they 
show  any  appreciable  amount  of  marine  erosion.  All  along 
the  Nantasket  and  Cohasset  shore  it  is  perfectly  obvious  that, 
save  where  the  rocks  are  very  finely  jointed,  or  a  dike  has 
yielded  to  the  ceaseless  pounding  of  the  waves,  the  ledges  are 
still  essentially  intact,  showing  still  the  roche  moutonnee  forms 
impressed  upon  them  by  the  ice-sheet,  even  the  glacial  striae 
being,  in  some  cases,  well  preserved  for  several  feet  below  the 
high-tide  level. 

Above  the  present  level  of  the  shore  evidences  of  marine 
erosion  on  the  hard  rocks  are,  so  far  as  I  have  observed,  wholly 
wanting.  If  the  sea  has  stood  at  higher  levels  in  post-glacial 
times,  the  evidence  must  be  sought  in  the  erosion,  not  of  the 
hard  rocks  —  granite,  felsite,  melaphyr,  etc.,  but  of  the 
non-lithified  or  drift  deposits.  Fortunately,  the  drumlins, 
which  are  such  a  prominent  feature  of  Boston  Harbor,  present 
in  their  firm  but  yielding  material  and  regular  outlines  condi- 
tions exceedingly  favorable  for  making  and  preserving  a  record 
of  even  a  very  brief  occupation  of  a  higher  level  by  the  sea. 
Any  one  who  notes  the  extensive  erosion  of  the  drumlins  by 
the  sea  at  its  present  level  and  the  comparative  stability  of  the 
erosion  scarps,  cannot  doubt  that  if  similar  features  —  terraces 
and  scarps  —  had  ever  been  developed  on  the  drumlins  at  higher 
levels,  some  indications  of  them  would  still  be  traceable.  A 
general  study  of  the  drumlins  of  the  Boston  Basin  has  satisfied 
me    that  undoubted    horizontal  erosion-marks    are  a   common 


164 

feature  up  to  a  height  of  100  feet  or  more  above  the  sea.  They 
exist  as  strongly-marked  and  approximately  horizontal  and 
longitudinal  scarps  and  terraces,  frequently  bearing  bowlder 
pavements  and  showing  a  general  correspondence  in  height  with 
the  sand  plains  of  the  region.  They  have  been  observed  in  all 
parts  of  the  Boston  Basin  ;  but  ai'e,  perhaps,  most  favorably 
exposed  for  study  on  several  of  the  drumlins  northeast  of 
Boston,  including  Breed's  Hill,  Pleasant  Hill,  Mt.  Revere,  Mt. 
Washington,  and  Powder-Horn  Hill.  They  are  also  a  promi- 
nent feature  of  several  drumlins  on  the  South  Shore,  including 
Baker's  and  Otis  Hills,  in  Hingham,  and  have  been  observed 
on  some  of  the  drumlins  of  the  Nantasket-Cohasset  area.  These 
may  be  briefly  described,  commencing  on  the  north. 

On  the  southwest  side  of  Telegraph  Hill,  in  Hull,  a  sloping, 
bowlder-strewn  terrace,  from  20  to  30  feet  above  the  sea  and 
backed  by  a  steep  declivity,  extends  directly  across  from  shore  to 
shore.  The  only  terraces  on  Point  Allerton  Great  Hill  are  such  as 
have  probably  been  formed  by  the  gradual  accumulation  of  soil 
by  rain-wash  against  stone  walls  at  the  lower  edges  of  culti- 
vated fields.  Three  such  artificial  terraces  may  be  readily 
traced  along  the  southern  slope  at  heights  of  from  55  to  75  feet. 
The  20-  to  30-foot  terrace  is  well  developed  on  the  south  and 
southwest  sides  of  Sagamore  Hill,  opposite  the  steamboat 
wharf,  and  it  may  be  traced  along  the  southwest  side  of  the 
northern  drumlin  of  World's  End.  A  strongly  marked  terrace 
extends  for  a  shoi^t  distance  on  the  northeast  side  of  Planter's 
Hill,  at  a  height  of  55  feet.  Occasional  indications  of  terraces 
have  been  noted  on  the  southwestern  slopes  especially  of  other 
drumlins  in  Nantasket  and  Cohasset,  including  Strawberry 
Hill,  Bumkin  Island,  Turkey  Hill,  Scituate  Hill,  etc.  But  the 
only  others  requiring  particular  description  are  the  series  on  the 
northeast  side  of  Booth  Hill,  in  Scituate. 

This  great  drumlin  extends  southeast  from  Bound  Brook, 
near  North  Scituate,  for  about  one  mile.  Its  northern  base  is 
closely  skirted  by  the  railroad,  and  on  this  side  it  immediately 


165 


:||V 


overlooks  the  salt  marshes  trihu- 
tary  to  the  Gulf.    Going  up  from 
the  marsh  we  come,  at  a  height 
of  25  feet,  to  a  distinct  and  level 
terrace,  which  is  several  hundred 
feet  wide  midway  of  the  length  of 
the  hill,  where  the  church  stands 
upon  it,  but  narrows  toward  either 
end.    The  road  running  from  the 
church  over  the  hill  rises,  at  the 
cemetery,  15  or  20  feet  to  a  well- 
defined  terrace  and  bowlder  pave- 
ment from  100  to  200  feet  wide. 
In  the  vicinity  of  the   church  the 
main     highway    between    North 
Scituate  and  Scituate  Harbor  runs 
on  the  lower  terrace .    But  farther 
west,   near    the   school  house,  it 
rises  to  the  second  terrace.    From 
the   school  house   a  private  road 
ascends  the  hill  and  brings  us  at 
a  height    of   about   30    feet   (70 
feet  above  the  marsh)  to  a  third 
terrace,  which  is  at  least  300  feet 
wide    and    thickly    strewn    with 
bowlders.       This   terrace  cannot 
be  clearly   traced   so  far  east  as 
the  hio-hway  across  the  hill ;    and 
like  both  the  others,  it  dies  out 
near  the    west  end    of  the    hill. 
Following  the  private  road  about 
30  feet    higher  (100  feet  above 
the  marsh)  we  come  to  a  broad, 
level  plain,  which  seems   at   first 


to  form  the  summit  of  the  hill;    but  on  tracing  it  eastward 


166 

across  the  highway  the  hill  is  found  to  rise  abruptly  some 
20  feet  above  the  plain  to  the  true  summit,  on  which  Hatch's 
Rock  (Fig.  23)  stands.  The  plain  is  here  at  least  300  feet 
wide  ;  and  at  all  jDoints,  save  where  it  has  been  cleared  for 
cultivation,  it  is  strewn  with  bowlders.  From  this  terrace  the 
view  seaward  is  broad  and  unobstructed  ;  and  if  the  sea  has 
ever  stood  at  this  level  since  the  drumlin  was  formed,  its 
storm  waves  must  have  broken  with  such  resistless  force  against 
the  banks  of  till  as  to  require  only  a  short  time  to  carve  this 
broad  platform. 

Several  of  the  lower  drumlins  of  this  region ,  which  show  no 
terraces— Green,  Hoop-Pole,  and  Mann  Hills — are  so  flat-top^jed 
as  almost  to  suggest  that  their  oi-iginal  summits  have  been 
worn  away  by  the  sea  or  some  agent  of  horizontal  erosion. 

The  lowest  terraces  on  the  drumlins  consist  chiefly  and  the 
upper  ones  to  a  very  limited  extent  of  modified  drift,  being, 
properly,  narrow,  fringing  sand  plains.  But  that  the  drumlins 
of  this  region  also  exhibit  many  true  terraces  of  erosion,  and 
that  these  are  in  many  cases  of  such  magnitude  as  to  profoundly 
modify  the  normal  contours  of  the  hills,  there  can  be  no  doubt 
or  question.  Concerning  the  origin  and  real  significance  of  the 
cut  terraces,  however,  there  is  still  room  for  the  widest  differ- 
ence of  opinion.  As  stated  in  the  general  description  of  the 
topography  (page  7),  I  at  first  regarded  these  terraces  as  true 
shore-lines,  and  for  the  most  part  as  marine  shores,  finding  in 
them  evidence  of  an  important  postglacial  elevation  of  this 
coast.  But  a  more  extended  and  critical  study  of  these  features 
since  the  printing  of  this  work  began  has  convinced  me  that, 
as  already  pointed  out,  they  lack  the  essential  characteristics  of 
true  shores.  They  are  not  only  deficient  in  continuity  and 
uniformity  of  level ;  but  it  is  further  impossible  to  regard  them 
as  marine  shores,  because,  with  rare  exceptions,  like  Booth 
Hill,  they  are  on  the  southwest  sides  of  the  drumlins  and  do 
not  face  the  sea  ;  or  as  the  shores  of  temporary  lakes,  because 
they  are  often,  as  in  the  Village  of  Hull  and  on  the  World's 


167 

End,  in  very  narrow  valleys  between  the  drumlins,  that  is,  in 
the  most  sheltered  spots,  where  we  would  least  expect  erosion 
by  waves.  Still  another  difficulty  in  the  way  of  regarding  them 
as  ordinary  shore-marks  is  the  fact  that  they  occur  only  on  the 
sides  of  the  drumlins,  the  scai-ps  never,  so  far  as  observed, 
changing  their  directions  so  as  to  cut  across  the  ends  of  the 
hills.  That  is,  they  notch  or  break  the  transverse,  but  never 
the  longitudinal,  profile  of  the  drumlins.  We  may  safely 
conclude,  then,  that  they  have  not  been  formed  in  or  by  standing 
water. 

It  has  been  suggested  to  me  that  the  terraces  may,  perhaps, 
be  regarded  as  original  features  of  the  drumlins,  owing  their 
formation  to  the  molding  action  of  the  moving  ice,  the  drumlins 
having  been  grooved  or  fluted  by  the  ice  on  a  large  scale.  This 
explanation  would  account  for  their  uniform  parallelism  with 
the  major  axes  of  the  drumlins  ;  and  I  am  inclined  to  believe 
that  some  of  the  less  distinct  terraces  are  really  gigantic  glacial 
grooves,  formed,  of  course,  while  the  ice  still  covered  the 
drumlins,  and  not  by  the  edge  of  the  ice,  since  it  is  probable 
that  when  the  ice  had  melted  sufficiently  to  uncover  the  tops  of 
the  drumlins  its  motion  had  practically  ceased.  Starting  with 
the  normal  type  of  drumlin,  we  recognize  departures  from  it  in 
several  directions.  Thus  some  are  lono-  and  narrow  or  rido;e- 
like,  while  in  others  the  longitudinal  and  transverse  axes  are 
nearly  equal,  and  still  others,  though  presenting  the  normal 
proportions  of  length  and  breadth,  are  unusually  flat-topped, 
like  Booth  Hill  and  many  of  the  lower  drumlins,  including 
Hoop  Pole  and  Green  Hills.  These  variations  are  probably 
original  and  due  in  part  to  the  contours  of  the  underlying- 
ledges  and  the  character  of  the  till  and  in  part  to  the  varying 
thickness  and  motion  of  the  ice  ;  and  no  reason  is  apparent  why 
the  latter  cause,  at  least,  may  not  give  us  departures  from  the 
regular  curvature  of  the  transverse  profiles. 

As  a  rule,  however,  the  terraces  are  too  deeply  incised  and 
much  too   sharply  defined  to  be  explained  in  this  way.     It  is 


168 

highly  improbable  that  in  the  case  of  a  terrace  due  to  glacial 
molding  we  should  pass  abruptly,  over  a  well-defined  edge, 
from  the  steep  erosion  slope  or  scarp  behind  the  terrace  to  the 
normal  slope  of  the  drumlin  above  the  scarp  ;  for  it  is  incon- 
ceivable that  the  erosive  action  of  a  continuous  sheet  of  ice 
could  be  so  sharply  diiFerentiated.  The  scarps  are  not  simply 
a  little  steeper  than  the  normal  slopes,  but  they  are  in  most 
cases  as  steep  as  the  material  will  maintain,  as  steep  and 
sharply  defined,  in  fact,  as  the  scarps  of  till  on  the  modern 
shore  which  have  been  protected  for  a  few  years  from  the  action 
of  the  sea. 

We  are  thus,  apparently,  forced  to  refer  the  terraces  to  the 
erosive  action  of  running  water.  It  is  improbable  that  strictly 
subglacial  streams  would  cross  the  drumlins  at  all :  and  if  they 
did,  the  natural  result  would  be  to  produce  channels  rather  than 
terraces.  The  same  would  be  true  of  superglacial  streams, 
wherever  they  cut  through  the  ice  into  the  underlying  drumlins  ; 
and  such  channels,  notching  their  summits,  have  been  observed 
on  some  of  the  drumlins  of  eastern  Massachusetts.  The  only 
alternative  view  that  now  seems  worthy  of  consideration  is  that 
which  refers  the  terraces  to  the  erosive  action  of  lateral  streams. 
When  the  ice  had  retreated  from  the  summits  of  the  higher 
drumlins  and,  presumably,  had  ceased  to  flow,  the  heat  reflected 
from  the  ground  would  naturally  melt  away  the  edge  of  the  ice 
sufficiently  to  afford  pathways  for  the  superglacial  streams,  and 
drainage  channels  for  glacial  waters  impounded  north  of  or 
behind  the  drumlins.  These  lateral  streams,  having  banks  of 
ice  on  one  side  and  till  on  the  other,  would  necessarily  erode 
the  latter  in  the  manner  indicated  by  the  existing  terraces. 
This  explanation  accounts  for  the  longitudinal  direction  of  the 
terraces  ;  for  the  observed  irregularities  in  height  and  level ;  for 
the  accumulations  of  modified  drift  sometimes  observed  at  the 
eastern  ends  of  tlie  terraces  ;  and  especially  for  the  occurrence 
of  the  terraces  chiefly  on  the  southern  slopes  of  the  drumlins, 
since  it  is  on  these  slopes  that  the  heat  of  the  sun  would  be 


169 

most  effective  in  melting  away  the  ice,  the  surface  of  which 
would  naturally  slope  toward  the  drumlins.  The  terraces  are 
thus  a  joint  product  of  glacial  and  fluvial  conditions,  contempo- 
raneous with  the  modified  drift ;  and  like  that  record  the  rapid 
waning  of  the  ice-sheet  when  it  had  ceased  to  flow  but  yet  lin- 
gered in  the  valleys.  Some  gullies  or  vertical  erosion  channels 
appear  to  have  been  formed  on  the  drumlins  at  this  time  ;  and 
probably  the  many  other  depressions  and  hollows  observed  on 
their  slopes,  which  are  often  ill-defined,  but  commonly  some- 
what saucer-shaped,  are  the  result,  chiefly,  of  local  landslips 
dating  from  this  period  when  the  till  was  first  laid  bare  and  was 
yet  unprotected  by  vegetation. 

THE    MODEEiSr    SHORE. 

When  the  sea  finally  retired  from  its  highest  postglacial 
level,  it  appears  to  have  subsided  quite  rapidly  to  a  level  some- 
what below  the  modern  beach.  Evidence  that  the  land  has  re- 
cently stood  higher  than  now  is  found  at  other  points  in  this 
region  chiefly  in  the  form  of  submerged  peat  beds  and  forests, 
and  the  former,  at  least,  would  probably  be  found  if  excava- 
tions were  made  in  the  Nantasket  marshes.  Although  the 
depressions  occupied  by  the  marshes  and  the  winding  channels 
of  Weir  River  Bay,  Strait's  Pond,  etc.,  are  clearly  submerged 
land  valleys,  as  previously  explained,  their  origin  —  the  erosion 
of  the  hard  rocks  —  undoubtedly  dates  from  the  strong  eleva- 
tion of  the  land  at  the  beginning  of  the  ice-age.  In  fact,  it  is 
not  in  land  erosion,  but  in  the  absence  of  marine  erosion  at 
certain  points  where  it  might  otherwise  be  expected  to  exist, 
that  the  evidence  sought  for  is  to  be  found.  It  is  a  fact  familiar 
to  all  that  the  drumlins  of  the  harbor  exhibit  marine  erosion 
wherever  freely  exposed  to  the  sea.  This  erosion  scarp  exists, 
in  general,  not  only  on  the  seaward,  but  also,  in  less  degree,  on 
the  landward,  sides  of  the  drumlin  islands.  Being  the  work, 
almost  exclusively,  of  the  waves,  it  varies  somewhat  with  the 


170 

breadth  and  depth  of  the  water,  and  is  most  marked  on  the  sides 
of  the  prevailing  winds.  It  is,  however,  almost  entirely 
wanting  on  the  eastern  ends  of  Thornbush  and  Telegraph  Hills, 
Little  Hog  Island,  Bumkin  Island,  the  western  part  of  White 
Head  and  Hampton  Hill,  as  well  as  the  eastern  shores  of 
World's  End  and  Planters'  Hill.  At  present,  of  course,  these 
points  are  bordered  by  shallow  water  and,  what  is  still  more 
important,  they  are  in  the  lee  of  Nantasket  Beach,  and  are  thus 
protected  from  the  ceaseless  beating  of  the  ocean  swell.  It  is 
perfectly  clear,  however,  that  this  broad  beach  has  been  formed 
since  the  existing  relations  of  the  land  and  sea  were  finally 
established  ;  and  if  the  postglacial  elevation  of  the  land  had 
ceased  when  the  present  level  was  gained,  the  sea,  sweeping 
freely  through  the  broad  passes  between  Strawberry  Hill  and 
Point  Allerton  on  the  north  and  White  Head  on  the  south,  and 
the  narrower  gaps  still  further  south,  would  inevitably  have 
worn  away  the  eastern  ends  of  the  masses  of  till  which  now 
enjoy  the  protection  of  the  beach.  Considerable  and,  at  some 
points,  ineffaceable  results  would  undoubtedly  have  been  accom- 
plished before  the  sea  succeeded  in  throwing  up  the  barrier 
beach  that  now  checks  its  own  depredations.  The  absence  of 
sensible  erosion  at  these  points  is,  however,  readily  explained, 
if  we  admit  that  the  comparatively  rapid  elevation  of  the  land 
probably  continued  until  it  stood  somewhat  higher  than  at 
present ;  and  that  a  slow  and  gradual  subsidence  has  since  been 
in  progress.  Whenever,  during  this  subsidence,  the  depres- 
sions between  the  drumlins  of  what  is  now  the  Nantasket 
Peninsula  were  brought  within  reach  of  the  waves,  barrier 
beaches  were  naturally  formed  across  them  ;  and  these  barriers 
have  kept  pace  in  their  upward  growth  with  the  subsidence,  so 
that  the  land  behind  Nantasket  Beach  has  been  continuously 
protected  from  the  action  of  the  surf.  This  view  relieves  us  of 
the  necessity  of  imagining  a  cordon  of  drumlins  outside  of  the 
present  beach  which  have  been  completely  washed  away, 
although    it  is  not  improbable  that  Harding's  Ledge  and  the 


171 

Black    Rock    Islets    are    the     foundations    of    such     vanished 
drumlins. 

It  is  obvious  to  the  most  casual  observation  not  only  that 
the  present  shore  is  strongly  marked  in  all  its  various  features, 
but  also  that  its  level  has  been  unchanged  for  a  long  time. 
This  is  seen  especially  in  the  essentially  finished  state  of  the 
salt  marshes,  which,  over  broad  and  continuous  areas,  have 
clearly  attained  their  maximum  elevation — extreme  high  tide; 
and  in  the  great  breadth  of  Nantasket  Beach.  The  front  of  the 
beach,  when  it  was  first  established,  probably  formed  a  series 
of  rather  strongly  concave  curves  connecting  tlie  drumlins,  all 
of  which  were  then  fully  exposed  to  the  surf.  The  over-wash 
of  the  beach  at  this  time  formed  what  is  now  its  rather  low  and 
marshy  inner  margin  ;  while  through  the  additions  made  by 
the  erosion  of  the  drumlins  the  line  of  the.  beach  has  been 
gradually  straightened  and  advanced  seaward  until  Strawberry 
Hill  and  the  drumlins  south  of  it,  although  their  sea-cliifs  are 
still  sharply  defined,  are  separated  and  protected  from  the 
breakers  by  a  wide  belt  of  sand  and  shingle.  The  Point 
Allerton  drumlins  are  still  within  reach  of  the  waves.  The 
little  hill  would  probably  have  been  completely  swept  away 
by  this  time  but  for  the  massive  sea-wall  which  has  been  built 
around  it.  Its  present  area  is  less  than  two  acres,  but  it 
probably  extended  once  as  far  as  the  beacon  and  had  an  esti- 
mated area  of  forty-five  acres — as  large  as  Boston  Common. 
Between  1847  and  1860  its  cliff  receded  65  feet,  equal  to  the 
loss  of  half  an  acre.  The  great  hill  has  lost  an  estimated  area 
of  fifteen  acres,  but  the  outward  growth  of  the  beach  has  now 
nearly  stopped  the  erosion.  The  finer  material  washed  from 
the  north  side  of  Telegraph  Hill  has  been  carried  to  the  west- 
ward to  build  Windmill  Point,  which  is  a  miniature  Cape  Cod. 
In  like  manner  the  waste  of  Green  Hill,  and  possibly  of  other 
drumlins  which  once  existed  in  that  vicinity,  has  formed  the 
barrier  beaches  between  Strait's  Pond  and  the  Atlantic  ;  and 
even  the  widest  of  these.  Crescent  Beach,  is  still  so  narrow  that 


172 

it  grows  chiefly  by  over-wash  during  great  storms.  The  same 
is  true  of  the  barrier  beaches  separating  Little  Harbor  from 
the  open  sea,  and  those  on  the  Scituate  shore.  The  supply  of 
fresh  material  being  now  virtually  cut  ofi^,  the  outward  growth  of 
the  beaches  must  substantially  cease  or  the  additions  consist  of 
much  finer  material,  such  as  might  be  brought  from  more 
distant  sources.  The  increasing  fineness  of  the  sand  on  Nan- 
tasket  Beach  is  apparent  from  the  fact  that  distinct  sand  dunes 
may  now  be  seen  along  the  front  of  the  beach  north  and  south  of 
Strawberry  Hill,  having  a  height  in  some  cases  of  ten  feet  or 
more,  while  they  are  entirely  wanting  on  the  back  side  of  the 
beach.  At  the  south  end  of  the  beach  the  sand  is  also  very  fine, 
and  abundant  traces  of  it  may  be  observed  at  heights  of  from  20 
to  30  feet  on  the  north  side  of  Atlantic  Hill ;  while  in  the  valley 
between  Atlantic  and  Willow  Ledge  Hills  a  considerable  bed, 
not  of  modified  drift,  but  of  the  same  fine  white  beach  sand 
may  be  traced  as  far  south  as  Atlantic  Avenue.  The  accumu- 
lation of  beach  sand  in  this  sheltered  spot  does  not  appear  to  be 
in  progress  now.  But,  although  it  could  be  used  as  an  argu- 
ment for  the  recent  elevation  of  the  coast,  I  am  disposed  to 
ascribe  it  to  strong  northerly  winds  in  past  years. 


173 


Supplement. 

During  the  time  that  has  elapsed  since  the  printing  of  this 
work  began,  some  additional  observations  have  been  made 
which,  although  not  involving  any  material  changes  in  my 
views  concerning  the  geological  structure  of  the  region,  it  is 
thought  best  to  indicate  briefly,  in  order  to  make  the  exposition 
of  facts  as  complete  as  possible. 

Besides  the  two  small  granite  quarries  on  the  Cohasset  and 
Scituate  shores,  referred  to  on  page  17,  there  is  a  third  quarry 
on  the  south  side  of  Scituate  Hill  which  has  afforded  some 
o;ood  stone  for  general  use. 

Undoubtedly  the  most  interesting  flow  of  melaphyr  in  the 
Nantasket  district  is  the  second  melaphyr  in  the  Valley  Beach 
section  described  on  page  52.  This  flow  is  of  particular 
interest,  not  alone  because  it  can  be  traced  in  one  direction  to  a 
distinct  edge,  but  also  on  account  of  the  numerous,  irregularly 
rounded,  amygdaloidal  masses  which  it  incloses.  I  have  referred 
to  these  masses  as  pseudo-bombs  ;  but,  although  well  satisfied,  as 
stated,  that  they  are  not  true  volcanic  bombs,  or  projected 
masses  of  lava,  it  is  very  gratifying  to  find  this  view  confirmed 
by  the  observations  of  Prof,  J.  D.  Dana  on  the  lavas  of  the 
Hawaiian  Islands  ;  and  I  am  grateful  to  Mr.  Geo.  P.  Merrill 
for  calling  my  attention  to  the  following  statements  by  Pro- 
fessor Dana.i 

The  aa  streams  are  remarkable  also  for  the  presence  of  lava- 
balls  of  concentric  structure  that  have  been  wrongly  called 
bombs.  These  lava-balls  ai-e  smoothish  exteriorly,  more  or  less 
rounded  and  bowlder-like,  and  vary  in  size  from  an  inch  or  less 
to  ten  feet  and  more. 

Some  of  these  lava-balls  have,  outside,  a  crust  of  hard  lava, 
and,  inside,  fragments  of  scoria ;  others  consist  of  concentric 
shells,  hard  and  scoriaceous  shells  alternating  with  one  another. 

1  Amer.  jouni.  sci.,  1887,  ser.  3,  vol.  34,  p.  364. 


174 

One  on  Hawaii,  near  Punaluu,  was  found  to  have  a  nucleus  of 
scoria  eighteen  inches  in  diameter,  and  around  this  successively  a 
stony  shell  of  three  inches,  a  scoriaceous  layer  of  from  one  to  two 
inches,  a  stony  shell  of  from  four  to  five  inches,  and  then  outside 
a  rough  lava  shell  six  inches  thick.  One  of  large  size,  broken 
open  on  one  side,  had  had  its  inside  filling  of  scoria  worked  out  by 
the  natives,  and  so  made  into  a  small  cave.  A  common  size  on 
Hawaii  is  from  three  to  five  feet  in  diameter ;  but  one  enormous 
lava-ball,  in  the  aa  field  west  of  Punaluu,  measured  24  by  12  by 
9  feet  in  its  extreme  dimensions,  and  contained  at  least  a  thou- 
sand cubic  feet.  Enough  of  its  hard  outer  shell  was  peeled  off 
to  ascertain  that  the  second  layer  was  quite  vesicular  or  scoria- 
ceous, and  the  next  layer  inside  hard  basalt  again.  These 
Hawaiian  lava-balls  lie  in  the  midst  of  the  other  blocks  of  the 
aa  stream,  proving  that  all  had  a  common  origin,  and  that  they 
are  not  projected  bombs,  and  hence  properly  not  bombs  at  all. 

Professor  Dana^  also  refers  in  the  following  terms  to  the 
paper  on  "Fragmentary  ejectamenta  of  volcanoes"  by  Dr.  H.  J. 
Johnston-Lavis'^,  who  has  studied  with  much  care  the  Vesuvian 
lavas  and  eruptions.  He  (Johnston-Lavis)  shows  that  the 
"volcanic  bombs"  of  writers  on  European  volcanoes  are  not 
bombs  any  more  than  those  of  Mt.  Loa ;  that  they  were 
not  projected  into  the  air ;  that  they  occur  scattered  over 
lava  streams  in  great  numbers  when  the  adjoining  country 
is  free  from  them,  and  occur  within  lava  streams ;  that 
they  vary  in  size  from  a  walnut  to  some  cubic  yards,  and  yet 
have  often  a  thin  shell  and  friable  nucleus  ;  that  they  occur  most 
commonly  by  far  on  lava  streams  whose  surface  is  rough  and 
scoriaceous  instead  of  corded.  He  regards  them  as  formed  of 
lapilli  that  fell  upon  the  flowing  lava,  and,  in  consequence  of  its 
forward  motion,  became  incorporated  with  it,  and  may  undergo 
partial  fusion,  but  usually  congeal  around  themselves  a  coating 
of  the  lava  in  which  they  are  involved.     Dr.   Johnston-Lavis 

'  Amer.  journ.  sci.,  1888,  ser.  3,  vol.  36,  p.  103. 
*  Proc.  geologists'  assoc,  London,  vol.  9,  no.  6. 


175 

also  points  out  that  the  ejected  blocks  of  solid  lava  are  wholly 
different  in  origin,  and  readily  distinguislied  from  the  so-called 
volcanic  bombs. 

The  red  slate  referred  to  on  page  55  as  outcropping  on  the 
edge  of  the  marsh,  west  of  the  Rockland  House,  is  probably  not 
a  tuff ;  but  it  appears  better  to  regard  it  as  a  slaty  layer  in  the 
conglomerate.  A  similar  layer  occurs  in  the  conglomerate 
south  of  Willow  Ledge  Pond  ;  and,  without  wishins;  to  suo-o-est 
the  exact  correlation  of  these  very  limited  deposits  of  finer  sedi- 
ment, it  has  appeared  best,  on  the  map,  to  indicate  the  con- 
glomerate in  the  vicinity  of  the  pond  as  extending  westward  far 
enough  to  embrace  the  second  outcrop  of  slate. 

There  appears  now  little  reason  to  doubt  that  the  large  out- 
crop of  hai'd,  slaty  rock  on  the  railroad,  west  of  the  Rockland 
House,  is  a  true  tuff;  and  it  should  probably,  in  spite  of  its 
finer  texture,  be  correlated  with  the  main  bed  of  tuff  at  the  base 
of  Atlantic  Hill. 

In  the  description,  on  page  67,  of  the  faults  crossing  Cres- 
cent Hill,  it  should  have  been  noted  in  connection  with  the  third 
fault,  which  accompanies  the  small  dike,  that  the  dike  itself 
marks  a  fault  plane  of  later  date  than  the  other  and  compensating 
with  reference  to  it.  The  two  faults  hade  in  opposite  directions, 
and  the  only  displacement  of  the  strata  actually  observable  is 
the  elevation  of  the  triangular  mass  of  melaphyr  between  the 
two  faults.  It  now  appears  probable,  also,  that  there  is  one 
more  fault  on  the  southern  slope  of  Crescent  Hill  than  has  been 
described  in  the  text ;  and  it  has  been  so  represented  on  the  map. 

There  appears  now  no  sufficient  reason  to  extend  the  fault 
which  is  supposed  to  cross  the  western  end  of  Conglomerate 
Plateau  southward  to  the  granite  and  the  boundary  fault,  as 
suggested  on  page  74  ;   and  it  has  not  been  so  drawn. 

The  position  of  the  conglomerate  overlying  the  melaphyr 
south  of  Conglomerate  Plateau  and  east  of  Round  Hill  appears 
to  be  slightly  synclinal,  probably  rising  both  to  the  east  and  west. 

It  now  appears  improbable  that  all  of  the  contacts  of  conglom- 


170 

ei'fite  and  mclaphyr  in  the  area  between  Hull  Street  and  Strait's 
Pond  are  duo  to  faults,  as  stated  on  page  75.  The  more  prob- 
able view  is  that  expressed  on  the  map,  viz.,  that  the  alternate 
contacts  are  faults.  The  conglomerate  of  Round  Hill  and  Con- 
glomerate Plateau,  with  a  oentlo  southerly  dip,  is  overlain  by 
the  second  nielaphyr  ;  and  tiie  outcrops  of  these  two  beds  are 
repeated  several  times  by  east-west  displacements  downthrovving 
to  the  north. 

Pecent  observations  have  thrown  additional  light  upon  the 
relations  of  the  dikes  of  the  central  belt  in  the  vicinity  of  Hull 
Street ;  and  nearly  all  the  dikes  in  (he  section  on  the  railroad  can 
now  be  traced  across  Conglomerate  Plateau.  Nos.  25  and  27  are 
clearly  the  same  dike  :  and  this  can  be  traced  now,  as  shown  on 
the  map,  to  the  intersection  of  Hull  Street  and  Atlantic  Avenue. 
Just  west  of  Hull  Street  it  appears  to  cross  the  course  of 
29  ;  and  what  appears  to  be  a  branch  from  the  latter  quite  clearly 
cuts  25  on  the  north  side  of  Hull  Street,  and  then  runs  parallel 
with  it.  No.  ol  also  branches  near  Hull  Street;  but  there 
seems  to  be  no  way  of  dctinitely  deciding  whether  the  dike 
which  ultimately  reaches  the  shore  of  Strait's  pond  is  29  or  31. 
No.  24,  on  East  Porphyrite  Hill,  is  clearly  a  continuation  of  38, 
on  Granite  Plateau  :  and  there  is  a  clear  path  for  it  across  Cliff 
Plateau,  although  it  does  not  appear  to  be  actually  exposed. 

For  the  elevations  of  some  of  the  drumlins  in  Cohasset  1  am 
indebted  to  Mr.  H.  W.  Nichols,  a.  student  in  the  Geological 
De[)artmcnt  of  the  Massachusetts  Institute  of  Technology  ;  and 
for  many  of  the  names  on  the  maps  I  am  under  obligations  to 
Col.  E.  T.  Bouve.  He  has  prepared  a  very  interesting  chap- 
ter on  the  ancient  land-marks  of  Hingham  and  Cohasset  for  the 
forthcoming  history  of  Hingham,  and  has  kindly  permitted 
me  to  use  the  proof-sheets. 

I  also  gladly  embrace  the  opportunity  which  this  supplement 
affords  to  acknowledge  the  important  aid  in  the  prosecution  of 
this  work  which  I  have  received  during  the  past  year  from  Mr. 
Thomas  A.  Watson.     He  has  not  only  made  possible  a  great 


177 

ccoiiotny  of  my  time  wliilo  in  tho  fl(;l(I  ;  \)\it.  Ijc  has  proved  lii/n- 
Holfan  uriiisiKilly  acute  and  accurate  obHCjrver,  vvitli  a  rea(Jy  arj(J 
clear  coinprehenHion  of  the  bearin^H  of  neaHy  every  new  fact  ; 
and  njy  work  \iuh  been  at  Korne  jjointH  rnateriaJJy  Htrcngtiiened 
by  tlie  data  and  conclusions  wliir-h  hf;  lias  \'n-c]y  placed  at  my 
disposal. 


#aasi0ttitl  papers 


OF     I'  H  E 


iv. 


GEOLOGY  OF  THE  BOSTON  BASIN 


WILLIAM    0.  CROSBY. 


VOL.  L 


PATiT   II.— HINGHAM. 


-K» — -. 


BOSTON : 

BOSTON    SOCIETY   OF   NATURAL   HISTORY 
1S94. 


GEOLOGY 


BOSTON   BASIN 


WILLIAM   0.    CROSBY. 


IN   TWO  VOLUMES. 


VOL.  I. 

PART  If.— HINGHAM. 


BOSTON  : 

BOSTON  SOCIETY  OF  NATUKAL  HISTORY. 
1894. 


HINGHAM. 

INTRODUCTION. 


The  town  of  Hingham,  Mass.,  forms  the  south  shore  of 
Boston  Plarbor  for  three  miles,  from  Weymouth  Back  River 
across  Hingham  Harbor  to  Weir  River  Bay ;  and  extends 
inland  from  six  to  seven  miles,  crossing  the  present  border  of 
the  Boston  Basin,  i.  e.,  the  southern  limit  of  the  sedimentary 
and  volcanic  rocks.  The  town  is  thus  divided  into  two  distinct 
and  very  unequal  geological  areas  ;  and,  as  the  general  map 
(PI.  1)  shows  very  clearly,  the  dividmg  line  is  quite  irregular. 
The  sedimentary  rocks  and  the  interbedded  lavas  are  limited 
almost  wholly  to  the  northwest  corner  of  the  town,  extending 
but  little  south  of  the  railroad  and  having  only  a  slight  areal 
development  east  of  the  harbor.  While  over  the  remainder  of 
the  town,  embracing  more  than  five  sixths  of  the  total  area, 
the  numerous  ledges  comprise  only  granitic  rocks  (granite, 
diorite,  and  felsite),  and  intersecting  dikes  of  diabase. 

The  granitic  area  of  Hingham  is  essentially  similar  to,  as 
well  as  continuous  with,  that  of  Cohasset  and  Nantasket  on  the 
east  and  Weymouth  and  Braintree  on  the  west,  the  entire 
South  Shore  district  being  a  unit  in  this  respect.  But,  as  was 
pointed  out  on  page  2,  the  sedimentary  and  volcanic  rocks 
borderinor  the  granite  on  the  north  and  forming  the  immediate 
shore  of  the  harbor  are  far  less  uniform  in  character  and  struc- 
ture, and  by  their  diversity  fully  warrant  or  necessitate  the 
division  of  the  South  Shore  into  several  distinct  areas,  which 
are  best  described  separately.  These  geological  areas  agree 
approximately  with  the  political  divisions,  the  geology  of  North 


180 

Hingham  contrasting  with  that  of  Weymouth  on  the  west  and 
still  more  with  that  of  the  Nantasket  areas  on  the  east.  The 
promontory  of  Kooky  Neck,  northeast  of  Planter's  Hill,  at  the 
mouth  of  Weir  River  Bay,  is,  however,  essentially  a  part  of  the 
Nantasket  area,  and  it  has  been  described  in  that  relation,  the 
true  or  natural  boundary  between  the  Hingham  and  Nantasket 
areas  being  marked  approximately  by  the  eastern  shore  of 
Hingham  Harbor. 

On  account  of  the  more  abundant  drift-deposits,  Hingham 
does  not  present  the  almost  continuous  rock  exposures  which 
characterize  the  Nantasket  area.  But,  fortunately,  a  different 
type  of  structure  prevails,  and  extended  outcrops  are  less  essen- 
tial to  the  correct  interpretation  of  the  geology.  Plication,  to 
a  large  extent,  takes  the  place  of  faulting,  the  sedimentary  and 
volcanic  rocks  being  involved  in  deep  and  almost  isoclinal 
folds  ;  and  hence,  although  the  actual  disturbance  and  displace- 
ment of  the  strata  are  probably  greater  in  Hingham  than  in 
Nantasket,  the  beds  are  more  continuous  and  more  easily 
traced  in  infrequent  outcrops.  Again,  while  Nantasket  shows 
repeated  alternations  of  beds  of  conglomerate  with  flows  of 
both  basic  and  acid  lavas  (melaphyr  and  porphyrite),  the 
porphyrite,  so  far  as  known,  is  wholly  wanting  in  Hingham 
and  the  melaphyr  is  limited  to  one  flow  or  bed  of  great  thick- 
ness ;  and  the  princij)al  problem  of  the  Nantasket  area  —  the 
identification  of  the  successive  flows  of  lava — is  really  not 
presented  at  all  to  the  students  of  Hingham  geology.  On  the 
other  hand,  while  the  sedimentary  rocks  of  the  Nantasket  area, 
south  of  the  beach,  are  almost  exclusively  conglomerate,  the 
Hingham  series  embraces  many  beds  of  sandstone  and  brownish 
slates  and  a  great  volume  of  pure  gray  slate  ;  and  the  special 
feature  of  the  geology  of  Hingham,  the  feature  in  which  it 
excels  not  only  Nantasket  but  the  entire  Boston  Basin,  is  the 
extended  series  of  alternating  beds  of  conglomerate,  sandstone, 
and  slate  which  it  presents  in  three  different  sections,  and  the 
seemingly  clear  exhibition  of  the  relations  of  this  conglomerate 


181 

series  to  the  great  slate  scries.  It  is  possible,  as  will  a[)pear 
later,  that  the  Hinghain  ledges  supplement  the  Nantjisket 
ledges,  the  basal  beds  of  conglomerate,  resting  upon  the 
granite  floor,  having  a  remarkably  fine  development  in  the 
latter,  while  the  former  afford  continuous  exposures  of  the 
upper  beds  of  conglomerate  and  the  overlying  slate. 

In  my  work  on  the  geology  of  Hingham  I  have  been  greatly 
assisted  in  various  ways  by  Mr.  T.  T.  Bouve.  In  fact,  we 
have  traversed  a  large  part  of  the  ground  together,  have  com- 
pared notes  at  nearly  every  step,  and  have  discussed  together 
all  the  interpretations  of  the  facts  occurring  to  either.  I  thus 
find  myself  wholly  unable  to  determine  in  all  cases  what  part 
of  the  work  is  really  my  own  ;  but  gratefully  acknowledge  my 
indebtedness  to  Mr.  Bouve  for  ideas  as  well  as  material 
assistance  in  the  field-work.  Mr.  Bouve  has  prepared  a 
general  account  of  the  geology  of  Hingham,  which  forms  a  part 
of  the  history  of  the  town  ;  and  the  Committee  in  charge  of 
this  work  have  kindly  permitted  the  Society  to  cooperate  in  the 
printing  of  the  three  special  maps  of  Hingham  accompanying 
this  paper. 

TOPOGRAPHY. 

Fundamentally,  or  so  far  as  the  hard  rocks  are  concerned, 
the  topography  of  Hingham  is  based  upon  the  westward  exten- 
sion of  the  broad  peneplain  which  we  have  traced  in  Cohasset 
and  Nantasket,  and  which  forms  the  entire  south  shore  of  Boston 
Harbor.  In  Hingham,  as  farther  east,  the  peneplain  is  proved 
by  the  generally  uniform  height  of  the  rocky  elevations  ;  and 
the  evidence  is  equally  clear  that  the  plain  itself  represents 
prolonged  preglacial  erosion  ;  while  the  deeply  channeled  and 
fragmentary  form  which  the  plain  now  presents  must  be 
attributed,  as  before,  mainly  to  the  comparatively  rapid  erosion 
attending  the  strong  elevation  of  the  land  at  the  beginning  of 
the  great  ice-age.      Owing  chiefly  to   the  large   proportion  of 


182 

slate  and  the  fact  that  the  conglomerate  and  sandstone  are  less 
generally  indurated  through  intimate  association  with  the 
volcanic  rocks,  the  sedimentary  rocks  of  Hingham  are  less 
prominent,  topographically,  than  those  of  Nantasket,  The 
slate,  where  occurring  in  large  bodies,  is  very  generally  eroded 
nearly  to  or  below  the  present  level  of  the  sea,  while  the  con- 
glomerate and  melaphyr  are  found  mainly  below  the  contour  of 
forty  feet,  and  never  rise  much  if  any  above  seventy  feet,  attain- 
iuo-  their  greatest  elevation  in  the  rids-e  alonij;  South  Street  in 
Hingham  Village.  The  granitic  area,  on  the  other  hand,  is,  as 
a  whole,  somewhat  more  elevated  than  in  Cohasset,  owing  prob- 
ably to  its  greater  distance  from  the  sea ;  the  average  or  normal 
elevation  of  the  ledges  south  of  the  railroad  ranging,  probably, 
from  60  or  70  feet  to  nearly  or  quite  150  feet  in  the  south  part 
of  the  town.  Although  the  hard  rocks  thus  show,  from  the 
lowest  slate  to  the  most  elevated  ledges  of  granite,  a  notable 
range  in  altitude,  there  are,  properly  speaking,  no  rock  hills 
except  such  as  have  resulted  from  the  division  of  the  peneplain 
and  its  reduction  to  a  fragnientary  condition  by  glacial  erosion. 
Especially  do  we  note  the  absence,  as  in  Cohasset  and  Nantasket, 
of  dominant  rock  hills  or  those  decidedly  overlooking  the  pene- 
plain. The  characteristic  features  of  an  ancient  topography — 
well-defined  ridges  with  culminating  summits  separating  broad 
level  valleys — are  wanting  here  and  in  Cohasset  and  through- 
out this  region  ;  but  we  find  instead  the  comparatively  narrow 
and  abrupt  valleys  and  the  broad,  plateau-like,  interstream 
surfaces  indicative  of  a  region  which,  having  been  worn  down 
to  its  base-level,  has  found  a  renewal  of  its  topographic  life  in 
a  comparatively  recent  elevation  and  is  only  now  well  started 
in  a  new  cycle  of  topographic  development.  This  topography 
is  geologically  ancient  only  in  the  sense  that  it  starts,  not  from 
an  elevated  sea-bottom,  a  virgin  land-surface,  but  from  an 
elevated  peneplain,  itself  the  last  expression  of  an  earlier 
topography  ;  and  if  the  present  stability  of  tlie  land  continues, 
the  features  of  the  modern  topography  will,   after  attaining  a 


183 

maximum  of  ruggedness  and  diversity,  be  gradually  effaced  in  a 
new  peneplain  at  the  existing  base-level. 

Although  the  average;  height  of"  the  rock  sui-face  gradually 
increases  as  vv^e  recede  from  the  shore,  and  es{)ecially  as  we  pass 
from  the  sedimentary  to  the  granitic  areas,  the  drumlins,  which 
are  planted  much  more  thickly  near  the  harbor  than  farther 
inland,  tend  to  neutralize  this  natural  erosion  gradient.  In 
Hingham,  as  in  Hull  and  Cohasset,  the  till,  so  far  as  exposed 
to  observation,  occurs  mainly  or  almost  wholly  in  the  form  of 
drumlins  ;  and,  as  the  general  map  so  plainly  shows,  no  less  than 
eleven  distinct  drumlins  rise  from  the  low  lands  bordering  the 
harbor  and  north  of  the  railroad,  including  the  two  double 
drumlins  of  World's  End  and  Planter's  Hill,  east  of  the  harbor. 
Of  the  whole  number,  fully  one  half  are  especially  typical  in 
form  and  of  nearly  the  first  magnitude  ;  and  since  they  rise 
directly  from  the  harbor  or  the  salt  marshes,  with  their  graceful 
slopes  unbroken,  save  by  an  occasional  erosion  terrace,  they 
are  more  impressive  than  the  inland  drumlins,  bordered  and 
obscured  by  elevated  ledges  and  sandplains.  Crow  Point,  at 
the  northwest  corner  of  Hingham  Harbor,  finds  its  origin  in 
a  linear  series  of  three  closely  connected  drumlins  ;  and  it  is 
noteworthy  that  the  other  four  drundins  on  the  west  side  of  the 
harbor  lie  north  of  or  upon  the  ridge  of  granite  extending  from 
the  south  end  of  the  harbor  to  Beal  Street.  South  of  the 
railroad  the  drumlins  are  much  more  scattering.  The  first  to 
attract  attention  is  the  remarkable  linear  group  or  chain  known 
as  the  Turkey  Hills,  on  the  boundary  between  Cohasset  and 
Hingham.  This  consists  (see  the  map)  of  one  main  drumlin 
of  the  first  magnitude  with  a  low  ridge  extending  southwesterly 
from  it  in  which  we  readily  recognize  three  small  and  approxi- 
mately equal  drumlins.  A  fifth  and  closely  connected  drumlin 
lies  immediately  south  of  the  main  hill ;  and  the  very  perfect 
detached  drumlin  concealed  in  the  woods  of  Turkey  Swamp 
may  be  referred  to  this  group.  Great  Hill,  south  of  West  Hing- 
ham, belies  its  name,  since  it  is  one  of  the  smaller  drumlins  of 


184 

the  town.  Its  height  does  not  exceed  125  feet,  and  it  rises  less 
than  60  feet  above  the  bordering  sandplains.  Nutty  HilL 
southwest  from  Great  Hill,  is  a  good  example  of  a  drumlin 
more  than  half  buried  in  the  sandplain.  Southwest  of  the 
Turkey  Hills  is  an  extended  area  of  elevated  rocky  Avoodland? 
lying  partly  in  Hingham  and  partly  in  Cohasset,  a  singularly 
well-preserved  section  of  the  ancient  peneplain.  But,  to  the 
west  of  this  tract,  in  the  valley  of  Weir  River,  before  we  come 
to  Prospect  Hill,  on  the  east  side  of  the  river  and  just  beyond 
the  southern  border  of  the  map,  several  rather  inconspicuous 
drumlin s  have  been  observed.  Prospect  Hill  is  well  named  ; 
for  it  is  one  of  the  largest  drumlins  in  the  Boston  Basin,  the 
hio-hest  point  in  Hingham  (218  feet)  ;  and  affords  a  wide 
prospect  in  all  directions.  The  general  map  gives  the  position, 
form,  size,  trend,  and,  so  far  as  known,  the  height  of  every 
recognized  drumlin  in  the  area  which  it  represents.^ 

The  rock  surface  of  Hingham  is  masked  not  only  by  the 
drumlins,  but  much  more  by  the  modified  drift,  which  is  far 
moi'e  abundant  than  in  Hull  and  Cohasset  and  is  broadly 
developed  in  level  plains  as  well  as  in  rounded  knolls  or 
kames  and  winding  ridges  or  eskers.  Although  the  more  or 
less  continuous  plains  of  sand  and  gravel  occurring  at  different 
and  increasing  heights  from  the  shore  southward  tend,  as  in 
Cohasset,  to  emphasize  the  plateau  character  of  the  peneplain, 
they  also  give  a  broadly  step-like  or  terrace  form  to  the  topog- 
raphy in  which  the  hard  rocks  certainly  do  not  share.  Some 
of  these  plains  gained  early  recognition,  as  witness  the  names 

1  The  elevations  in  nortliern  Hull,  including  Peildock's  and  Little  Hog  Islands,  were 
taken,  by  permission,  from  the  unpublished  charts  of  the  Harbor  Commissioners;  while 
those  in  southern  Hull,  including  Bumkin  ajid  Grape  Islands,  were  measured  with  a 
hand-level,  by  the  author.  The  chief  elevations  in  Hingham  and  Cohasset,  including 
nearly  all  those  conveniently  near  the  sea  or  salt  nuirshes,  were  very  accuratel}-  deter- 
mined by  levelling  through  the  kindness  of  Mr.  A.  E.  Woodward,  Mr.  Cyrus  C.  Babb, 
and  Mr.  H.  W.  Nichols,  formerly  students  of  civil  engineering  in  the  Massachusetts 
Institute  of  Technology;  and,  finallj',  the  elevations  in  southern  Ilinghani  ;ind  Cohas- 
set have  been,  for  the  most  part,  determined  barometrically  Ity  (he  author,  with  the 
assistance  of  Mr.  Nichols. 


185 

on  the  map  —  Lower  Plain,  Glad  Tidings  Plain,  and  Liberty 
Plain. 

Lower  Plain ,  with  a  normal  height  of  from  50  to  55  feet,  has  itw 
best  development  between  High  Street  and  the  railroad,  west  of 
Weir  River,  including  Hingham  Centre  and  the  district  about 
Great  Hill.  Toward  the  west  and  northwest  especially,  it  loses, 
very  largely,  the  character  of  a  plain,  dividing  into  irregular 
rounded  hummocks  and  winding  ridges  or  cskers  enclosing 
numerous  kettle-holes  and  small  ponds  and  bogs.  In  this 
fragmentary  form,  only  rarely  attaining  its  maximum  height, 
this  plain  has  a  particularly  interesting  development  in  the 
Hockley  district  between  West  Hingham  and  Weymouth  Back 
River.  The  fine  series  of  eskers  between  Beal's  Cove  and 
Stoddard's  Neck  should  be  referred  to  this  plain,  and  also  the 
plain  (40-50  feet)  so  well  developed  around  the  southern 
end  of  the  harbor.  Eastward,  along  the  line  of  the  railroad 
and  East  Street,  it  can  be  traced  into  Cohasset,  where  we  have 
already  recognized  it  as  the  principal  jolain  of  that  town.  There 
is,  however,  in  the  vicinity  of  the  coast,  a  distinctly  lower 
plain,  with  a  normal  height  of  from  20  to  30  feet.  This  is  very 
perfectly  developed  between  Beal  Street  and  Huit's  Cove  and 
may  be  traced  at  intervals  through  the  northwest  part  of  the 
town.  Glad  Tidings  Plain  extends  from  the  vicinity  of  High 
and  Free  Streets  south  to  the  northern  margin  of  Liberty 
Plain,  embracino-  Cushins;  and  Fullin^  Mill  Ponds  and  havino- 
a  normal  height  of  from  65  to  70  feet.  Liberty  Plain  lies  almost 
wholly  south  of  the  southern  boundary  of  the  map,  embracing 
Accord  Pond  and  extending  into  the  adjoining  towns.  It  rises 
very  abruptly  from  Glad  Tidings  Plain  to  its  normal  height  of 
130  or  135  feet. 

Each  plain  is  developed  to  some  extent  in  the  form  of  out- 
liers or  islands  on  the  next  lower  one,  and,  conversely,  kames, 
eskers,  or  limited  plateaus  rising  from  the  surface  of  either 
plain  may  be  regarded  as  representing  in  height  and  age  the 
next   higher  plain.     The  eskers  known  as   Break-neck  Hills, 


186 

beyond  the  southern  border  of  the  map,  on  Gushing  Street, 
hold  this  relation  to  Liberty  Plain,  Glad  Tidings  Plain 
may  thus  be  traced  as  far  north  as  Hobart  Street,  where 
it  forms  a  very  perfect  plateau  (Pigeon  Plain),  and  it  is 
probably  represented  by  the  high  ridges  or  eskers  (70-85  feet) 
between  Fort  Hill  and  Beal's  Cove. 

Although  the  sand  plains  testify  to  the  postglacial  flooding, 
if  not  to  an  actual  depression  of  the  land,  the  salt  marshes  and 
the  drowned  valleys  of  Weymouth  Back  River,  Hingham 
Harbor,  and  Weir  R,iver  Bay  are  a  sufficient  indication  that  the 
land  formerly  stood  higher  than  at  present,  and  that  the  exist- 
ing level  has  been  maintained  for  a  very  long  time.  It  i§ 
obvious,  then,  that  so  far  as  the  lithified  formations  are  con- 
cerned, the  relief  features  of  Hingham  may  be  summarized  as 
follows  :  Hingham  and  the  adjoining  towns  are  an  area  of  hard 
rocks  which,  in  preglacial  times,  had  been  slowly  worn  down 
nearly  to  its  base-level ;  and  such  topographic  ruggedness  as 
was  developed  in  this  old  peneplain  during  the  strong  elevation 
which  ushered  in  the  glacial  epoch  is  pretty  well  smothered  by 
the  marine  deposits  and  the  almost  continuous  mantle  of  drift. 

In  its  drainage  system  Hingham  is  almost  a  unit.  With  the 
exception  of  the  northwest  corner  of  the  town,  which  drains 
directly  into  the  harbor  and  Weymouth  Back  River,  and  the 
limited  basin  of  Fresh  River,  also  tributary  to  Weymouth  Back 
River,  nearly  the  entire  area  is  drained  by  Weir  River  and  its 
branches  ;  and  this  system  derives  but  very  little  water  from 
beyond  the  limits  of  Hingham.  It  rises  in  Valley  Swamp,  in 
Norwell,  and  in  Accord  Pond,  which  lies  in  the  three  towms  of 
Norvvell,  Abington,  and  Hingham.  The  streams  are  all  small, 
and,  although  the  total  fall  is  considerable,  the  drainage  is,  as 
a  whole,  decidedly  sluggish,  the  streams  meandering  through 
))road  level  meadows  and  swamps,  with  little  power  to  clear  out 
their  drift-encumbered  channels.  Strangely  enough,  Weir 
River  is  not  now  tributary  to  Hingham  Harbor,  but  when 
within  three  fourths  of  a  mile  of  the  head  of  the  harbor  and 


187 

one  fourth  of  :i  mile  of  the  Home  Meadows,  a  sak  rnarsli  wliich 
is  virtually  a  southward  continuation  of  the  harbor,  it  is 
abruptly  deflected  to  the  eastward  by  the  ridge  of  modified  drift 
along  East  Street,  a  spur  from  the  Lower  Plain,  and  pours  its 
waters  over  the  granite  ledges  into  Weir  liiver  Bay.  The 
facts  concerning  this  very  clear  case  of  diverted  drainage  will 
be  more  fully  presented  in  the  section  on  the  surface  geology. 
Not  only  the  streams,  but  also  the  areas  of  obstructed 
drainage,  the  ponds,  swamps,  and  marshes,  have  been  traced  out 
with  considerable  care;  and  all  these  features,  so  far  as  the 
scale  will  permit,  are  delineated  with  approximate  but  not 
uniform  accuracy  on  the  general  map.  With  the  exception  of 
Accord  Pond,  which  belongs  only  in  part  to  Hingham  and  is 
beyond  the  limits  of  the  map,  the  larger  ponds,  including 
Triphammer,  Fulling  Mill,  Gushing,  and  Foundry  Ponds,  are 
all  artificial,  originating  in  the  construction  of  dams  at  favor- 
able points  across  Weir  River  or  its  tributaries.  Round  Pond, 
on  the  lower  part  of  Fresh  River,  is  also  a  mill-pond.  Accord 
Pond,  the  principal  source  of  the  water  supply  for  Hingham 
and  Hull,  and  135  feet  above  the  sea,  owes  its  existence  to  the 
sandplain  and  connecting  esker  which  form  a  natural  dam 
directly  across  the  valley  in  which  the  pond  lies.  The  swamps, 
which  often  represent  ponds  in  course  of  extinction,  still 
embrace  occasional  limited  sheets  of  water ;  and  kettle-ponds, 
or  ponds  occupying  the  deeper  hollows  in  the  modified  drift, 
are  fairly  common  in  some  districts,  especially  over  the  broad 
area  of  modified  drift  west  of  Weir  River.  Some  of  these  are 
too  small  for  accurate  representation  on  the  map,  and  the 
majority  ai'e  wet-weather  ponds  only.  The  swamps  and 
marshes  represented  on  the  map  require  no  special  description. 
At  the  lower  end  of  every  fresh-water  marsh  or  swamp  there  is 
an  obvious  obstruction,  usually  of  modified  drift,  rarely  ledges. 
The  feeble  and  sluo-o-ish  character  of  the  streams  is  seen  in  the 
little  progress  they  have  made  in  trenching  these  drift  barriers. 
An  occasional  swamp  crosses  a  low  water-parting  and  drains  in 


188 

opposite  directions.  This  is  true  of  the  swamp  on  Rockland 
Street,  of  that  east  of  Huit's  Cove,  and  of  that  between  Hersey 
and  Central  Streets,  the  latter  being  tributary  to  both  Town 
Brook  and  Weir  River;  while  many  of  the  smaller  swamps, 
especially,  have  no  visible  drainage,  occupying  more  or  less 
irregular  kettles  or  depressions  in  the  sandplains.  Some  of 
what  Avere  originally  salt  marshes  are  now  fresh,  having  been 
reclaimed  by  the  construction  of  artificial  dikes  or  barriers. 
These  are  the  "damde  meddowes"  of  the  early  inhabitants,  and 
the  two  most  important  examples  are  those  on  Weir  River 
Bay,  south  of  Rocky  Neck,  and  west  of  Pleasant  Hill.  The 
considerable  swampy  tracts  which  have  been  reclaimed  by 
artificial  drainage  are  not  represented  on  the  map. 

The  beach  deposits,  which  are  such  a  prominent  feature  of 
Hull  and  Cohasset,  are  almost  entirely  wanting  on  the  more 
sheltered  shores  of  Hingham  ;  and  there  is  a  corresponding 
absence  of  erosion  of  the  drumlins  and  other  drift  formations. 
The  World's  End  is  joined  to  Planter's  Hill  by  a  short  barrier 
beach,  but  the  most  important  beach  of  any  kind  is  that  on  the 
north  shore,  northwest  of  Pleasant  Hill,  separating  one  of  the 
"damde  meddowes"  just  referred  to,  from  the  sea.  The  prin- 
cipal drowned  or  submerged  valley  lying  wholly  in  the  town  is 
Hingham  Harbor.  The  silting  up  of  this  basin  has,  over  the 
greater  part  of  its  area,  reached  the  eel-grass  stage  ;  and  it 
would  probably  have  become  a  salt  marsh  long  ago  if  the 
scouring  action  of  the  tides  had  been  checked  by  a  barrier  beach 
across  its  mouth.  The  same  is  true  of  Weymouth  Back  River. 
The  tidal  scouring  is  particularly  efficient  here  on  account  of 
the  contracted  form  of  the  basin. 


MAPS. 

This  part  is  illustrated  by  one  general  and  three  special 
maps.  The  general  map  is  but  a  continuation,  on  the  same 
sheet,    of  that  for    Hull     and  Cohasset,    and    lience    may    be 


189 

described  in  similar  terms.  The  priMcipal  obje(;t  of  this  iiiap  is 
to  show  the  general  distribution  and  relations  of  the  hard  rocks, 
the  surface  geology,  and  the  topography.  This  map,  as  has 
been  previously  explained,  affords,  even  in  the  absence  of 
contour-lines,  a  general  idea  of  the  relief-features.  The  out- 
lines of  the  principal  elevations  and  depressions  are  shown  in 
the  drunilins  and  the  drainage  system  ;  and  the  remainder  of 
the  surface  consists  either  of  the  sandplains  with  the  kettles  and 
kames,  or  of  ledges,  the  isolated  remnants  of  the  ancient  pene- 
plain. It  thus  requires  only  a  little  imagination  to  see  in  the 
map  the  actual  form  of  the  surface.  The  delineation  of  the 
surface  features  —  the  drumlins,  streams,  swamps,  etc. — is 
far  from  being  uniformly  accurate.  They  have  been  traced  out 
with  the  greatest  care  in  the  northern  part  of  the  town,  and  in 
general  where  the  country  is  most  open  and  accessible.  Some 
of  the  wooded  areas,  remote  from  the  streets  and  destitute  of 
land-marks,  have  been  only  very  imperfectly  explored.  This 
is  especially  true  of  the  large  tract  of  rocky  woodland  lying 
between  King  Street  and  Scituate  Pond  in  Cohasset  and  Union 
Street  and  Beechwood  River  in  Hingham,  embracing  more 
than  four  square  miles  and  crossed  by  only  one  road  —  the 
most  complete  wilderness  in  the  Boston  Basin  outside  of  the 
Blue  Hills.  For  many  of  the  names  of  the  hills,  streams,  and 
other  natural  features  which  appear  upon  this  map  I  am  espe- 
cially indebted  to  Mr.  Edward  T.  Bouve's  most  interesting- 
contribution  to  the  town  history  on  the  "ancient  landmarks"  of 
Hingham  and  Cohasset.  The  special  maps,  having  been 
printed  in  co-operation  with  the  Town  of  Hingham  and  before 
the  plan  of  this  work  was  fully  decided  upon,  do  not  agree  in 
scale  and  topographic  detail  with  the  special  map  of  Nantasket, 
the  most  notable  difference  being  the  absence  of  contour-lines 
and  the  outlines  of  the  swamps  and  marshes,  and  the  repre- 
sentation (in  black  characters)  of  the  actual  outcrops,  as  well 
as  the  theoretical  distribution  (in  colors)  of  the  sedimentary 
and  volcanic  rocks.     These  special  maps  represent   the  three 


190 

sections  of  special  geological  interest,  exclusive  of  Rocky  Neck  ; 
and,  territorially,  they  are  continuous  from  Crow  Point  via 
Huit's  Cove,  Beal's  Cove,  and  West  Hingham  to  the  southern 
end  of  the  harbor. 


THE  GRANITIC  ROCKS  OF  HINGHAM. 

On  account  of  the  more  continuous  development  of  the  sand- 
plains,  the  outcrops  of  the  granitic  rocks  over  the  greater  part 
of  Hingham  are  less  frequent  than  in  Cohasset ;  and  nowhere 
in  Hingham  do  we  find  any  section  of  these  rocks  comparable 
in  clearness  and  continuity  with  that  along  the  Cohasset  shore. 
There  can  be  no  question,  however,  that  the  granitic  rocks  of 
Hingham  are  essentially  similar  in  most  respects  to  those  of 
Cohasset.  They  embrace  in  the  same  general  proportions  the 
three  principal  types — granite,  diorite,  and  felsite.  The  diorite 
is,  in  every  case,  clearly  the  oldest  rock,  its  relations' to  the 
granite  being  the  same  as  in  Cohasset.  Considerable  attention 
has  been  given,  especially  by  Mr.  Bouve,  to  tracing  the 
distribution  of  the  diorite,  with  the  object,  originally,  of  repre- 
senting it  by  a  separate  color  on  the  map  ;  but  tliis  lias  been 
found  wholly  impi-acticable.  Its  relations  to  the  granite  are  so 
intimate  and  intricate  that,  in  the  absence  of  perfect  and  con- 
tinuous exposures  and  a  map  on  an  inconveniently  large  scale, 
no  lines  could  be  drawn  which  would  not  include  considerable 
granite  or  exclude  a  large  pai't  of  the  diorite.  We  have  learned, 
however,  as  the  result  of  this  attempt  to  map  the  diorite,  that, 
while  occasional  inclusions  of  diorite  (often  very  small  inclusions, 
it  is  true),  may  be  observed  in  almost  every  good  exposure  of 
granite  in  Hingham,  this  rock  occurs  abundantly  only  in  a  fairly 
well-defined  east-west  belt  near  the  northern  edge  of  the  granite. 
The  limits  of  this  belt  are  roughly  indicated  by  the  distribution 
of  the  V-shaped  characters  on  the  general  map.^     Commencing 

1  The  diorite  of   Cohasset  is  siinilavly  represented;  but  tliis  feature  of  the  map  had 
not  been  adopted  when  tlie  Cohasset  text  Avas  printeil. 


191 

at  the  Cohasset  boundary,  in  tlie  vicinity  of"  tlic  railroad,  I  he 
diorite  is  observed  most  abundantly  in  the  ledi«,(!s  aloiio-  Hull 
Street,  Weir  River  Lane,  Kilby  and  East  Streets  ;  and  tiicn, 
after  a  partial  gap  of  more  than  a  rnile  where  the  ledges  of  all 
kinds  are  very  generally  concealed  by  the  Lower  Plain,  north- 
west of  Fort  Hill,  near  West  Hingham,  for  a  short  distance, 
until  all  the  ledges  are  again  blotted  out  by  the  modified  drift. 
It  should  not  be  supposed,  however,  that  these  marks  represent, 
in  every  case,  continuous  ledges  of  diorite,  for  the  most  of  the 
ledges  are  of  a  mixed  character,  granite  penetrating  or  inclosing 
diorite.  This  irregular  belt,  it  Avill  be  observed,  is  in  about  the 
same  latitude  as  the  principal  occurrences  of  diorite  in  Cohasset, 
and  on  the  same  east-west  line  are  found  the  chief  outcrops  of 
diorite  in  Weymouth  ;  but  south  of  this  belt  we  have  observed 
no  important  or  notable  masses  of  diorite.  In  lithological 
character  the  diorite  of  Hingham  is  scarcely  to  be  distinguished 
from  that  of  Cohasset,  showing  similar  variations,  except  that 
it  nowhere  exhibits  a  distinct  flow-structure.  It  is  usually 
finely  crystalline  and  dark  colored  ;  but  occasionally  it  is 
coarser,  with  the  hornblende  either  very  clearly  and  prominently 
developed  or  mainly  wanting,  giving  a  light-colored,  feldspathic 
variety  which  might  be  readily  mistaken  for  syenite  or  even 
granite.  Epidote  is,  as  usual,  the  principal  secondary  mineral, 
occurring  chiefly  as  narrow  and  irregular  segregations  and 
veinlets,  especially  along  the  joint  cracks. 

Although  the  granite  of  Hingham  almost  certainly  embraces 
the  three  distinct  types — distinct  in  age  and  character — observed 
on  the  Cohasset  shore,  few  ledges  have  been  observed  in  which 
the  relations  of  the  two  older  types — the  basic  hornblendic 
o;ranite  of  medium  texture  and  the  coarse  acid  granite — are 
clearly  exposed.  Perhaps  the  most  favorable  exposure  of  this 
kind  is  that  afforded  by  Button  Island,  in  Hingham  Harbor. 
The  bedrock  of  the  island  is  wholly  granite,  with  only  a  thin 
covering  of  drift.  The  granite  is  chiefly  an  unusually  coarse 
and  distinct  example  of  the  second  or  more  acid  type,  enclosing 


192 

small  fragments  of  fine-grained  diorite.  But  it  also  encloses 
on  the  south  and  west  shores  much  laro-er  masses  of  what 
appears  at  first  sight  to  be  the  older  and  more  basic  granite  of 
medium  texture.  The  contacts  are,  however,  somewhat  ambigu- 
ous, appearing  to  favor  the  view  that  the  finer-grained  granite 
is  the  newer,  and  hence  to  be  correlated  with  the  third  or 
microgranitic  type.  By  far  the  greater  part  of  the  granite 
evidently  belongs  to  the  newer  variety,  the  typical,  sparingly 
hornblendic,  usually  coarsely  and  distinctly  crystalline,  gray  to 
pink  or  red  granite  of  the  South  Shore  district.  This  is 
especially  true  where  the  granite  is  not  moat  intimately  associated 
with  the  diorite.  The  hornblendic  element,  as  in  Cohasset,  is 
very  generally  replaced  partially,  sometimes  wholly,  by  mica 
(chiefly  biotlte) .  This  is  seen  very  clearly  in  the  abandoned 
quarry  on  Long  Bridge  Lane,  which  is,  with  one  exception,  the 
only  point  in  Hingham  where  the  granite,  or  any  rock,  has 
been  systematically  quarried.  The  feldspar  (orthoclase)  is 
commonly  gray  or  pink,  more  rarely  green,  at  least  in  part; 
and,  very  locally,  as  in  some  of  the  ledges  along  Thaxter 
Street,  it  is  porphyritically  developed. 

Undoubtedly,  one  of  the  most  interesting  exposures  of 
granite  in  Hingham  is  the  small  quarry  about  one  third  of  a 
mile  northeast  of  Abington  Street,  in  the  southwest  corner  of 
the  town,  and  nearly  two  miles  beyond  the  limits  of  the  map. 
On  going  in  from  Abington  Street  by  the  quarry  road,  the 
ordinary,  coarse,  biotite  granite  gives  way,  with  apparent 
abruptness,  to  a  finely  crystalline,  homogeneous,  light  gray 
variety,  which  is  rich  in  quartz,  and  contains  but  little  of  an 3^ 
dark  accessory.  The  other  limits  of  the  fine  granite  were  not 
observed,  but  it  appears  to  cover  a  considerable  area.  The 
feature  of  particular  interest  which  it  presents  is  the  remarkably 
perfect  parallel  joint-structure.  In  fact,  it  is  the  jointing  that 
o-ives  the  granite  its  value  as  a  quarry-stone.  The  parallel 
jointing  is  traceable  over  an  area  at  least  500  feet  long  north 
and  south,  and  half  as  broad ;  but  it  is  most   perfectly  devel- 


193 


oped  in  the  more  northerly  and  hirger  of  the  two  small  quarries. 
The  joints  of  one  system  are  phenomenally  perfect,  close,  and 
parallel,  dividing  the  granite  into  almost  absolutely  plane  sheets 
varying  in  thickness  from  half  an  inch  to  two  feet  or  more,  hut 
mostly  from  four  to  twelve  inches.  The  trend  of  the  joints  is 
approximately  N.  S.  (S.  5°W.),  and  the  hade  about  vertical 
(W.  0°-3°.)  The  granite  thus,  in  a  general  view,  simulates  a 
bedded  rock,  like  sandstone,  very  closely  ;  and  the  surfaces  of 
the  sheets  are  so  plane  and  smooth  and  the  grain  so  perfect, 
that  blocks  suitable  for  building  purposes  are  obtained  with 
remarkable  ease,  the  joint-structure  serving  the  same  useful 
purpose  in  this  granite  as  in  the  Roxbury  puddingstone.  It  is, 
of  course,  needless  to  dwell  upon  the  obvious  support  which 
this  example  lends  to  the  earthquake  theory  of  parallel 
jointing.^ 

Irregular  dikes  and  masses  of  the  more  finely  crystalline  and 
micro-crystalline  granite,  and  of  felsite,  are  frequently 
observed  cutting  through  the  coarser  granites  and  also  the 
diorite,  precisely  as  in  Cohasset.  Perhaps  the  most  interesting 
exposure  of  these  older  rocks  is  that  on  the  summit  of  Fort 
Hill,  where  the  diorite  has  been  laid  bare  in  grading  the  street. 
The  diorite  is  very  sharply  and  clearly  intersected  by  numerous 
narrow,  branching  dikes  of  a  finely  crystalline  pinkish  granite. 
The  dikes  are  broken  by  slight  faults  and  enclose  angular 
frao^ments  of  the  diorite.^  On  the  north  side  of  East  Street, 
between  Andrew  Heights  and  Kilby  Street,  the  diorite  is  cut 
by  an  irregular  dike  of  a  gray  felsite  from  eight  to  ten  inches 
in  width.  This  proves  on  analysis  to  be  a  basic  felsite,  agree- 
ing in  composition  better  with  syenite  than  granite.  At  many 
points  on  Andrew  Heights  and  along  Kilby,  East,   and  other 

'  Proc.  B.  S.  ISr.  H.,  V.  22,  p.  72-85. 

*  Since  the  above  paragraph  was  written,  this  beautiful  and  instructive  exhibition 
of  the  relations  of  the  granite  and  diorite  has  been  entirely  and  ruthlessly  obliterated 
by  the  farther  grading  of  the  street ;  a  fact  which  every  student  of  Hingham  geology 
will  sincerely  regret. 

OCCAS.  PAPERS.   B.   S.  N.   H.   IV.   13. 


194 

streets  as  far  as  the  Cohasset  boundary,  the  ordmary  coarse 
granite  can,  in  spite  of  the  lichens,  be  seen  to  form  irregular 
dikes  in,  or  to  enclose  angular  fragments  and  masses  of,  the 
diorite.  The  greenish  gray  quartz-porphyry  obscurely  exposed 
in  South  Street,  east  of  Hersey  Street,  in  Hingham  Village,  is 
probably  a  dike  in  the  granite  ,  but  considerable  digging  would 
be  required  to  prove  it.  In  the  rear  of  the  first  house  on 
Lincoln  Street  and  Fountain  Square,  north  of  the  Unitarian 
Church,  is  a  mass,  clearly  eruptive  through  the  coarse  granite, 
of  a  compact,  flinty,  purplish  felsite,  which  has  been  proved  by 
analysis  to  agree  with  the  granite  in  composition. 

The  felsite  of  Hingham  is  not  wholly  intrusive,  or  in  the 
form  of  dikes.  The  gray  felsite,  on  the  north  side  of  Beal 
Street,  at  the  Avestern  end  of  the  granite,  is  quite  probably 
part  of  a  surface  flow  ;  and  the  beautiful  red  felsite  occurring 
so  plentifully  in  the  form  of  bowlders  in  the  vicinity  of  Lincoln 
and  Thaxter  Streets  is  unquestionably  effusive.  The  former 
varies  from  a  slightly  greenish  gray  or  white  to  a  pinkish  tint, 
and  encloses  many  more  or  less  distinct  fragments  of  similar 
or  darker  felsite  and  an  occasional  fragment  of  granite.  The 
breccia  structure  thus  resulting  is  so  marked  in  a  portion  of 
the  rock  that  it  was  at  first  mistaken  for  conglomerate  ;  and 
the  isolated  elliptical  area  on  this  part  of  the  map  marked  and 
colored  as  conglomerate  is  really  felsite.  The  exposures  of 
the  felsite  are  not  sufficient  to  show  clearly  its  relations  to 
either  the  granite,  which  it  seems  to  overlie,  or  the  melaphyr, 
which  probably  once  covered  it. 

The  red  felsite  of  Thaxter  and  Lincoln  Streets  is  by  far  the 
most  attractive  of  all  the  older  rocks  of  Hingham.  Li  fact, 
it  is  the  most  beautiful  variety  of  felsite  in  eastern  Massa- 
chusetts ;  and  it  is  also  unique  in  being  the  only  felsite  in 
the  entire  south  shore  district  which  is  certainly  effusive. 
Unfortunately,  it  occurs  in  an  area  which  is  covered  almost 
continuously  by  salt  marshes  and  drundins,  and  there  is  prac- 
tically no  opportunity  to  study  it  171  situ.     This  rock,   which 


has  attracted  attention  since  tlie  first  settlement  of  the  town, 
was  described  by  Prof.  Edward  Hitchcock,  in  his  final  rejjort 
upon  the  geology  of  Massachusetts,  under  the  head  of  por- 
phyry, as  occurring  in  ridges  a  little  north  of  the  village. 
Such  ridges  do  not  exist  now ;  and  the  statement  of  this 
accurate  observer  should,  perhaps,  be  interpreted  as  referring 
to  ledges  along  the  line  of  Lincoln  Street  which  have  probably 
been  effaced  by  the  subsequent  grading  of  the  street.  At  the 
present  time,  the  only  actual  exposure  of  the  rock  in  siiu  is 
an  obscure  outcrop  in  the  roadway  at  the  junction  of  Crow 
Point  Lane  and  Downer  Avenue.  This  was  formerly  a  some- 
what protruding  ledge,  but  it  does  not  now  rise  above  the 
level  of  the  street,  and  might  easily  be  overlooked.  It  is  in 
an  area  colored  as  melaphyr  on  the  map,  and  it  is  regarded 
as  a  boss  of  the  felsite  projecting,  as  the  result  of  erosion, 
through  the  sheet  of  melaphyr.  The  character  for  felsite  also 
appears  on  the  map  (PI.  9)  behind  Mr.  Bradley's  barn,  west  of 
Thaxter  Street  arid  south  of  Lincoln  Street.  No  actual  ledge 
of  felsite  has  been  observed  here,  but  a  number  of  excep- 
tionally large,  angular  bowlders  in  the  drift,  some  of  which 
can  be  seen  on  the  surface,  while  others  were  exposed  in  a 
temporary  excavation,  suggest  that  the  ledge  is  close  by. 
Immediately  north  and  west  of  this  point  there  are  outcrops 
of  melaphyr,  and  no  bowlders  of  felsite  can  be  seen.  ITence 
the  northern  boundary  of  the  felsite  may  be  regarded  as 
accurately  located  at  this  point.  But  the  boundary  between 
it  and  the  granite  on  the  south  is  a  mere  matter  of  conjecture, 
although  the  line  on  the  special  map  extending  from  Broad 
Cove,  south  of  the  solitary  house  on  Lincoln  Street,  across 
Bradley's  Hill  and  Thaxter  Street,  is  probably  an  approxima- 
tion to  it.  The  distribution  of  the  bowlders  certainly  suggests 
an  east-w^est  belt  of  the  red  felsite  along  the  northern  edge  of 
the  granite,  and  passing  under  Bradley's  Hill.  That  it 
extends  east  under  Broad  Cove  is  at  least  probable ;  and 
that  it    extends   west    beneath    Squirrel    Hill    we    have    some 


196 

evidence  in  the  ledge  of  felsite  at  the  western  base  of  this  hill 
and  some  500  or  600  feet  south  of  Lincoln  Street.  At  this 
point,  however,  the  felsite,  although  still  of  a  reddish  or 
purplish  color,  is  much  more  compact  and  homogeneous. 

Although  there  is  no  opportunity  to  observe  its  relations  to 
other  rocks,  the  eifusive  or  volcanic  nature  of  the  red  felsite 
forming  the  numerous  bowlders  east  of  Squirrel  Hill  and  the 
solitary  ledge  already  referred  to,  on  Downer  Avenue,  is 
abundantly  proved  by  its  structural  features.  It  exhibits 
throughout  a  distinct  but  not  conspicuous  banding  or  flow- 
structure,  which  is  usually  rather  fine,  but  sometimes  quite 
coarse  and  often  somewhat  contorted,  discontinuous,  or  other- 
wise irreo-ular.  The  confused  and  irreo-ular  character  of  much 
of  the  banding  is  due  in  part  to  the  enclosure  of  angular  frag- 
ments of  the  same  or  a  very  similar  felsite.  The  enclosed 
fragments  vary  from  a  small  fraction  of  an  inch  to  several  inches 
in  diameter,  and  are  very  irregularly  distributed,  so  that  while 
the  greater  part  of  the  felsite  is  comparatively  free  from  them, 
they  vary  in  the  remainder  from  thinly  scattering  to  densely 
crowded,  occasional  masses  of  the  rock  being  packed  so  full  of 
fragments  that  the  banding  is  completely  obliterated  and  it 
closely  resembles  an  ordinary  breccia.  This  fragmental  character 
of  the  felsite  has  suggested  to  several  observers,  including  the 
present  writer,  that  it  is  a  metamorphic  conglomerate.  But 
having  observed  precisely  similar  structures  (banding  and 
brecciation)  in  modern  obsidian,  I  have  been  for  a  long  time 
thoroughly  convinced  that  this  metamorphic  theory  is  untenable, 
and  that  the  felsite  is  a  true  volcanic  rock,  a  devitrified  obsidian. 
Among  the  arguments  against  its  sedimentary  origin  are  the 
fxcts  that  the  fragments  are  all  of  the  same  kind  of  rock  ;  that 
they  are  never  assorted  or  show  in  any  way  the  action  of  water  ; 
and  that  the  felsite  appears,  as  has  been  proved  b}^  analyses  in 
other  cases,  to  be  chemically  essentially  intact  and  homogeneous, 
still  retaining  in  every  part  the  full  [)r()portion  of  alkalies 
required  for  an  acid  feldspar,  which  would  be  very  unusual  in  a 


197 

clastic  rock.  The  fact  that  the  fvag:ments  or  so-called  pehhles 
show  a  gradation  in  distinctness  from  those  that  are  very  sharply 
defined  to  those  that  are  perfectly  blended  with  the  enclosing 
felsite,  is  only  what  would  be  expected  when  fragtnents  of 
glass  (obsidian)  are  enveloped  in  melted  glass.  The  only 
particularly  obvious  indications  of  chemical  change  in  the  felsite 
since  its  eruption  are,  first,  the  red  color,  which  may  be 
original,  but  is  probably  due  in  part  at  least  to  the  peroxidation 
of  iron  during  devitrification  ;  and,  secondly,  the  occurrence  in 
the  rock  of  inconspicuous  streaks  and  masses  of  quartz,  either 
vitreous,  chalccdonic,  or  jaspery, — silica  i-eplacing  the  less 
stable  portions  of  the  glass.  Red  felsites  are  not  uncommon  in 
eastern  Massachusetts  ;  but  the  only  occurrence  at  all  closely 
resembling  the  Hingham  felsite  is  the  red  felsite  near  the 
Neponset  River,  in  Hyde  Park.  Structurally  they  are  strik- 
ingly similar,  except  that  the  banding  of  the  Hyde  Park  variety 
is  rather  coarser,  and  it  is  more  generally  brecciated  ;  but  the 
Hingham  felsite,  with  its  deeper  and  brighter  color  is  the  more 
beautiful  of  the  two. 


THE  BEDDED  ROCKS  OF  NORTHERN  HINGHAM. 

GENERAL    RELATIONS    AND    ORIGIN. 

It  is  unnecessary  to  repeat  what  has  been  stated  under  this 
heading  for  the  Nantasket  area ;  but  the  one  topic  may  be 
regarded  as  supplementary  to  the  other.  The  sedimentary 
rocks  of  Hingham  present,  as  previously  stated,  a  greater  variety 
than  those  of  Nantasket,  embracing  besides  the  conglomerate 
many  intercalated  beds  of  sandstone  and  slate  of  both  brownish 
and  greenish  tints,  and  the  great  body  of  gray  slate  overlying 
this  conglomerate  series.  The  contemporaneous  volcanic  rocks 
or  effusive  lavas,  on  the.  other  hand,  are  less  varied,  including 
no  acid  or  fragmental  varieties,  but  consisting  wholly  of  rather 


198 

typical  melaphyr.  Furthermore,  the  melaphyr  appears  to  be 
limited  to  one  heavy  bed  near  the  base  of  the  conglomerate 
sei'ies  ;  and  the  repeated  alternations  of  sediments  and  lavas  so 
characteristic  of  the  Nantasket  area  are  wanting  in  Hingham. 
The  stratigraphic  contrast  is  so  great  that  a  satisfactory 
correlation  is  impossible  with  the  data  now  at  command.  It 
appears  probable,  however,  as  stated  in  the  introduction,  that 
the  two  areas  are  complementary,  the  Hingham  series  beginning 
with  approximately  the  same  beds  with  which  the  Nantasket 
series  ends.  This  view  naturally  leads  us  to  regard  the  melaphyr 
of  Hingham  as  probably  equivalent  to  the  great  bed  of  melaphyr 
near  the  top  of  the  Nantasket  section  ;  and  certainly  the  thick 
beds  are  far  more  likely  than  the  thinner  ones  to  extend  over 
considerable  areas. 

Although  the  great  slate  series,  consisting  of  gray  slate  with- 
out intercalated  sandstone  and  conglomerate,  appears  to  cover 
a  considerable  area  in  the  northwestern  corner  of  Hingham,  it 
is  well  exposed  only  on  the  shores  of  Huit's  Cove  and  Beal's 
Cove.  These  exposures,  however,  are  sufficient  to  show,  first, 
that  the  slate  is  certainly  conformable  with  and  essentially  a 
continuation  of  the  conglomerate  series  ;  and,  secondly,  tliat  it 
probably  overlies  these  coarser  rocks.  But  the  a])parent  absence 
of  fossils  in  the  slate  and  the  entire  lack  of  outcrops  connecting 
the  Hingham  beds  with  tlie  fossiliferous  Cambrian  slates  of 
Weymouth  and  Braintree  leave  us  no  certain  clue  to  the  geo- 
logical age  of  the  Hingham  strata,  except  what  is  afforded  by 
their  relations  to  the  granitic  rocks  and  the  composition  of  the 
conglomerate.  Fortunately,  however,  this  evidence,  which 
will  be  fully  set  forth  in  a  later  section,  is  sufficiently  clear  to 
permit  us  to  say  provisionally  that,  while  the  granite  is  certainly 
newer  than  tlie  Paradoxides  beds  of  Braintree,  the  conglomerate 
scries  of  Hingham,  like  that  of  Nantasket,  and  lience  the  over 
lying  slate,  must  be  newer  than  the  granite. 

The  history  of  the  Hingham  strata  may,  then,  be  outlined 
as   follows :    subsequently  to    the   deposition    and    plication   of 


199 

the  Paradoxides  slates,  subsequently  to  tlic  eruption  (1ii()ii;j;]i 
them  in  succession  of  the  granitic  rocks, — diorite,  granite,  and 
t'elsite, — and  subsequently  to  the  extensive  erosion  which  has  so 
largely  swept  away  these  ancient  sediments,  began  the  pro- 
gressive subsidence,  accompanied  in  its  earlier  stages  by 
volcanic  activity,  during  which  were  formed  the  conglomerate 
and  interbedded  lavas  of  the  Nantasket  area.  Only  the  latest 
bed  of  lava  poured  out  in  this  part  of  the  basin  is  exposed  in 
Hingham,  west  of  Rocky  Neck;  but  the  cessation  of  the 
igneous  outbursts  was  folio  w^ed  by  the  o-reat  con  "glomerate 
series  of  Hingham,  with  its  mterstratified  beds  of  sandstone 
and  slate  ;  and  the  variable  character  of  the  strata  is  a  plain 
indication  that  the  physical  conditions  were  far  from  uniform 
during  this  period.  The  conglomerate  occurs  mainly  in  beds 
from  twenty  to  eighty  feet  thick  alternating  with,  usually, 
thinner  beds  of  gray  sandstone  and  red  or  gray  slate,  for  a 
total  thickness  of  nearly  one  thousand  feet.  In  fact,  no  other 
part  of  the  Boston  Basin  affords  such  clear  and  abundant  evi- 
dence of:  (1)  frequent  changes  from  beach  or  shallow  water 
deposits  (conglomerate  and  sandstone)  to  those  formed  in 
deep  and  quiet  water  (slate),  and  vice  versa;  and  (2)  the 
oscillations  of  the  earth's  crust,  upon  which  these  changes 
usually  depend.  This  intermittent  conglomerate  series  appears 
to  pass  somewhat  gradually  and  with  perfect  conformity  up 
into  the  great  slate  series,  which  consists  throughout  of  a  fine, 
dark  gray  slate,  and  has  an  apparent  thickness  of  at  least  one 
thousand  feet,  without  any  interstratified  beds  of  coarser 
material.  This  is  sufficient  to  prove  that  the  oscillations  of 
level  attending  the  formation  of  the  conglomerate  series  were 
followed  by  a  profound  and  prolonged  subsidence  ;  for  during 
all  the  time  when  the  slate  was  being  slowly  and  quietly  depos- 
ited, Hingham  must  have  formed  the  floor  of  a  comparatively 
deep  ocean.  But  this  tranquility  could  not  last  forever  ;  for 
the  subterranean  forces  were  slowly  gathering  strength,  and 
the  formation  of  the  slate  was  undoubtedly  terminated  by  the 


200 

advent  of  an  epoch  of  severe  compression  of  the  earth's  crust 
in  this  region,  in  consequence  of  which  the  sedimentary  rocks 
were  elevated  to  form  dry  lard,  folded,  faulted,  and  injected 
by  dikes  of  diabase.  During  all  the  long  ages  since  this 
geographical  revolution,  Hingham  has  been  mainly,  if  not 
continuously,  a  land  area ;  and  the  slate,  together  with  the 
underlying  conglomerate,  has  sutFered  enormous  erosion. 
These  rocks  have  thus  been  completely  removed  from  large 
areas  of  granite  which  they  once  covered ;  and  they  are 
preserved  to  us  now  only  where  they  were  most  deeply 
folded  or  faulted  down  between  the  granitic  masses,  and  thus 
protected  from  erosion. 

GENERAL    STRUCTURE    OF    NORTHERN    HINGHAM. 

The  eastern  shore  of  Hingham  Harbor  is  not  only  the 
natural  boundary  line  between  the  geological  districts  of 
Nantasket  and  northern  Hingham,  but  it  probably  marks 
the  position,  as  shown  on  the  general  map,  of  one  of  the 
great  transverse  faults  of  the  South  Shore  ;  and  it  certainly 
corresponds,  as  already  explained,  to  a  very  important  con- 
trast in  geological  structure,  plication  being  as  characteristic 
of  the  Hingliam  area  as  liudting  is  of  the  Nantasket  area. 
As  the  general  map  so  clearly  shows,  the  key  to  the  structure 
of  the  volcanic  and  sedimentary  rocks  of  Hingham  is  the 
oblong  area  of  granite  and  felsite  lying  north  of  the  railroad 
and  Beal  Street,  and  bearing  the  three  drumlins  of  Bradley's, 
Squirrel,  and  Baker's  Hills.  The  general  position  of  this 
mass  is  unquestionably  anticlinal.  This  is  most  obvious  at 
the  western  extremity,  where  the  melaphyr  and  the  sedi- 
mentary strata  curve  around  the  granite  and  dip  away  from 
it  on  both  sides.  Southward  from  this  point,  between  Beal 
Street  and  Beal's  Cove,  the  structure  is  monoclinal ;  and  the 
ledges  afford  a  nearly  continuous  section  across  the  entire 
conss'lomerate  series  and  a  considerable  thickness  of  the  over- 


201 

lyini^  slate,  the  hitter  undoubtedly  marking  the  po.sitiijii  of  a 
synclinal  axis  ;  but  the  south  side  of  the  syn(;line  is  prohal)ly 
cut  off  by  the  boundary  fault,  for  we  seem  to  pass  abruptly 
from  the  slate  to  the  granite.  In  the  vicinity  of  Hockley 
Lane  a  transverse  fault  appears  to  separate  this  normal  suc- 
cession of  the  strata  from  an  inverted  succession  which  extends 
thence  eastward  to  Main  Street  or  beyond.  The  melaphyr  is 
now  on  the  south  side,  overlying  the  conglomerate  ;  and  these 
stratified  rocks,  although  occupying  a  synclinal  position 
between  the  granite  on  the  north  and  south,  are,  we  must 
suppose,  bounded  on  both  sides  by  important  dislocations  and 
terminated  on  the  east  by  the  great  faidt  along  the  east  side 
of  Hingham  Harbor. 

Northwest  from  the  western  extremity  of  the  granite  axis,  a 
very  steep,  narrow,  and  broken  monocline  separates  the  granite 
from  the  great  trough  holding  the  main  body  of  slate.  This 
faulted  monocline  is  marked  by  a  second  band  of  melaphyr, 
which  broadens  toward  the  northeast,  forming  the  large  quad- 
rangular area  of  this  rock  east  of  Huit's  Cove.  This  is  the 
largest  exposure  of  melaphyr  in  Hingham,  and,  although  it 
appears  to  be  bounded  on  all  sides  by  downthrow  faults,  the 
quaquaversal  dips  of  the  bordering  strata  show  that,  in  a  lesser 
degree,  it  is  essentially  similar  in  its  structural  relations  to  the 
granitic  area.  On  the  west  side,  the  upper  bed  of  conglom- 
erate and  the  slate  are  seen  to  dip  away  from  the  melaphyr. 
On  the  north,  the  downthrow  of  the  sedimentary  rocks  is  suffi- 
cient to  conceal  the  conglomerate,  and  the  slate  lies  with 
conformable  strike  against  the  melaphyr.  On  the  south,  the 
narrow  monocline  separating  this  body  of  melaphyr  from  the 
granite  broadens  somewhat,  until  it  reaches  the  fault  at  the 
northwest  end  of  Sqiiirrel  Hill,  where  it  changes,  perhaps 
abruptly,  to  a  broad  shallow  syncline  of  melaphyr  and  con- 
glomerate on  the  south,  separated  by  a  strike  fault  from  a 
gentle  southerly  m.onocline  of  conglomerate  and  sandstone  on 
the  north.     These  features  probably  extend  east  under  Broad 


202 

Cove  and  Otis  Hill ;  and  the  monocline  can  be  clearly  traced 
still  farther  in  the  ledges  of  Melville  Garden  and  in  the  three 
sedimentary  islands  of  the  harbor.  Westward  from  the 
Garden,  however,  this  monocline,  of  east-west  strike  and 
southerly  dip,  changes  gradually  but  rapidly  to  a  north-south 
strike  and  westerly  dip,  plunging  down  against  the  great  mass 
of  melaphyr  already  described. 

It  is  obvious  from  this  sketch  of  the  geological  structure  that 
Avhile,  as  previously  stated,  folds  of  various  types  are  the 
dominant  form  of  displacement  and  give  character  to  the  area, 
the  flexures  are  profoundly  modified  by  longitudinal  and 
transverse  faults.  The  correct  interpretation  of  these  main 
structure  lines  is  evidently  essential  to  the  determination  of  the 
stratigraphic  elements  or  the  normal  succession  of  the  strata. 
Of  the  four  sections  accompanying  the  special  maps,  three  are 
approximately  complete,  viz.:  (1)  the  section  south  of  the 
granite,  through  the  village;  (2)  the  section  from  Beal  Street 
to  Beal's  Cove  ;  and  (3)  the  section  from  Melville  Garden  west 
toward  Huit's  Cove.  They  agree  in  their  main  features  and 
especially  in  showing  repeated  alternations  of  coarse  and  fine 
sediments.  But  a  more  careful  comparison  reveals  the  fact 
that  they  cannot  be  exactly  correlated  or  synchronized  ;  and  we 
are  obliged  to  recognize,  even  in  this  limited  area,  important 
lateral  changes  in  the  character  or  thickness  of  individual  strata  ; 
sandstone  at  one  point  being  represented  by  conglomerate  or 
shale  at  another,  and  so  on.  The  following  table  of  the  strata 
of  Hingham  has  been  compiled  from  the  first  and  second 
sections  referred  to  above,  and,  with  the  foregoing  qualifica- 
tions, may  be  regarded  as  substantially  correct.  The  individual 
beds  are  subject  to  constant  variations  in  thickness  ;  and  since 
the  outcrops  are  unfavorable  to  exact  measurements,  the  thick- 
nesses given  are,  as  the  round  numbers  indicate,  sim])ly 
approximations.  Some  of  the  beds  attain  the  maxinnim 
thickness  in  the  one  section  and  some  in  the  other  ;  and  hence 
the  totals  do  not  correspond  to  the  actual  sections, 


203 


Table  of  the   IIin(/ham  Strata. 

Granitic  rocks  (diorite,  granite,  and  felsitc). 

1.  Conglomerate  (basal).     Thickness  uncertaio. 

2.  Melaphyr 120-240  feet 

3.  Fine  conglomerate  and  sandstone,  alternating      .     .     .     120-200 
i.  Gray  slate 40-60 

5.  Conglomerate,  sandstone,  and  slate,  alternating  .     .     .  100-170 

6.  Gray  and  red  slate 90-130 

7.  Conglomerate        30-  50 

8.  Red  slate 20-40 

9.  Conglomerate       40-  50 

10.  Red  slate • 20-30 

11.  Conglomerate  .     .    ' 75-100 

12.  Red  slate 50-75 

13.  Sandstone  and  conglomerate,  alteruatiug 200-300 

905-1445 

14.  Gray  slate 500  4- 

Dikes  of  diabase  intersecting  the  bedded  rocks,  owing  chiefly, 
it  is  jjrobable,  to  the  less  continuous  outcrops,  but  partly,  no 
doubt,  to  the  fewer  faults,  are  much  less  conspicuous  in  Hing- 
hani  than  in  Nantasket.  They  probably  agree  with  the 
Nantasket  dikes  in  dating  from  the  plication  and  faulting  of  the 
strata.  The  most  important  distinction  is  that  between  the 
great  masses  of  coarsely  crystalline  diabase  scores  or  hundreds 
of  feet  in  breadth  and  very  irregular  in  outline,  and  the  ordinary, 
narrow,  wall-like  dikes  of  finely  crystalline  diabase.  The 
latter,  at  least,  belong  chiefly  to  the  east-west  systems  of 
Nantasket.  No  clear  intersections  have  been  observed  ;  and  no 
dikes  which  could  certainly  be  referred  to  the  youngest  or 
north-south  system  of  Nantasket. 

LITHOLOGY. 

The  rocks  of  Hinghani  are,  lithologically,  so  similar  to  those 
of  Nantasket  that  a  very  brief  treatment  of  this  topic  will 
suffice ,  the  main  purpose  of  these  studies  being  to  decipher  the 


204 

structure  and  physical  history  of  the  region  and  not  to  under- 
take a  minute  investigation  of  the  different  kinds  of  rocks. 
The  granitic  rocks  (diorite,  granite,  and  felsite)  have  been 
described  in  sufficient  detail  in  the  preceding  pages.  Mr. 
Merrill  has  kindly  examined  thin  sections  from  some  of  the 
more  typical  dikes,  and  finds  them  all  to  be  diabase  essentially 
similar  to  that  of  the  Nantasket  dikes.  He  has  also  found  that 
the  melaphyr  is  in  essential  agreement  with  the  common  basic 
variety  of  Nantasket.  It  has  commonly  a  dark  greenish  color, 
due  to  chlorite  and  epidote,  but  limited  portions  are  often 
brownish  or  purplish  owing  to  the  local  peroxidation  of  the  iron. 
It  varies  in  texture  from  almost  perfectly  compact  or  even  slaty 
to  distinctly  and  coarsely  amygdaloidal.  The  amygdules  con- 
sist chiefly  of  quartz  and  epidote,  but  include  also  chlorite, 
feldspar,  and  calcite.  The  quartz  is  usually  crystalline,  but 
sometimes  chalcedonic  or  jaspery.  These  secondary  minerals 
also  occur  commonly  in  veinlets  and  irreo:ular  seo'res'ations  : 
but  the  brecciation  so  characteristic  of  the  great  bed  of  melaphyr 
near  the  top  of  the  Nantasket  series  is  rarely  distinctly  observed 
in  Hingham ;  although  the  scoriaceous  structure  which  one 
naturally  looks  for  in  the  superficial  portions  of  a  flow  is  plain 
enough  at  some  points,  as  in  the  mass  of  melaphyr  near  the 
junction  of  Downer  Avenue  and  Crow  Point  Lane.  The 
Hingham  bed,  like  the  Nantasket  bed  just  referred  to,  is 
probably  composite,  a  succession  of  flows,  but  the  only  observed 
facts,  besides  its  great  thickness,  Avhich  clearly  point  to  this 
conclusion,  are  the  intercalated  bed  of  sandstone  and  conglom- 
erate which  is  exposed  south  of  Beal  Street,  about  midway 
between  the  street  and  the  little  pond  (PI.  8)  ;  and  a  small 
amount  of  banded  slate  or  possibly  tufi^  which  a  recent  excavation 
has  exposed  in  the  melaphyr  immediately  west  of  West  Hingham 
Station.  The  occurrence  last  mentioned  is  the  only  thing 
resembling  volcanic  tuff*,  or  in  .any  way  suggestive  of  ex])losive 
eruptions,  which  has  been  observed  in  Hingham. 

The   conglomerate   of  Hingham   is   composed   for   the   most 


205 

part  of  small  and  well-rounded  pebbles,  chiefly  of  difi^'crcnl. 
varieties  of  felsite  and  granite.  It  is  a  typical  puddingstoiie, 
becoming  a  breccia  only  where  the  basal  conglomerate  rests 
directly  upon  the  granite.  Being  less  intimately  associated 
with  melaphyr  than  the  Nantasket  conglomerate,  it  is,  as  a 
whole,  less  indurated,  possessing  less,  probably,  of  a  distinctly 
siliceous  cement.  This  is  especially  noticeable,  also,  in  the 
arenaceous  layers,  which  in  Nantasket  have  the  flinty  hardness 
of  red  quartzite,  but  in  Hingham  are  more  like  normal  sand- 
stone. The  well-rounded  and  assorted  pebbles  are  usually  less 
than  an  inch  and  rarely  more  than  two  or  three  inches  in 
diameter.  But  at  one  locality,  on  the  eastern  shore  of  Huit's 
Cove,  the  conglomerate  is  exceptionally  coarse  and  irregular 
in  structure,  containing  rounded  pebbles  or  bowlders  of  granite, 
etc.,  of  all  sizes  up  to  a  yard  in  diameter.  It  is  interesting  to 
note  also  that  some  of  the  pebbles  at  this  point  are  an  impure 
gray  limestone,  and  that  limited  portions  of  the  rock  have  a 
distinctly  calcai^eous  cement.  The  finer  portions  of  the  con- 
glomerate frequently  become  gradually  but  distinctl}^  arenaceous  ; 
and  most  of  the  beds  show  repeated  alternations  of  true 
conglomerate  with  coarse,  pebbly  sandstone,  so  that  it  is  quite 
impossible  to  map  the  two  rocks  separately.  Although  the 
conglomerate  series  is  so  largely  arenaceous,  there  is  compara- 
tively little  pure  or  typical  sandstone ;  the  most  important 
occurrence  of  this  kind  being  the  bed  of  gray  or  brownish  gray 
sandstone  over  a  hundred  feet  thick  in  the  Hersey  Street 
section.  The  sandstone  is  usually  gi'ay  to  light  brown  in  color, 
rarely  distinctly  ferruginous,  and,  as  stated,  rarely  a  good 
quartzite.  The  brown  color  is  probably  due  more  to  the 
admixture  of  grains  of  red  felsite  than  to  a  ferruginous  cement. 
The  argillaceous  rocks  of  Hingham  are  quite  varied.  The 
more  limited  beds  of  slate  included  in  the  conglomerate  series  are 
usually  of  some  shade  of  red,  brown,  or  purple  ;  but  the  thicker 
beds  are  in  large  part  of  a  greenish  or  a  greenish  gray  color,  the 
iron,  evidently,  being  less  highly  oxidized.     It  is  difficult,  in  the 


206 

-conglomerate  series,  to  draw  the  line  between  slate  and  sand- 
stone,  and  part  of  what  has  been  mapped  as  slate  might  be 
otherwise  classified.  The  great  slate  series  above  the  conglom- 
erate consists  throughout  of  a  homogeneous  gray  slate,  a  hard, 
firm,  but  not  strictly  impalpable  rock.  It  is  distinctly  siliceous 
in  composition,  and  a  fine  granular  structure  is  often  apparent 
under  a  lens.  Although  usually  more  or  less  distinctly  banded, 
the  stratification  being  marked  by  laminae  of  alternating  colors, 
the  Hinffham  slates,  whether  in  the  conolomerate  series  or  in 
the  slate  series,  are  rarely  shaly  or  exhibit  a  lamination  cleavage  ; 
and  only  to  a  limited  extent,  as  in  the  vicinity  of  Huit's  Cove, 
is  the  true  slaty  cleavage,  transverse  to  the  bedding,  well 
developed.  A  very  perfect  cuboidal  or  rhomboidal  joint- 
structure  can.  be  seen  in  many  exposures. 

DETAILED    STRUCTURE    OF    NORTHERN    HINGHAM. 

We  are  now  ready  for  a  systematic,  ledge-to-ledge  study  of 
the  bedded  rocks  of  Hingham,  following,  as  at  Nantasket,  the 
topographic  order  of  the  outcrops.  The  village  area,  bordering 
the  railroad,  south  of  the  granite  axis,  is  in  most  respects  a 
convenient  and  natural  starting  point  for  a  detailed  examina- 
tion of  the  sedimentary  and  volcanic  rocks.  The  section 
between  Bcal  Street  and  Beal's  Cove  is  more  complete,  and 
presents  a  normal  rather  than  an  inverted  succession  of  the 
strata,  but  the  ledges  are  less  continuous,  and  the  beds  are 
much  less  easily  traced  along  the  strike.  Moreover,  it  is  a 
wooded  and  swampy  tract,  while  the  village  area  is  elevated 
and  open,  and  readily  accessible,  in  spite  of  the  numerous 
houses  and  fences. 

The    Village  Area. 

This  is  the  area  represented  on  the  first  of  the  special 
maps   (PI.  7),  embracing  all  the  ledges  between  Main  Street 


207 

and  Hockley  Lane.  Topographically  it  is  essentially  ouc.  con- 
tinuous ridge,  save  where  it  is  crossed  by  Town  Urook  :im(1  ihc 
railroad  at  West  Ilinghanj  Station. 

The  most  contiiuious  exposures  of  this  narrow  belt  are  in  the 
vicinity  of  Hereey  Street ;  and  the  dip  in  this  part  of  the  area, 
especially,  is  quite  constant  —  S.  70°.  The  most  northei'ly 
outcrop  along  this  line  is  the  ledge  of  conglomerate  (13)^  about 
150  feet  west  of  Hersey  Street,  in  the  rear  of  the  second  and 
third  houses  from  the  corner,  on  South  Street.  A  few  yards 
south  of  this  ledge,  in  the  rear  end  of  these  lots  and  extending 
into  the  adjoining  lot  on  the  south,  is  an  outcrop  of  dark  red 
slate  (12).  The  slate  must  cross  Hersey  Street  at  the  first 
bend  ;  and  it  is  exposed  repeatedly  along  the  base  of  the  con- 
glomerate escarpment  from  200  to  300  feet  east  of  the  street. 

Going  up  Hersey  Street  from  the  railroad,  the  first  rock 
actually  seen  is  the  conglomerate  (11)  bordering  the  red  slate 
just  referred  to  on  the  south.  It  commences  a  hundred  feet  or 
so  west  of  the  street ;  and  its  northern  border  forms  a  con- 
tinuous escarpment  from  15  to  20  feet  high,  due  to  the  erosion 
of  the  red  slate  and  extending  about  500  feet  east  of  the  street. 
This  escarpment  is  the  northern  edge  of  an  area  from  600  to  1,000 
feet  in  length  (E.— W.)  and  500  feet  in  breadth,  over  whicli 
the  ledges,  as  the  map  shows,  are  almost  continuous,  and  the 
thickness  of  the  several  beds  admits  of  accurate  measurement. 
The  conglomerate  just  described  has  a  breadth  of  from  75  to  90 
feet ;  and  it  is  followed  on  the  south  in  succession  by  about  20 
feet  of  red  slate  (10)  ;  from  40  to  50  feet  of  conglomerate  (9)  ; 
25  feet  of  red  slate  (8)  ;  from  35  to  45  feet  of  conglomerate 
(7)  ;  from  115  to  135  feet  of  red  and  gray  slate  (6)  ;  from 
100  to  130  feet  of  gray  sandstone  with  -some  gray  slate  (5)  ; 
and  40  feet  of  conglomerate  (5). 

This  Hersey  Street  section  is  broken  by  several  large  and 
irregular  dikes   (see  PL    7)  ;  but  the  breadth  of  the    trap    is 

1  The  numbers  in  parentheses  refer  to  the  general  table  of  Hingham  strata  on  page 
202. 


208 

not  included  in  tlie  foregoing  measurements  of  the  beds  Avbicli 
it  intersects.  Farther  south  on  the  line  of  Hersey  Street  the 
sandplain  conceals  everything  for  nearly  500  feet ;  and  the  first 
outcrops  in  that  direction  are  granite  and  diorite,  with  other 
large  masses  of  diabase. 

Most  of  the  beds  which  we  have  crossed  on  Hersey  Street 
can  be  traced  east  by  satisfactory  outcrops  to  Lafayette  Avenue. 
The  strike  gradually  changes  in  this  direction,  however,  from 
nearly  due  east-west  to  east-southeast,  as  the  map  shows.  Of 
the  most  northerly  conglomerate  (13)  there  is  only  one  small 
and  rather  uncertain  exposure  nearly  GOO  feet  from  Hersey 
Street.  The  first  red  slate  (12),  after  a  gap  of  700  feet,  is 
well  exposed  behind  Mr.  Lane's  house  on  South  Street.  It 
forms  an  abrupt  escarpment  or  cliflffrom  15  to  20  feet  high,  and 
has  also  been  found  in  excavations  50  feet  or  more  north  of  the 
cliff.  The  dip  at  this  point  is  only  25°,  which  fully  accounts 
for  the  increased  breadth  of  the  bed  as  shown  on  the  map. 
South  of  this  slate  we  are  able  to  identify  in  almost  continuous 
ledges  the  following  beds:  conglomerate  (11);  red  slate 
(10) ,  not  clearly  exposed,  but  represented  by  a  blank  depressed 
space  of  the  proper  width;  conglomerate  (9)  ;  red  slate  (8)  ; 
conglomerate  (7)  ;  red  and  gray  slate  (6)  ;  and  gray  sand- 
stone and  slate  (5).  The  more  southerly  beds  along  this  line 
have  approximately  the  same  dip  as  on  Hersey  Street  —  S. 
70° ;  but  toward  the  north  the  dip  diminishes  to  50°  and  25°. 

The  first  slate  (12)  and  the  first  and  second  conglomerates 
(11  and  9)  are  well  exposed  also  in  the  angle  east  and  south  of 
Lafayette  Avenue.  The  structure  here  is  much  more  compli- 
cated and  interesting.  Immediately  in  the  angle  of  the  avenue 
the  slate  is  in  contact  with  the  conglomerate  (11)  as  usual; 
but  100  feet  farther  east  it  rests  in  like  manner  against  a 
parallel  mass  of  granite  ;  while  the  conglomerate  abuts  squarely 
against  the  granite  and  the  great  dike  of  diabase  from  50  to  70 
feet  wide  which  interru})ts  the  granite  at  this  point.  The 
granite,  which  has  a  breadth  of  from  40  to  50  feet  north  of  the 


200 

trap,  is  coarsely  crystalline,  and  there  is  not  the  slip^htest  indica- 
tit)n  tliat  it  is  eruptive  through  the  sedimentary  rocks.  No  veins 
or  apophyses  of  granite  penetrate  the  slate,  and  the  normal  coarse 
texture  of  tlie  granite  is  unchanged  near  the  contact.  But  the 
abrupt  way  in  which  the  diabase  ends  against  the  conglomerate 
and  the  obvious  partial  dislocation  or  lateral  shifting  of  tlie 
slate  are  certainly  very  suggestive  of  faulting.  Again,  the 
slate,  although  its  strike  is  parallel  with  the  granite,  clearly 
dips  against  the  latter  :  and  in  all  the  outcrops  it  shows  gi'cat 
disturbance,  being  strongly  contorted  and  faulted,  or,  locally, 
completely  crushed.  The  contact  between  the  granite  and  slate 
is  probably  a  strike-fault  with  the  downthrow  to  the  north  ;  and 
all  the  sedimentary  beds  are  probably  cut  by  a  transverse  fault 
along  the  west  side  of  the  granite,  which,  as  the  arrows  on  the 
map  indicate,  downthrows  to  the  west  south  of  the  granite-slate 
contact  and  to  the  east  north  of  it.  Along  its  contact  with  the 
slate  the  granite  continues  scarcely  200  feet,  ending  as  abruptly 
as  it  began.  In  the  direct  line  of  the  o^ranite  we  here  find 
broad  outcrops  of  the  slate,  which,  with  a  southerly  dip  of 
80°,  is  exposed  almost  continuously  for  its  normal  breadth  ; 
while  the  dike  of  diabase,  as  before,  abuts  squarely  against  the 
conglomerate.  Evidently,  the  granite  and  trap  are  bounded 
on  the  east  as  well  as  on  the  west  by  a  fault ;  and  these  two 
faults  are  obviously  reversed  in  throw  or  compensating.  To 
avoid  their  indefinite  extension,  they  are  represented  as  con- 
verging northward  ;  but  that  is  not  strictly  required  by  the 
observed  facts.  The  main  point,  however,  is  that  the  facts 
appear  to  justify  us  in  regarding  the  granite  and  its  included 
diabase  as  a  somewhat  rectangular  block  which  has  been  ele- 
vated relatively  to  the  bordering  strata,  the  uplift  having  been 
sufficient  to  carry  the  sediments  normally  overlying  this  mass 
of  granite  above  the  present  plane  of  erosion.  Southward  this 
block  of  granite  is  continuous  with  the  main  area  ;  and  it  is 
simply  a  displaced  portion  of  the  general  granitic  floor  upon 
which  the  sediments  rest.     The  evidence  for  the  second  fault  is 

OCCAS.  TAPERS  B.  S.  N.  H.  IV.  14. 


210 

materially   sti'engtheiied   by  the  marked  change  in  the  strike, 
from  south  of  east  to  north  of  east,  as  we  cross  it. 

In  following  the  sedimentary  rocks  farther  east  we  find  an 
outcrop  of  conglomerate  in  front  of  the  school  house  on  Elm 
Street,  nearly  opposite  the  end  of  Central  Street,  and  directly 
in  the  line  of  strike  of  the  slate  ;  while  several  exposures  of 
the  red  slate  are  found  in  the  field  north  of  the  school  house. 
These  facts  clearly  suggest  the  third  transverse  fault  shown 
on  the  map.  The  character  of  this  fault  is  wholly  uncertain. 
No  outcrops  of  slate  or  conglomerate  have  been  observed 
south  of  Elm  Street ;  and  the  map  is  here  largely  hypo- 
thetical. Quite  certainly  the  fault  does  not  downthrow  to  the 
west  as  the  arrow  indicates  ;  but  it  is  more  probably  a  hori- 
zontal thrust-fault,  the  eastei-n  extension  of  the  strata  having 
been  shoved  bodily  to  the  northward  from  100  to  150  feet. 
The  rock  showing  in  the  field  south  of  Elm  Street  and  east  of 
Central  Street  is  diorite  and  probably  iyi  situ.  This  indicates 
a  gradual  narrowing  of  the  sedimentary  belt ;  but  beyond 
this  point  there  are  no  outcrops  of  any  kind  for  more  than 
half  a  mile  ;  and  where  and  how  the  sedimentary  rocks  termi- 
nate we  can  only  conjecture.  As  previously  stated,  the  gen- 
eral map  is  constructed  in  accordance  with  the  view  that  they 
cross  Main  Street,  pass  beneath  the  sandplain  occupied  by 
the  cemetery,  and  end  in  the  Home  Meadows  against  the 
great  boundary  fault  between  the  Hingham  and  Nantasket 
areas,  supposed  to  coincide  in  position  and  direction  with  the 
east  shore  of  Hingham  Harbor. 

Returning  to  Hersey  Street  and  tracing  this  series  of  strata 
westward,  we  find  that  all  the  beds  which  can  be  traced  more 
than  200  feet  from  the  street  are  distinctly  flexed  to  the  north, 
the  flexure  amounting  to  a  horizontal  shift  of  about  100  feet. 
It  is  necessary  to  suppose,  of  course,  that,  as  shown  on  the 
map,  the  entire  series  is  involved  in  the  displacement ;  and 
there  are  some  indications  that  the  beds  are  actually  com- 
pressed or  pinched  on  the  bend,  as  if  they  had  experienced  a 


211 

transverse  thrust  or  a  tendency  t()  shear.  But,  unfortunately, 
there  is  at  this  point  a  complete  hiatus  of  nearly  /}()()  feet 
in  the  outcrops ;  and  the  full  extent  and  form  of  the  fold 
can  not  be  made  out.  That  the  east-west  strike  is  quickly 
resumed  is  shown,  however,  by  the  ledges  between  this  and 
West  Hingham  Station ;  and  it  is  especially  satisfactory  to 
find  that  the  very  first  exposures  show  the  thick  bed  of  gray 
sandstone,  the  most  unique  bed  of  the  entire  sedimentary 
series,  and  therefore  the  most  to  be  relied  upon  in  correlation. 

About  100  feet  back  of  the  school  house  and  400  feet  from 
Soutii  Street,  an  artificial  excavation  has  uncovered  the 
abrupt  face  of  a  ledge  which  shows  75  feet  of  greenish  gray 
sandstone  (5)  with  distinct  slaty  partings,  dipping  S.  70° 
beneath  40  feet  of  conglomerate  (5)  ;  while  fragments  in  the 
soil  indicate  that  the  conglomerate  is  overlain  in  turn  by  slate 
(4).  The  conglomerate  and  sandstone  are  easily  traced  to 
the  westward  several  hundred  feet,  where  they  form  the 
northern  portion  of  a  group  of  ledges  affording  the  following 
nearly  continuous  section,  beginning  on  the  south  : — granite,  in 
several  obscure  outcrops  ;  then,  after  an  interval  of  a  hundred 
feet  or  more,  with  one  uncertain  exposure  of  melaphyr,  come 
without  an  appreciable  break,  120  feet  of  melaphyr  (2)  ;  120 
feet  of  fine  conglomerate  and  sandstone  (3)  ;  50  feet  of  slate, 
mostly  gray,  but  changing  to  red  on  the  north  (4),  the  red 
rock  being  well  exposed  behind  one  of  the  houses  on  South 
Street;  40  feet  of  conglomerate  (5)  and  nearly  100  feet  of 
gray  slaty  sandstone  (5).  The  isolated  and  obscure  outcrop 
of  gray  slate  a  few  yards  from  the  line  of  South  Street  (see 
map)  undoubtedly  marks  the  extension  of  the  great  bed  (6)  of 
the  Hersey  Street  section  ;  and  on  the  north  side  of  the  street, 
in  the  rear  of  the  second  house  from  West  Hingham  Station, 
we  have  a  satisfactory  exposure  of  the  conglomerate  ( 7 )  which 
borders  this  slate  on  the  north.  The  only  remaining  outcrop 
in  this  direction  lies  some  300  feet  farther  to  the  northwest, 
beyond  the  railroad  and  on  the  north  side  of  the  brook.     It 


212 

is  a  mass  of  conglomerate  about  10  feet  across,  lying  in  the 
meadow,  and  is  possibly  only  a  bowlder.  Assuming  it  to  be 
in  situ,  or  nearly  so,  it  probably  represents  the  most  north- 
erly conglomerate  (13)  of  the  Hersey  Street  section. 

The  section  afforded  by  this  group  of  ledges,  it  will  be 
observed,  overlaps  on  the  south  and  supplements  the  section 
along  Hersey  Street,  the  two  together  affording  a  nearly  com- 
plete section  of  the  conglomerate  series.  The  melaphyr  of  this 
section  is  the  typical  variety,  greenish  and  purplish  in  color,  and 
compact  to  highly  amygdaloidal  and  scoriaceous  in  texture. 
The  amygdules  consist  chiefly  of  epidote  and  quartz,  and  are 
usually  small,  or,  if  larger,  rather  scattering  and  not  crowded. 
As  a  whole  the  rock  is  very  massive,  but  portions  of  the  bed 
show  irregular  veinlets  and  segregations  of  epidote  and  fer- 
ruginous quartz  ;  and  the  distribution  of  the  amygdaloidal  and 
scoriaceous  melaphyr  is  such  as  to  suggest  that  this  may  be  a 
composite  flow.  The  contact  between  the  melaphyr  and  the 
arenaceous  conglomerate  is  very  clearly  exposed  for  about  ten 
feet  on  the  highest  part  of  the  ridge.  It  is  straight,  exactly 
parallel  with  the  strike  of  the  conglomerate,  and  shows  only 
minute  irregularities,  variations  of  an  inch  or  so  from  the  mean 
line.  The  melaphyr  does  not  penetrate  the  conglomerate  any 
more  distinctly  than  the  conglomerate  penetrates  the  melaphyr. 
Some  irregular  cracks  in  the  lava  appear  to  have  been  filled 
with  fine  sand,  which  is  now  highly  ferruginous  ;  but  aside  from 
these  there  are  no  distinct  inclusions  of  the  sedimentary  rocks 
in  the  melaphyr.  On  the  other  hand,  a  few  small  pebbles  of 
melaphyr  similar  to  this  were  observed  in  the  conglomerate, 
within  a  few  inches  of  the  contact.  The  conglomerate  and 
sandstone  exhibit  no  special  alteration  or  unusual  induration  at 
this  point ;  and  in  every  way  the  facts  are  favorable  to  the  view 
that  the  melaphyr  is  contemporaneous  and  not  intrusive. 

Between  the  ledges  just  described  and  those  next  to  the  west 
there  is  a  gap  of  six  hundred  feet,  occupied  by  the  valley  of 
Town  Brook  and  the  railroad.      But  innnediately  l)eyond  West 


213 

Hingham  Station,  the  melaj)liyr  outcrops  proiiiiticntly,  toriuing 
the  summit  and  abrupt  southern  slope  of  a  hill  which  is  flanked 
by  modified  drift  on  the  north.  It  is  essentially  similar  in 
character  to  the  melaphyr  east  of  the  railroad,  except  that  a 
larger  proportion  of  the  rock  is  compact  or  not  distinctly 
amygdaloidal,  some  of  the  exposures  resembling  a  massive  slate. 
Irregular  segregations  of  epidote,  etc.,  are  common;  and,  as 
already  stated,  the  melaphyr  encloses  at  one  point  a  broken  or 
faulted  layer  a  few  inches  in  thickness  of  a  beautifully  laminated 
or  banded  slate.  This  is  well  exposed  at  the  present  time  in 
the  part  of  the  ledge  nearest  to  the  station,  where  a  new  street 
is  being  graded  up  the  hill  from  West  Street.  So  far  as  can  be 
determined,  it  is  near  the  middle  of  the  bed  of  melaphyr  ;  and  it 
is  certainly  far  from  any  large  body  of  slate.  The  most  natural 
explanation  appears  to  be  that  it  is  a  thin  layer  of  tuff,  similar 
to  some  of  the  tuff  beds  of  Nantasket,  and  hence  another 
indication  that  the  bed  of  melaphyr  is  composite,  consisting  of 
two  or  more  flows.  The  melaphyr  is  exposed  along  the  strike 
(W.  by  N.)  in  frequent  ledges  for  about  1,000  feet,  or  to 
within  500  feet  of  Hockley  Lane,  and  is  of  similar  character 
throughout. 

On  the  north  side  of  the  first  hill,  in  the  angle  between  the 
melaphyr  and  West  Street,  are  several  outcrops  of  conglomerate  ; 
but  the  contact  is  not  exposed  here.  North  and  west  from  this 
point  the  stratified  rocks  are  almost  entirely  concealed  by  the 
undulating  modified  drift.  I  have  recently  discovered,  how- 
ever, that  the  small  ledges  of  melaphyr  on  the  west  slope  of  the 
main  hill,  lying  between  the  two  *large  groups  of  ledges  and 
divided  by  the  east-west  fence,  as  shown  on  the  map,  are 
bordered  on  the  north  side  by  fine  conglomerate  and  sandstone 
precisely  similar  to  the  rocks  in  contact  with  the  melaphyr  east 
of  the  railroad  ;  and  the  contact,  which  is  clearly  exposed,  is  of 
the  same  character.  It  is  very  evident  that  the  sedimentary 
rocks  were  deposited  over  the  melaphyr,  for  they  fill  cracks  in 
it  and  are  partly  made  up  of  debris  derived  from  it.      Near  the 


214 

melaphyr  the  fragments  of  that  rock  are  large  and  angular  ; 
but  the  conglomerate  as  a  whole  is  composed  chiefly  of  felsite 
and  granite,  and  cannot  be  classed  as  a  tuff.  The  dip  is  as 
usual  about  S.  70°.  Irregular  veinlets  of  iron  oxide  are  seen 
in  both  the  sandstone  and  the  melaphyr ;  and  cannot  be 
regarded  as  evidence  that  the  melaphyr  is  intrusive.  The 
northern  border  of  the  melaphyr,  as  drawn  on  the  map,  is 
shown  by  these  recent  observations  to  be  too  far  to  tlie  north  at 
this  point ;  and  it  is  possible  that  a  small  transverse  flexure  or 
fault  separates  this  contact  ledge  from  the  remaining  outcrops 
of  melaphyr  in  the  next  field  to  the  west.  In  this  field  it  is 
quite  noticeable  that  the  melaphyr  is  more  compact  toward  the 
granite  on  the  south  or  the  supposed  base  of  the  flow,  and 
more  amygdaloidal  and  scoriaceous  toward  the  sedimentary 
rocks  on  the  north  or  the  supposed  top  of  the  flow.  Going 
north  from  these  ledges  into  the  adjoining  field  we  find,  after 
an  interval  of  nearly  75  feet,  or  15  feet  beyond  the  fence,  60 
feet  of  fine  conglomerate  and  sandstone  with  an  apparent  dip 
S.  45°  ;  50  feet  concealed  ;  and  10  feet  of  sandstone.  Tlicse 
exposures,  which  are  marked  on  tlic  map,  must  evidently  be 
referred  to  the  same  bed  (3)  which  we  have  elsewhere  found  in 
contact  with  the  melaphyr  ;  and  more  recently  I  have  discovered 
north  of  these,  small  and  obscure  exposures  of  purple  slate 
(4),  dipping  S.  70°;  and  conglomerate  (5).  These  scanty 
outcrops  serve  to  show  that  the  Village  series  of  strata  probably 
extends  without  essential  change  this  far  to  the  west. 

We  come  now  to  the  interesting  group  of  conglomerate 
ledges  in  the  field  south  of  the  melaphyr  and  Hockley  Lane, 
and  bordered  on  the  east  and  south  by  granite  (see  the  map). 
The  relations  of  the  granite  and  conglomerate  are  very  inti- 
mate, and  could  be  fully  known  only  by  removing  all  the 
superficial  detritus  from  this  area.  Along  the  south  side  of  the 
field,  especially,  and  on  both  sides  of  the  fence,  we  pass 
repeatedly  and  abruptly  from  the  one  rock  to  the  other  ;  and 
there  are  probably  several  or  many  l)0sscs  or  knobs  of  granite 


215 

projectinii^  throui2^h  a  thin  and  approximately  horizontal  bed  of 
conglomerate  ;  although  the  surface  relations  of  tlu;  two  rocks 
might  be  partly  explained  by  faulting.  That  the  granite  is  not 
younger  than  and  eruptive  through  the  conglomerate  is  proved 
beyond  the  shadow  of  a  doubt  by  the  nature  of  the  contact  and 
the  composition  of  the  conglomerate.  The  latter  is  made  up 
very  largely,  especially  where  it  lies  directly  upon  or  against 
the  granite,  of  the  angular  and  half  rounded  debris  of  exactly 
the  same  coarsely  crystalline  red  granite  ;  and  the  fragments, 
which  range  from  single  grains  of  quartz  and  feldspar  to 
masses  two  feet  or  more  in  diameter,  are  sometimes  only  imper- 
fectly separated  from  the  parent  ledge,  the  finer  sediment 
appearing  to  penetrate  cracks  in  the  granite.  The  relations  are 
essentially  the  same  as  on  Rocky  jSTeck  and  Granite  Plateau  of 
the  Nantasket  area,  where  the  basal  conglomerate  rests  upon 
the  granite  ;  and  they  also  agree  with  the  contacts  resulting 
from  the  rapid  deposition  of  coarse  sediments  over  a  disinte- 
grated land-surface,  as  recently  described  by  Professor 
Pumpellyi  and  Professor  Emerson.^  On  purely  structural 
grounds,  this  conglomerate  should  certainly  be  correlated  with 
the  basal  conglomerate  of  Nantasket ;  which  would,  apparently, 
separate  it  widely  from  all  the  other  Hingham  strata.  The 
melaphyr  marked  on  the  map  as  outcropping  in  the  midst  of 
this  basal  conglomerate,  probably  overlies  it.  It  is  amygda- 
loidal  and  otherwise  similar  to  the  rest  of  the  Hingham  mela- 
phyr, but  may,  of  course,  be  the  equivalent  of  any  of  the 
earlier  basic  flows  of  Nantasket. 

This  completes  the  detailed  examination  of  the  ledges  of  the 
Village  area.  If,  as  appears  necessary,  we  regard  the  series 
as  inverted,  the  general  structure  and  the  relations  to  the 
granite  must  be  approximately  as  represented  in  the  section 
accompanying  the  map.  A  long,  narrow  block  of  strata  is 
faulted  down  between  walls  of  granite,  the  drop  on  the  north 

>  Bull.  geol.  soc.  America,  vol.  2,  p.  209-224. 
*  Bull.  geol.  soc.  America,  vol.  2,  p.  451-456. 


216 

being  so  much  greater  than  that  on  the  south  as,  in  conjunction 
with  a  horizontal  or  plicating  stress,  to  overturn  the  beds.  Or  it 
may  be  otherwise  described  as  an  inverted  syncline  deeply 
folded  down  in  the  granite  and  the  northern  half  carried  away 
by  the  fault  which  elevated  the  granitic  axis  on  the  north. 
The  map  shows  two  areas  of  slate  along  the  northern  border  of 
the  conglomerate  series.  These  are  not  based  upon  any  out- 
crops ;  but  the  topography  is  favorable  to  the  occurrence  of 
slate  here,  and  they  are  introduced  in  accordance  with  the 
foregoing  explanation  of  the  structure.  The  same  is  the  case 
also  with  the  conglomerate  shown  south  of  the  melaphyr,  east 
of  the  railroad  ;  but  the  conglomerate  which  is  represented  in 
the  section  as  lying  upon  the  granite  is  the  basal  conglomerate 
already  described   south  of  the  melaphyr,  near  Hockley  Lane. 

The   Beal's    Cove  Area. 

This  is  the  area  indicated  by  the  second  special  map 
(Plate  8) .  It  is  crossed  diagonally  by  Beal  Street  and  embraces 
all  the  ledges  between  Beal's  Cove  and  Lincoln  Street. 
Topographically  it  belongs  chiefly  to  the  broad  lower  or  lowest 
sandplain,  but  includes  Tucker's  Swamp  and  the  western 
slopes  of  Baker's  and  Squirrel  Hills.  This  area  has  been 
already  described  as  affording,  between  the  granite  on  Beal 
Street  and  Beal's  Cove,  the  most  complete  and  normal  section 
of  the  Hingham  strata. 

The  western  extremity  of  the  granite  axis  is,  fortunately, 
well  defined,  as  the  map  shows,  by  numerous  outcrops  north 
of  and  in  the  street  and  the  obscure  but  reliable  .outcrops 
between  the  houses  south  of  the  street.  The  granite  is  over- 
lain at  this  point,  as  already  described,  by  several  irregular 
patches  of  effusive  felsite,  including  the  mass  wrongly  marked 
as  an  outlier  of  conglomerate.  One  proof  that  not  only  this 
felsite  but  also  that  farther  east,  in  the  vicinity  of  Thaxter  and 
Lincoln  Streets,  are  part  of  a  surface  flow  or  truly  effusive  has 
not  been  stated;  viz.,  that  it  occurs  only  near  the  junction  of 


217 

the  gnuiitc  and  tlic  bedded  rocks,  wlucli,  it  iw  so  evident,  were 
deposited  ii|)ou  the  granite.  In  otlier  words,  we  find  the 
felsite  ii[)on  what  we  know  to  liave  been  tlie  ancient  snrface  of" 
the  granite,  from  which  the  sedimentary  deposits  have  recently 
Ijeen  denuded  ;  and  v\^]ierever  erosion  has  cut  below  this  surface 
the  effusive  felsite  is  wholly  wanting  and  we  observe  only  an 
occasional  narrow  and  irregular  dike  of  intrusive  felsite. 

Along  the  northwest  side  of  the  granite  there  is  a  line  of 
cono'lorncrate  ledt»:es  sufficient  to  indicate  a  narrow  but  continu- 
ous  or  nearly  continuous  band  of  conglomerate  separating  the 
granite  from  the  overlying  melaphyr.  This  conglomerate 
undoubtedly  extends  considerabl}^  farther  than  it  is  marked,  the 
more  northerly  outcrops  having  escaped  observation  at  the 
time  the  map  was  drawn.  One  of  these  is  on  the  west  side  of 
the  winding  road  known  as  Hawke's  Lane  running  south  from 
Lincoln  Street  to  an  isolated  house  ;  and  another,  which  will 
be  referred  to  later,  is  on  the  south  side  of  Lincoln  Street  west 
of  the  lane.  This  conglomerate  is  a  true  puddingstone,  con- 
sisting of  rounded  fragments  of  granite,  felsite,  and  melaphyr  : 
but  the  contact  with  the  granite  is  not  clearly  exposed,  and  we 
can  only  conjecture  that  the  relations  of  the  two  rocks  are, 
perhaps,  the  same  as  south  of  Hockley  Lane.  This  supposed 
basal  conglomerate  can  not  be  traced  around  the  extremity  of 
the  granite  axis,  and  the  narrow  belt  of  it  marked  along  the 
southern  margin  of  the  granite  is  unsupported  by  a  single  out- 
crop, finding  its  justification  simply  in  the  depression  between 
the  granite  and  melaphyr ;  while  the  fact  that  outliers  of 
melaphyr  rest  upon  the  granite  north  of  Beal  Street  is  rather 
against  its  existence.  In  the  two  sections  accompanying  the 
map,  it  is  omitted  from  the  one  extending  south  from  Beal 
Street  to  Beal's  Cove ;  although  represented,  as  the  facts 
justify,  in  the  other,  running  northwest  from  the  granite. 

South  of  the  granite,  the  main  bed  of  melaphyr  is  well 
exposed  in  two  large  groups  of  ledges.  It  is  the  typical  green, 
basic   variety,   and  is  abundantly  but   coarsely   and  irregularly 


218 

amygdaloidal,  with  many  irregular  veinlets  and  segregations 
of  impure  epidote,  etc.,  portions  of  the  rock  being  of  a  distinctly 
scoriaceous  character.  A  coarse  flow-structure  is  sometimes 
indicated  by  tlie  arrangement  of  the  amygdules  in  parallel  sheets 
or  layers,  which  have  the  normal  dip  of  the  section,  S.  60°  or 
more.  As  the  map  shows,  the  breadth  of  the  belt  varies, 
perhaps,  with  the  dip,  from  about  160  to  more  than  300  feet. 
In  the  western  group  of  ledges,  a  little  north  of  the  middle  of 
the  belt,  a  thin  layer  of  conglomerate,  passing  upward  into 
sandstone,  the  entire  thickness  being,  possibly,  less  than  ten 
feet,  interrupts  the  continuity  of  the  melaphyr.  This  is  the 
normal  reddish  conglomerate,  consisting  chiefly  of  granite  and 
felsite  ;  and  it  can  not  be  classed  as  a  tufl"  or  a  fragmental 
melaphyr.  The  dip  of  the  conglomerate  is  S.  60° ;  and  there 
is  no  reason  to  doubt  that  it  is  essentially  conformable  with  the 
melaphyr  above  and  below.  But  since  neither  contact  is 
clearly  exposed,  the  facts  are  evidently  insuflicieut  to  reveal 
the  true  significance  of  the  conglomerate.  We  can  not  deter- 
mine conclusively  whether  it  is  a  true  interbedded  conglomerate, 
or  simply  an  outlier  of  the  very  similar  overlying  conglomerate, 
displaced  by  faulting.  The  latter  view  would  readily  explain  the 
unusual  breadth  of  the  melaphyr  at  this  point ;  and  it  may  be 
added  that,  independently  of  this  conglomerate,  there  is  no 
special  indication  that  the  melaphyr  is  a  composite  flow. 

In  the  eastern  group  of  ledges,  in  the  angle  between  Beal 
Street  and  Portuguese  Lane,  the  contact  of  the  melaphyr  and 
overlying  conglomerate  is  satisfactorily  exposed,  and  is  decidedly 
favorable  to  the  view  that  the  melaphyr  is  contemporaneous, 
i.  e.,  that  the  conglomerate  has  been  deposited  upon  its  surface. 
The  melaphyr  is  here  very  coarse  and  scoriaceous  ;  and  the 
numerous  cracks  and  inequalities  of  its  surface  arc  filled  with 
the  finer  pai't  of  the  conglomerate,  in  such  a  way  as  to  jilace 
the  relations  of  the  two  rocks  beyond  question.  The  conglom- 
erate contains,  however,  comparatively  little  debris  that  can  be 
referred    to  this    melaphyr ;   and    this  fact,   together    with    the 


210 

roiin^h  cliaracter  of  the  inelaphyr,  points  to  the  conclusion  that 
it  sidi'ered  but  little  erosion  before  tlie  congloinerute  was  spread 
over  it,  and  hence  that  it  is  probably  a  submarine  flow. 

The  conglomerate  dips  south  or  away  from  the  melaphyr  at 
the  normal  angle ;  and  these  eastern  ledges  overlap  and  su[)ple- 
nicnt  the  extensive  and  prominent  outcrops  farther  west,  in  the 
vicinity  of  the  small  pond,  the  entire  breadth  of  the  bed  being 
thus  satisfactorily  exposed.  This  bed  is  throughout  a  normal, 
massive,  well-indurated  puddingstone,  embracing  very  little 
arenaceous  material.  The  pebbles  are  mainly  from  one  to  two 
or  three  inches  in  diameter  :  but  in  certain  layers  the  maximum 
rises  to  six  inches  or  even  a  foot.  The  southerly  displacement 
of  the  conglomerate  in  passing  from  the  eastern  to  the  western 
ledges  is  very  noticeable,  and  evidently  related  to  the  increased 
breadth  of  the  melaphyr  in  that  dii'ection.  Two  explanations 
have  been  suggested  for  the  latter:  first,  a  diminished  dip, 
which  would  cause  a  curvature  of  the  overlying  strata,  as  shown, 
on  the  map,  even  though  they  retain  throughout  their  normal 
high  inclination  ;  secondly,  a  strike-fault,  as  indicated  by  the 
conglomerate  enclosed  in  .the  melaphyr,  which  miglit  have  the 
same  result  as  regards  the  overlying  beds,  or,  ending  against 
transverse  faults,  might  involve  an  actual  dislocation  of  the 
sedimentary  series.  Unfortunately,  the  entire  absence  of  out- 
crops along  this  north-south  line  makes  it  impossible  to  choose 
between  these  explanations  ;  and  it  may  well  be  that  both  of 
them  are  true. 

With  this  introduction,  we  can  pass  rapidly  over  the  entire 
section  from  the  granite  southward,  since  it  is,  in  a  large 
measure,  a  repetition  of  the  Village  section.  The  strike  ranges 
from  S.  65°  to  S.  80°  E.  ;  and  the  dip,  except  perhaps  with 
the  melaphyr,  varies  but  little  from  S.  80°.  The  thicknesses 
are  chiefly  approximations,  the  beds,  either  on  account  of  not 
being  sharply  defined  or  from  lack  of  cpntinuity  in  the  outcrops, 
rarely  admitting  of  exact  measurement.  The  numbers  refer  to 
the  general  table  of  Hingham  strata  and  also  indicate  the  corre- 


220     . 

lation  of  this  section  with    the  Village  section,   although    the 
sequence  here  is  normal  and  not  reversed,  as  follows  : — 

Granite  and  felsite,  with  outliers  of  melaphyr.  Conglomerate 
(1),  not  observed.  Melaphyr  (2),  140  feet  (minimum). 
Conglomerate  (3),  medium  to  coarse  puddingstone,  very  much 
coarser  and  thicker  than  in  the  Village  section,  175  feet. 
Slate  (4),  not  exposed,  but  i-epresented  by  a  depression,  65  (?) 
feet.  Fine  conglomerate  alternating  repeatedly  witli  sandstone 
(5) ,  170  feet.  The  broad  outcrop  south  and  east  of  Portuguese 
Lane  shows  that  this  is  a  complete  mixture  or  blending  of 
coarse  sandstone  and  fine  conglomerate  ;  while  in  the  Village 
section  there  are  two  distinct  beds — 40  feet  of  conglomerate 
without  sandstone  and  100  feet  of  sandstone  without  conglome- 
rate. Purplish  and  gray  slate  (6),  probably  100  feet. 
Concealed  by  kames  and  swamp  ;  and  only  one  outcrop,  east 
of  Portuguese  Lane.  Conglomerate  (7),  35  feet.  A  single 
ledge  on  the  west  side  of  the  lane,  passing  south  into  the 
solitary  outcrop  of  the  next  bed.  Red  slate  (8),  40  feet. 
This  crosses  the  lane  just  north  of  the  point  where  it  divides  ; 
and  a  few  yards  east  of  the  junction  is  the  principal  exposure 
of  the  next  stratum.  Fine  conglomerate  and  sandstone  (9), 
50  feet.  Red  slate  (10),  not  exposed,  but  there  is  room 
between  the  nearest  outcrops  of  conglomerates  9  and  11  for  25 
or  30  feet  of  slate.  Conglomerate  (11) ,  fine  but  without  much 
sandstone,  about  80  feet ;  forms  extensive  ledges  east  of  the 
lane.  Red  slate  (12),  about  50  feet ;  several  good  outcrops 
east  of  the  lane.  Fine  conglomerate  and  sandstone  (13),  a 
broad  belt  of  uncertain  width.  If  correctly  mapped,  the 
thickness  must  be  about  300  feet.  The  more  prominent  ledges 
are  chiefly  fine  conglomerate  ;  but  there  is  undoubtedly  a  large 
proportion  of  sandstone.  Outcrops,  it  will  be  observed,  are 
almost  wholly  wanting  in  the  southern  half  of  this  belt ;  and 
the  southern  limit  is  quite  uncertain.  Gray  slate  (14),  of  un- 
known thickness.  The  exposures  begin  near  the  north  shore  of 
Beal's  Cove  ;  and  along  the  sliorc  of  the  river  the  outcrop  is  con- 


221 

tinuous  from  the  end  of  Portuguese  Lane  south  to  the  point,  the 
total  exposed  thickness  being,  ])robal)ly,  not  less  than  500  feet. 
The  slate  is  of  a  very  uniform  character, — gray,  rather  coarse, 
very  thinly  and  evenly  bedded,  well  jointed,  but  without  marked 
cleavage.  Some  layers  are  finely  pitted  by  weathering  and 
wave-action,  indicating,  possibly,  some  lime  in  the  rock  ;  and 
there  are  some  indistinct  concretions.  But  although  the  section 
appears  in  eveiy  way  favorable  for  the  occurrence  of  fossils,  I 
have  searched  for  them  in  vain.  In  the  most  northerly  outcrop 
of  the  slate  it  is  distinctly  and  repeatedly  interstratified  with 
layers  of  hard,  gniy,  and  rather  coarse  sandstone,  from  two 
inches  to  two  feet  thick.  Since  such  intercalations  of  coarse 
material  are  wholly  wanting  in  the  remaining  500  feet  of  the 
slate,  this  outcrop  is  regarded  as  marking  the  transition  from 
the  underlying  conglomerate  and  sandstone  (13)  to  the  slate  ; 
and  it  is  the  chief  fact  relied  upon  in  drawing  the  boundary  on 
the  map.  It  is  unnecessar}^  to  dwell  upon  its  importance  as 
evidence  that  the  slate  conformably  overlies  the  conglomerate 
series.  It  may  well  be,  however,  that,  as  in  the  case  of  the 
conglomerate  in  the  Huit's  Cove  section,  to  be  described  in  the 
following  pages,  this  intercalated  coarse  material  is  underlain 
by  a  considerable  thickness — 100  feet  or  more — of  slate. 

It  is  impossible  to  carry  the  section  farther  south  ;  for  the 
east  and  south  shores  of  Beal's  Cove  are  wholly  composed  of 
modified  drift  which  rises  from  30  to  80  feet  above  the  water, 
and  extends  southeast  across  the  Hockley  district  to  the  rail- 
road, blotting  out  everything  but  an  occasional  ledge  of  granite. 
Following  the  east  shore  of  Weymouth  Back  River  south 
from  Beal's  Cove,  no  ledges  of  any  kind  are  observed  for 
about  800  feet  from  the  south  angle  of  the  cove,  and  then, 
when  nearly  opposite  the  south  side  of  Whale  Island,  we 
reach  the  beginning  of  an  outcrop  of  coarse  granite  which  is 
almost  continuous  alongc  the  shore  for  500  feet.  The  granite 
is  divided  by  three  large  east-west  dikes  ;  but  there  is  not  a 
trace  of  slate  or  any  sedimentary  rock.     Between  the  nearest 


222 

outcrops  of  slate  and  granite  there  is  thus  a  gap  of  fully  one 
fourth  of  a  mile,  and  concerning  the  exact  position  and  the 
character  of  the  contact  we  know  simply  nothing. 

The  stratigraphic  continuity  of  the  section  from  the  granite 
north  of  Beal  Street  across  the  melaphyr  and  the  conglomerate 
series  to  Beal's  Cove  is  unquestionable ,  and,  although  the 
relations  of  the  Beal's  Cove  slate  to  the  conglomerate  series 
are  not  so  fully  and  clearly  exposed  as  could  be  desired,  the 
perfect  agreement  in  strike  and  dip  and  the  indications  of  a 
gradual  transition  afforded  by  the  interstratification  of  sand- 
stone and  slate,  appear  to  justify  us,  as  already  stated,  in 
regarding  it  as  a  part  of  the  same  conformable  sequence  of 
strata.  If,  as  this  view  requires,  the  slate  conformably  over- 
lies the  conglomerate  series,  Beal's  Cove  must  mark  the 
position  of  a  synclinal  axis  ;  and,  since  the  slate  must,  in  this 
case,  be  much  newer  than  the  granite,  the  south  side  of  the 
syncline  is  probably  cut  away  by  a  fault  with  the  downthrow  to 
the  north,  for  there  is  evidently  not  room  enough  south  of  the 
axis  for  a  repetition  of  the  conglomerate  series  and  melaphyr. 
Between  the  granite  north  of  Beal  Street,  which  still  holds  its 
normal  relation  to  the  bedded  rocks  deposited  upon  it,  and  the 
granite  against  which  they  end,  as  the  result  of  faulting,  on 
the  south,  we  have,  then,  a  steep  monocline  (one  half  of  a 
syncline)  and,  as  previously  stated,  one  complete  section  of 
the  Hinghara  strata. 

Eastward  from  Beal's  Cove  and  south  of  Tucker's  Swamp, 
there  are,  fortunately,  sufficient  outcrops  to  connect  the  section 
just  described  with  the  ledges  in  the  vicinity  of  Hockley  Lane. 
The  single  small  outcrop  of  the  true  slate  (14)  east  of  the 
cove  is  much  nearer  the  water  than  represented,  and  evidently 
below  the  highest  bed  on  the  west  side  of  the  cove.  About 
1,000  feet  east  of  the  salt  marsh  bordering  the  northern  arm 
of  the  cove,  and  immediately  beyond  where  the  boundary  of 
Tucker's  Swamp  bends  to  the  north,  we  come  to  a  con- 
siderable exposure  of  the  newest  conglomerate  and  sandstone 


900 


(13),  with  a  large  but  imperfectly  exposed  dike  of  diabase, 
while  farther  east  we  are  able  to  identify  slates  12  and  10  and 
conglomerates  11,  9,  and  7  ;  and  north  of  the  swamp,  apparently 
conglomerates  5  and  3,  in  very  obscure  outcrops.  Several  of 
these  outcrops  are  not  marked  on  the  map,  and  others  are 
incorrectly  marked.  It  is  evident,  however,  that  some  of  these 
beds,  if  they  continue,  must  abut  against  the  end  of  the 
raelaphja-  of  the  Village  section.  This  is  suggestive  of  a 
transverse  fault.  Farther  north  the  outcrops  are  so  imperfect 
on  both  sides  of  the  line  that  a  lack  of  correspondence  or 
continuity  can  not  be  easily  proved.  Farther  south  alono-  the 
supposed  fault  line,  however,  we  find  evidence  which,  so  far 
as  it  goes,  is  entirely  satisfactory.  This  is  afforded  by  the 
slate  which  is  mapped  as  forming  a  short  northwest  and 
southeast  line  of  outcrops  south  of  the  end  of  Hockley 
Lane.  This  slate  is  the  purplish  variety,  with  occasional  thin 
streaks  of  sandstone.  It  strikes  in  the  direction  named  and 
dips  N.  E.  70°.  In  the  direction  of  the  strike  the  slate  is 
exposed  very  near  the  granite,  and  it  is  evident  that  it  is  cut 
off  by  the  granitic  rocks  on  both  the  east  and  south.  The 
most  careful  search  fails  to  reveal  any  evidence  whatever  that 
the  granite  is  intrusive  in  the  slate  ;  but  the  most  satisfoctory 
explanation  of  their  relations  is  that  proposed  on  the  map. 
This  slate  has  been  somewhat  doubtfully  referred  to  the  great 
bed  (14)  ;  and,  judging  from  its  dip,  it  belongs  on  the  south 
side  of  the  synclinal  axis,  which  implies  a  shallowing  of  the 
trough  in  this  direction.  The  marked  southeasterly  strike  may 
be  regarded  as  in  some  way  the  result  of  the  transverse  dis- 
placement. 

Having  now  brought  the  strata  of  the  Beal's  Cove  area  face 
to  face  with  those  of  the  Village  area,  it  is  readily  apparent 
that  they  can  be  correlated  only  by  completely  reversing  the 
one  series  or  the  other.  This  is  most  obvious  in  the  case  of 
the  melaphyr,  which  is  on  the  south  in  the  Village  area  and 
on  the  extreme  north  in  the  Beal's  Cove  area.      The  entirely 


224 

normal  character  of  the  Beal's  Cov^e  section  leaves  no  room  to 
doubt  that,  as  previously  stated,  it  is  the  Village  section  which 
requires  reversing.  This  transverse  fault  is,  then,  the  one 
important  dividing  line  for  the  entire  district  between  the  Home 
Meadows  and  Beal's  Cove.  West  of  this  line  we  have  a  steep 
but  normal  monocline,  terminated  against  the  granite  on  the 
south  by  faulting  ;  while  east  of  it  a  great  displacement  along 
the  northern  border  of  the  bedded  rocks,  accompanied  by  a 
severe  plicating  strain,  has  completely  inverted  and  dislocated 
this  end  of  the  series.  It  is  interesting  to  observe,  also,  that 
this  change  or  break  occurs  opposite  the  southwest  angle  of  the 
granite  axis  on  the  north,  showing  that  this  mass  of  granite  is, 
as  previously  pointed  out,  a  dominant  or  controlling  factor  in 
the  structure  of  this  region.  The  foregoing  outline  of  tlie 
structure  of  this  narrow  southern  trough  will,  perhaps,  be  more 
readily  or  fully  comprehended  if  its  various  phases  are  pre- 
sented in  succession,  as  follows  :  From  the  Home  Meadows 
to  Beal's  Cove  there  was  originally,  or  would  have  been  but 
for  the  faulting,  one  continuous  syncline.  This  is  broken 
transversely  by  the  Hockley  Lane  fault.  The  western  half 
remains  an  open  syncline  :  but  the  greater  part  of  its  southern 
slope  is  carried  away  by  a  strike-fault,  which  brings  up  the 
underlying  granite  in  that  direction.  The  eastern  half,  owing 
to  the  stronger  compression,  being  in  the  narrowest  part  of  the 
granite  vise,  becomes  an  inverted  isocline  with  the  axial  plane 
dipping  to  the  south  ;  and  its  northern  side  is  partly  carried 
away  and  partly  concealed  by  a  strike-fixult,  bringing  up  the 
ofranite  axis  wliich  these  strata  once  covered. 

In  attempting  to  trace  the  various  strata  of  the  Beal's  Cove 
section  westward,  we  find  them  passing  at  once  beneath  almost 
continuous  deposits  of  modified  drift,  including  several  high 
kames.  The  slate  series  and  the  more  southern  members  of 
the  conglomerate  series  are  thus  hopelessly  cut  off.  But  the 
nielaphyr  and  tl)e  strata  immediately  above  it  emerge  sufficiently 
from  the  sea  of  drift  so  that  their  relations  to  the  extremity  of 


225 

the  granite  axis  can  be  obacrvo(^.  It  is  very  [)lain  that  tlio.se 
beds  curve  regularly  around  the  granite,  following  its  north- 
western as  faithfully  as  its  southern  margin.  The  broadening 
of  each  bed  as  it  rounds  the  angle  is  due  to  the  natuial  dim- 
inution of  the  dip  at  this  point.  North  of  Beal  Street  the  belt 
of  melaphyr  is,  at  first,  apparently,  scarcely  100  feet  wide,  l)ut 
it  gradually  broadens  northward,  possibly  as  the  residt  of  a 
diminished  dip.  It  is  seen  in  contact  with  the  basal  conglom- 
erate at  several  points.  The  most  northerly  and  most  satisfactory 
exposure  of  the  contact  is'  in  a  small  excavation  on  the  south- 
west side  of  Hawke's  Lane,  where  the  conglomerate  is  not 
marked  on  the  map  ;  but  in  every  case  the  appearances  are 
best  explained  by  regarding  the  melaphyr  as  contemporaneous 
rather  than  intrusive.  It  fills  the  inequalities  in  the  surface 
of  the  conglomerate  ;  but  does  not  properly  penetrate  that  rock. 
Although  the  great  bed  of  conglomerate  (3)  overlying  the 
melaphyr  is  well  exposed  for  the  entire  distance  between  Beal 
Street  and  Lincoln  Street,  not  a  single  good  contact  could  be 
found. 

Scattered  through  the  woods  and  swamp,  stratigraphically 
above  this  conglomerate,  are  numerous  outcrops  of  conglom- 
erate, sandstone,  and  slate  which  it  is  difficult  to  connect 
satisfactorily  in  continuous  belts.  These  are  somewhat 
generalized  on  the  map,  and  the  correlation  indicated  there  is 
probably  not  entirely  correct.  It  can  hardly  be  doubted, 
however,  that  we  have  here,  in  normal  sequence,  the  red 
slate  (4),  conglomerate  and  sandstone  (5),  and  red  slate  (G)  ; 
and  then  follow  in  succession  conglomerate,  slate,  conglomerate, 
melaphyr,  and  conglomerate,  as  shown  on  the  map.  South  of 
Beal  Street,  however,  and  disregarding  this  melaphyr,  it  would 
not  be  difficult  to  divide  the  nearly  continuous  line  of  outcrops 
so  as  to  identify  or  represent  every  bed  in  the  Beal's  Cove 
section  up  to  the  highest  conglomerate  (13),  The  interpreta- 
tion of  this  part  of  the  Beal's  Cove  area  is  one  of  the  puzzles 
of  Hingham  geology.     The  general  structure    appears    to    be 

OCCAS.    PAPERS    B.    S.    N.    H.  IV.  15 


226 

synclinal ;  and  the  map  and  section  are  constructed  in  accord- 
ance with  this  view.  The  dip,  for  the  most  part,  varies  but 
slightly  from  90° ;  but  still,  the  advocate  of  the  synclinal  theory 
can  find  some  ground  for  its  support  in  the  attitude  of  the 
strata.  It  should  be  pointed  out,  however,  that  the  dip  of  the 
ledo-e  of  slate  on  the  north  side  of  Beal  Street  is  W.  85°,  and 
not  E.  85°,  as  marked ;  while  the  ledges  southwest  of  this 
point  and  east  of  the  Alms  House  are  vertical.  According  to  the 
map  and  section  the  western  strata  are  a  repetition  of  those  on 
the  east  side  of  the  belt ;  and  the  syncline,  although  closely 
appressed,  is  shallow,  holding  only  the  lower  half  of  the  con- 
glomerate series,  up  to  and  including  the  second  slate  (6)  ;  and 
as  it  approaches  the  shore  it  is  merged  rapidly  with  the  east- 
west  monocline  or  rather  with  the  main  syncline. 

Although  this  interpretation  seemed  the  most  satisfactory  at 
the  time  when  the  special  map  and  section  were  drawn,  sub- 
sequent observation  and  reflection  have  caused  the  alternative 
view  to  appear  more  acceptable;  viz.,  that  all  the  beds  of  the 
Beal's  Cove  section  pass  in  regular  order  around  the  granite, 
the  structure  being  monoclinal  in  both  directions  from  the 
o-ranite.  This  later  and  present  view  is  expressed  on  the 
general  map,  and  hence  the  two  maps  are  not  in  agreement 
here.  The  ledges  of  slate  south  of  the  Alms  House  and  nearest 
to  the  shore  appear  at  first  sight  to  stand  in  the  way  of  the 
later  interpretation.  But  the  strike  of  these  outcrops  is  really 
much  more  northerly  than  mapped,  being  directly  toward  the 
most  westerly  ledges  east  of  the  Alms  House.  Certain 
irregularities  in  the  strike  were  made  the  most  of  to  bring  the 
ledges  into  conformity  with  the  earlier  explanation  ;  and  the 
dip,  which  is  W.  85°,  was  also  regarded  as  a  local  irregularity. 
I  am  now  disposed  to  refer  these  ledges  to  the  highest  bed  of 
red  slate  (12),  placing  them  on  the  northwest  side  of  the  great 
bend,  but  very  near  the  turning  point.  And  the  still  higher 
beds,  the  massive  conglomerate  (13)  and  the  gray  slate  (14), 
instead  of  bearing  away  to  the  westward,  as  if  to  cross  the  river. 


227 

are  now  regarded  as  bending  to  the  north  with  the  whore  and 
following  the  lower  beds  around  the  bend.  This  gi-eat  bend  is, 
of  course,  the  extremity  of  the  great  central  and  dominant 
anticline  of  Hingham,  and  that  it  is  the  extremity  is  proved  by 
the  undeniable  fact  that  the  axis  here  plunges  steeply  dowr)- 
ward  to  the  west.  Southward  from  this  axis  the  •  beds  foi"m  a 
monocline  of  from  80°  to  85°,  extending  to  Beal's  Cove  ;  and  on 
the  northwest  side  a  monocline  of  from  85°  to  90°  extends  out 
beneath  the  level  sandplain. 

At  the  time  of  my  first  observations  in  this  locality,  fifteen 
years  ago,  I  was  deeply  impressed  by  the  fact,  that  there  are, 
northwest  of  the  granite,  two  similar  and  parallel  ridges  of 
conglomerate  and  melaphyr,  with  an  intervening  valley  com- 
posed chiefly  of  sandstone  and  slate.  This  strong  topographic 
suggestion  of  a  syncline  biased  all  my  later  observations,  until 
the  })resent  writiiig  compelled  a  broader  and  more  rigid  examina- 
tion of  the  facts,  and  the  absence  of  any  real  geological  evidence 
of  a  synclinal  structure  became  apparent. 

Whichever  view  of  the  structure  of  the  beds  on  the  north- 
west side  of  the  axis  is  accepted,  important  strike-fiiults  nnist 
be  introduced  to  explain  the  second  belt  of  melaphyr.  This 
melaphyr  is  essentially  similar  to  that  on  the  east  side,  against 
the  granite,  and  one  naturally  regards  it  at  first  as  marking  a 
denuded  anticline.  Its  eastern  edge,  however,  is  quite  cleai'ly 
transgressive  with  reference  to  the  bordering  strata ;  and  it 
is  necessary  either  to  introduce  a  fault  here  or  to  regard  the 
melaphyr  as  a  dike.  No  reliable  dips  have  been  observed  in  the 
conglomerate  on  the  west  side  of  this  melaphyr  ;  but  it  may  be 
reasonably  correlated  with  that  on  the  shore  of  Huit's  Cove, 
which  dips  westward  away  from  the  melaphyr  and  beneath  the 
slate  series.  This  belt  of  melaphyr  is  a  direct  prolongation  of 
the  great  body  of  melaphyr  lying  east  of  Huit's  Cove,  and  can 
not  be  regarded  as  intrusive  unless  we  are  prepared  to  ascribe 
that  origin  to  the  entire  area,  or  in  fact  to  all  the  inelaphyr  of 
Hingham,      The  fault  separating  the  melaphyr  from   the  slate 


228 

series  on  the  west  conceals  for  the  entire  distance  all  but  the 
highest  bed  of  conglomerate,  and  this  also  is  cut  out  or,  rather, 
concealed  for  a  part  at  least  of  the  distance  between  the  cove 
and  Lincoln  Street,  the  slate  lying  here  directly  against  the 
melaphyr.  The  area  crossed  by  the  western  portions  of  Beal 
and  Lincoln  Streets  is  an  unbroken  sandplain,  and  the  broad 
expanse  of  slate  represented  here  is  unsupported  by  a  single 
outcrop.  The  map  finds  abundant  justification,  however,  in 
the  topography  and  in  the  extensive  outcrops  of  slate  on  the 
shores  of  Huit's  Cove,  since,  if  continued,  and  we  have  no 
reason  to  suppose  the  contrary,  these  beds  must  cross  this  area. 
The  tongue  of  melaphyr  can  not  be  traced  south  of  a  point  500 
feet  north  of  Beal  Street  ;  and  whether  the  bounding  faults 
actually  meet,  as  mapped,  and,  if  so,  whether  the  united  fault 
extends  farther  to  the  southwest,  it  is  impossible  to  determine. 
The  general  interpretation  of  the  geologic  structure  here  pro- 
posed makes  it  unnecessary  to  suppose  that  any  of  the  Hingham 
strata  extend  far  into  Weymouth.  All  of  wliich  tends  to 
emphasize  the  importance  of  Weymouth  Back  River  as  a 
geologic  boundary  ;  and  we  may  reasonably  assume  that  this 
valley  follows  a  fault  comparable  in  magnitude  and  structural 
importance  with  that  along  the  east  side  of  Hingham  Harbor, 
separating  areas  which  are  strongly  contrasted  in  their  geologic 
features. 

The  Lincoln  Street  and  Broad  Cove  Area. 

This  area  is  represented  by  the  southern  part  of  the  third 
special  map  (Plate  9),  or  all  that  part  southeast  of  the  melaphyr 
and  south  of  the  great  dike.  It  embraces  but  few  ledges, 
being  covered  almost  continuously  by  the  Squirrel  Hill  and 
Bradley  Hill  drumlins  and  the  low  sandplain  and  broad 
meadows  into  which  they  sink. 

The  monocline  northwest  of  Squirrel  Hill  can  be  readily 
traced  to  the  northeast  across  Lincoln  Street,  as  the  map 
shows,  as  far  as  Huit's  Cove  Lane.      All  the  beds,  so  far  as 


229 

observed,  are  vertical  ;  and  it  is  obvious  that  tiiere  is  room  for 
not  more  than  half  of  the  conglotnerate  series  between  the 
mclaphyr  on  the  southeast  and  that  on  the  northwest.  The 
lower  beds,  from  the  melaphyr  (2)  and  the  great  conglomerate 
(3)  to  the  third  conglomerate  (7),  are  clearly  continuous. 
Hence  there  can  be  but  little  doubt  that  the  missing  beds 
belong  in  the  upper  half  of  the  section,  or  that  their  absen(;e  is 
due  to  a  great  fault  which  has  elevated  the  melaphyr  on  the 
northwest.  The  map  represents  the  melaphyr  between  the 
conglomerate  series  and  the  granite  as  broadening  rapidly 
toward  the  northeast,  and  this  would  seem  to  indicate  a 
marked  flattening  of  the  dip.  Outcrops  are  wholly  wanting, 
however,  south  of  Lincoln  Street  and  east  of  Hawke's  Lane  ; 
and  it  now  seems  much  better  that  this  area  should  be 
colored  as  granite.  But  even  then  there  would  be  a  decided 
increase  in  the  breadth  of  the  melaphyr  east  of  the  lane. 

The  steep  monocline  can  not  be  traced  beyond  Huit's  Cove 
Lane,  The  first  one  of  the  two  transverse  faults  represented 
here  appears  to  be  justified  by  the  offsetting  of  the  strata  as 
seen  in  the  actual  ledges  ;  and  it  is  probably  less  important 
than  the  other,  which  is  based  upon  the  general  interruption  of 
the  outcrops  and  especially  upon  the  fact,  that  to  the  eastward 
of  this  line,  so  far  as  the  scattering  ledges  allow  us  to  judge, 
an  entirely  different  type  of  structure  prevails.  The  dips,  of 
both  the  melaphyr  and  the  sedimentary  rocks,  are  everywhere 
low  and  indicate  broad  shallow  folds.  In  the  absence  of 
outcrops  immediately  east  of  the  north  end  of  Huit's  Cove 
Lane,  we  can  not  know  whether  this  change  takes  place 
abruptly  or  gradually,  by  faulting  or  otherwise  ;  but  the  fault 
line  on  the  map  calls  attention  to  the  ftict  that  a  change 
occurs  somewhere  in  that  vicinity,  and  it  also  makes  it  easier 
to  interpret  the  outcrops  along  the  line  of  Lincoln  Street  east 
of  Huit's  Cove  Lane. 

The  melaphyr  south  of  Lincoln  Street  and  west  of  Thaxter 
Street,    at    the    northern    base    of    Squirrel    Hill,    is    mostly 


230 

greenish,  sometimes  reddish  or  purplish,  abundantly  and 
rather  coarsely  amygdaloidal  in  certain  parts,  with  segregations 
of  epidote  and  ferruginous  silica,  and  in  every  essential 
respect  similar  to  that  which  we  have  followed  all  the  way 
around  the  granite  from  Portuguese  Lane.  The  dip,  so  fsir  as 
can  be  judged  by  the  flow  structure — the  sheets  and  layers 
of  amygdules — is  nearly  horizontal.  The  occurrence  of 
felsite  behind  Mr.  Bradley's  barn,  just  west  of  Thaxter  Street, 
as  already  described,  makes  it  necessary  to  curve  the  southern 
boundary  of  the  melaphyr  to  the  north  here.  The  last  exposure 
of  this  melaphyr  is  in  Thaxter  Street,  a  few  yards  south  of 
Lincoln  Street;  but  it  is  assumed,  in  the  absence  of  evidence 
to  the  contrary,  to  form  a  continuous  belt  bordering  the  granite 
as  far  east  as  the  harbor.  The  melaphyr  in  the  angle  east  of 
Huit's  Cove  Lane  and  north  of  Lincoln  Street  is  of  precisely 
the  same  character,  except  that  it  is,  perhaps,  more  profusely 
amygdaloidal  ;  and  the  layers  of  amygdules  indicate  very 
plainly  a  gentle  southerly  dip,  suggesting  that  the  conglomerate 
on  Lincoln  Street,  between  Thaxter  Street  and  the  Lane,  in 
which  the  dip  has  not  been  made  out,  lies  in  a  shallow 
syncline,  and  that  the  melaphyr  is  continuous  beneath  it. 
This  correlates  the  conglomerate  with  the  great  bed  (3), 
which  we  have  elsewhere  found  lying  directly  upon  the 
melaphyr.  It  is  the  broad  and  open  character  of  this  fold,  as 
thus  indicated,  that  has  led  me  to  extend  the  conglomerate 
color  so  far  beyond  the  observations. 

The  large  outcrop  of  melaphyr  west  of  the  junction  of 
Downer  Avenue  and  Crow  Point  Lane  is  quite  varied  in 
character.  It  is  not  conspicuously  amygdaloidal,  and  the 
lower  and  northern  part  of  the  mass,  although  somewhat 
hrecciated  and  containing  irregular,  angular  segregations  of 
bright  red  jasper,  is  mainly  quite  compact  and  massive. 
Above,  however,  the  melapli}^-  is  very  scoriaceous,  with 
numerous  jaspery  and  epidotic  segregations  and  veinlcts  and 
distinct  indications  of  a  true  conglomerate  with  well-rounded 


231 

pebbles  lying  upon  and  filling  the  inequalitieK  of  its  surface. 
We  have  exposed  here,  apparently,  the  original  upper  surface 
of  the  nearly  horizontal  bed  of  melaphyr.  Care  is  required  to 
distinguish  the  segregations  of  jasper  from  the  inclusions  of 
red  felsite.  Several  small  fragments  of  felsite  similar-  to  that 
a  hundred  yards  away,  at  the  junction  of  Downer  Avenue  and 
Crow  Point  Lane,  and  one  mass  a  yard  or  more  in  diameter  of 
a  slightly  more  granitic  character,  were  observed  imbedded  in 
the  melaphyr,  indicating  very  plainly  that  this  ancient  basic 
lava  broke  through  the  still  more  ancient  acid  lava  on  its  way 
to  the  surface.  The  outcrop  in  the  street,  as  previously 
explained,  probably  represents  a  knob  or  boss  of  felsite 
projecting  through  the  flow  of  melaphyr  where  erosion  has  cut 
most  deeply  into  the  latter.  The  extension  on  the  general 
map  of  these  two  belts  of  melaphyr  eastward  across  Broad 
Cove  and  Otis  Hill  to  the  harbor  is,  no  doubt,  somewhat 
hazardous ;  but  no  reason  is  apparent  for  terminating  the 
melaphyr  west  of  the  shore,  and  its  extension  finds  some,  if 
not  sufficient,  justification  in  the  bowlders  of  melaphyr 
observed  on  Button  Island.  The  only  rock  observed  in  place 
on  this  islet,  as  previously  stated,  is  granite  ;  but  the  numerous 
large  and  angular  bowlders  of  amygdaloidal  melaphyr  on  the 
north  and  northwest  shores  prove  that,  although  this  rock  does 
not  form  part  of  the  island,  it  must  underlie  the  portion  of 
the  harbor  immediately  to  the  northward.  The  melaphyr 
forming  these  bowlders  is  similar  to  that  in  the  vicinity  of 
Lincoln  Street,  being  of  greenish  and  reddish  tints,  coarsely 
and  distinctly  amygdaloidal  with  epidote  and  quartz,  and 
exhibiting  numerous  irrej^ular  seo-reo-ations  or  veinlets  of  the 
same  minerals. 

The  great  dike  represented  on  the  special  map  as  extending 
west  from  Downer  Avenue  across  Planter's  Fields  Lane  to  the 
eastern  angle  of  the  Huit's  Cove  melaphyr  is  probably  the  best 
interpretation  of  the  outcrops  which  it  embraces.  These  are 
all   of  precisely    the    same   character,    a  typical    coarsely    and 


232 

uniformly  crystalline,  massive  diabase,  the  texture  being  pro- 
portional to  the  magnitude  of  the  dike.  Another  argument 
for  the  dike  is  that  the  strata  on  opposite  sides  of  this  line  of 
outcrops  are,  as  the  map  shows,  clearly  and  entirely  at  variance 
in  dip  and  strike,  and  a  fault  coinciding  with  the  dike  is  a 
structural  necessity.  Whether  the  dike  actually  ends  against 
the  melaphyr  as  represented,  it  is  impossible  to  determine, 
since  it  passes  in  this  direction  beneath  an  extensive  wet 
meadow  and  outcrops  are  wholly  wanting.  The  coarse, 
holocrystalline,  and  homogeneous  character  of  the  diabase 
utterly  forbids  connecting  the  dike  with  the  melaphyr  as  a 
possible  channel  of  supply  for  the  latter.  The  melaphyr  is 
certainly  older  and  the  dike  is  just  as  clearly  newer  than  the 
bordering  sedimentary  rocks  ;  and  the  smaller  dikes  running 
south  through  the  sandstone  and  conglomerate  are  probably,  as 
indicated  on  the  map,  branches  of  the  mahi  dike. 

The  strata  between  the  great  dike  and  the  melaphyr  of 
Downer  Avenue  and  Lincoln  Street  form,  apparently,  a  low 
monocline  of  15° — 30°.  We  commence  on  the  avenue,  north 
of  Planter's  Fields  Lane,  with  outcrops  of  a  purplish  and  gray 
slate  passing  south  into  gray  sandstone  with  fine  [)ebbly 
layers,  the  whole  dipping  S.  30°.  The  sandstone  can  be 
traced  west  across  the  fields,  as  mapped,  and  is  seen  to  change 
gradually  upward  into  the  small-pebbled  conglomerate  which 
outcrops  so  prominently  along  the  entire  distance,  especially 
west  of  the  lane.  The  dip  of  the  conglomerate  was  not 
clearly  observed  ;  but  it  passes  on  the  south,  through  sandstone 
to  a  finely  banded  slate,  75  feet  in  breadth,  with  a  southerly 
dip  of  only  15°.  South  of  the  slate  are  several  slight 
exposures  of  sandstone  before  we  come  to  the  conglomerate 
crossed  by  the  lane,  near  the  melaphyr.  This  outcrop  is  more 
extensive  than  marked,  extending  200  feet  southwest  of  the 
lane. 

The    correlation  of  these  beds    is   not  easy.      Their  surface 
exposures  or  developments,  as    compared  with  other  sections, 


Occas, Papers,  Boston  Soc.Nat.  Hist.,  YoI.IV; 


O  O  O  O  ^  y 

oo  o  oo    toncfCoTneroUe. 

o  o  o   <^  ^ 


Sandstone. 


Slate 


HHwl    Dikes 

ScaleIinch  =5oo  feet 


Downer 


Crow  Point  and  theNb 


Plate  10 


lin^ 


Langley  Id. 


Sarah  Id. 


^HBORiNG  Islands 


233 

are,  of  course,  greatly  broadened  by  the  relatively  low  dips  ; 
and  there  is  nothing  to  show  conclusively  with  which  part  of 
the  Real's  Cove  section,  for  example,  they  should  be  identified. 
No  dips,  however,  have  been  observed  south  of  the  slate  ;  and 
it  is  possible  tliat  the  low  and  wavering  dip  of  the  slate  itself 
should  be  inter[)reted  as  marking  a  synclinal  axis,  the  sand- 
stone and  conglomerate  on  the  south  being  a  repetition  of  that 
on  the  north.  The  conglomerate  could  then,  perhaps,  be 
correlated  with  the  first  great  bed  (3)  and  regarded  as  passing 
up  over  the  melaphyr,  which  would  thus  mark  an  anticlinal 
axis  ;  and  we  shoidd  be  able  to  dispense  with  the  fault  between 
the  conglomerate  and  melaphyr. 

The  Melville  Garden  and  Planter  s  Fields  A.rea. 

This  area  embraces  the  district  east  of  the  Huit's  Cove 
melaphyr  and  north  of  the  great  dike  and  Otis  Hill,  including 
a  large  part  of  the  tract  known  as  Planter's  Fields,  Melville 
Garden,  Pleasant  Hill,  the  smaller  drumlins  forming  Crow 
Point,  and  the  adjacent  islands  of  Hingham  Harbor — Ragged, 
Sarah,  and  Langlee.  From  the  western  base  of  Pleasant  Hill 
eastward,  north  of  Melville  Garden,  to  Downer  Landing, 
the  drumlins  are  continuous,  as  shown  on  the  general  map, 
and  the  hard  rocks  are  wholly  concealed  ;  while  curving  around 
the  south  and  west  sides  of  this  drift  area,  as  previously 
noticed,  is  a  well-exposed  belt  of  strata,  the  third  and  last 
cjeneral  section  of  the  cono-lomerate  series. 

The  most  perfect  outcrops  are  those  afforded  by  the  islands,^ 
which,  it  may  be  noted  in  passing,  are  a  beautiful  illustration 
of  the  dependence  of  relief  upon  geologic  structure.  The  shores 
of  these  islets  are  almost  continuous  exposures  ;  and  the  atti- 
tude of  the  sti'ata  is  exceedingly  constant,  the  strike  being  neai-ly 
due  east-west  and  the  dip  S.  35°-40°.  Langlee  Island  is  an 
approximately  rectangular  mass  600    feet  long   and    400   feet 

1  See  the  uncolored  special  map  (PL  10). 


234 

wide,  with  two  detached  half-tide  ledges  on  the  eastei-n  end, 
making  the  extreme  length  800  feet.  The  two  small  bays  in- 
denting the  western  shoi'e  correspond  to  depressions  crossing  the 
island  parallel  with  the  strike  and  due,  no  doubt,  to  the  erosion 
of  the  softer  strata.  Commencing  with  the  lowest  or  most 
northerly  beds,  the  island  presents  the  following  section  : — 

Coiiiiiomerate,  coarse  to  mediuin,  100  feet,  equal  to  thickness  of  GO  feet. 
Concealed,  probably  slate,  35      "         "       "         "  "    20      " 

Conglomerate,  medium  to  fine,       170      "         "       "         "  "100       " 

Streak  of  sandstone  near  the  northern  edge. 
Concealed,  probably  slate,  70      "         "       "         "  "  40      " 

Conglomerate,  60      "         "       "         "  "  35      " 

435  255 

Along  the  north  side  of  each  depression  there  is  a  dike  of 
uncertain  width,  the  north  dike  having  a  northerly  hade  of  15°  ; 
but  it  is  improbable  that  the  depressions  are  due  wholly  or 
even  chiefly  to  the  erosion  of  the  dikes.  Langlee  Island  is 
separated  by  about  600  feet  of  water  from  Sarah  Island,  an 
ol)long  mass  275  feet  in  breadth  and  835  feet  in  extreme  length, 
with  a  h)ng,  lialf-tide  ledge  about  100  feet  from  and  parallel 
with  the  south  shore.  It  is  constituted  as  follows,  in  ascending 
order :  Sandstone,  gray  and  coarse,  85  feet,  equal  to  thick- 
ness of  50  feet,  with  a  streak  of  conglomerate  near  the  middle 
of  the  bed.  Conglomerate,  medium  to  fine,  190  feet,  equal  to 
thickness  of  115  feet,  with  a  streak  of  sandstone  near  the 
northern  edge.  Water,  probably  concealing  sandstone  and 
slate,  75  feet,  equal  to  thickness  of  45  feet.  Conglomerate, 
forminof  half-tide  ledfje. 

West  of  Sarah  Island,  with  about  300  feet  of  water  interven- 
ing, is  Ragged  Island,  235  feet  in  breadth  and  735  feet  in 
extreme  length,  with  two  linear  ledges  parallel  to  the  southern 
shore  and  100  feet  distant.  These  two  islands  are  thus 
essentially  similar  in  form  ;  and  they  present  identical  sections, 
except  that  in  the  depression  south  of  Ragged  Island  a  little 


235 

sandstone  is  exposed,  making  it  probable  tliat,  as  stated,  this 
break  covers  a  bed  of  sandstone,  or  of  sandstone  and  slate. 
The  sandstone  forming  the  north  shore  encloses  from  5  to  10 
feet  of  fine  conglomerate,  which  becomes  finer  toward  the  west, 
changing  to  sandstone.  It  encloses  masses  of  banded  slate  of 
various  colors,  some  of  which  are  possibly  large  pebbles  ;  but 
the  greater  number,  it  is  clear,  can  not  be  explained  in  that  way, 
and  must  be  referred  to  the  irregular  deposition  of  fine  silt  with 
the  coarse.  In  most  cases  the  stratification  planes  of  the  slate 
coincide  exactly  with  the  bedding  of  the  sandstone  and 
conglomerate.  The  correlation  of  the  two  sections  is  unques- 
tionable ;  but  when  we  carry  the  line  of  strike  from  either 
island  across  to  the  other  a  lateral  displacement  is  observed, 
each  longitudinal  feature  on  Ragged  Island  being  about  100 
feet  farther  south  than  its  continuation  on  Sarah  Island.  This 
want  of  alignment  seems  to  be  best  explained  by  a  transverse 
fault  between  the  two  islands  ;  with  the  downthrow  to  the  east, 
as  show^n  on  the  map.  If  such  a  fault  exists,  the  vertical 
displacement  must  be  about  75  feet. 

Some  300  or  400  feet  southwest  of  Ragged  Island,  a  i)ile  of 
angular  blocks  of  fine  conglomerate  projects  from  the  mud  flat 
when  the  tide  is  out.  These  masses  probably  represent  an 
underlying  ledge,  although  it  is  possible  they  were  derived  from 
the  Ragged  Island  bed. 

In  the  .general  line  of  strike  of  Ragged  Island  is  the  high  and 
massive  ledge  of  conglomerate  forming  the  headland  and  the 
north  shore  of  Walton's  Cove,  in  Melville  Garden.  The  con- 
glomerate has  a  breadth  of  at  least  150  feet  in  the  garden, 
probably  passing  beneath  the  water  on  the  south  ;  and  it  may 
be  safely  correlated  with  the  main  bed  on  the  southern  islands. 
More  careful  observation,  however,  shows  that  there  is  here, 
again,  a  lack  of  alignment,  the  southern  border  of  the  conglom- 
erate on  Ragged  Island  coinciding  in  direction  very  closely  with 
the  northern  border  of  the  conglomerate  in  the  garden.  This 
means  a  horizontal  displacement  of  perhaps   150  feet,   and  a 


236 

vertical  slip  of  at  least  100  feet,  with  the  downthrow  to  the 
east,  the  transverse  fault  between  Sarah  and  Ragged  Islands 
being  repeated  between  Ragged  Island  and  the  main  land. 

Northwest  of  the  bridge  across  the  cove,  the  cono-lomerate 
slopes  steeply  down  to  the  water  in  one  broad  diagonal  joint- 
face.  On  the  north  side  of  the  headland  at  the  eastern  end  of 
the  ledge  the  conglomerate  shows  very  distinctly  a  southerly 
dip  of  35°.^  The  shore  retreats  here,  tiie  headland  giving  way 
to  a  small  sandy  beach,  at  the  north  end  of  which  a  grav  sand- 
stone outcrops  with  the  same  dip  as  the  conglomerate.  It  is 
very  evident  that  the  sandstone  is  really  as  broad  as  the  beach, 
that  the  base  of  the  abrupt  northern  face  of  the  conglomerate 
marks  the  contact  of  the  two  rocks,  and  that  this  sandstone  is 
the  continuation  of  that  formina;  the  northern  shores  of  Rao-wed 
and  Sarah  Islands.  Between  the  buildings  in  the  northern  part 
of  the  garden  is  an  exposure  of  conglomerate  (see  map)  which  is 
stratigraphically  below  the  sandstone  :  and  westward  on  this 
line,  across  Downer  Avenue  and  east  of  Whiton  Avenue,  it 
outcrops  sufficiently  to  prove  a  bed  of  considerable  thickness. 
In  its  eastward  extension  this  bed  must,  of  course,  pass  wholly 
to  the  north  of  the  southern  islands.  The  outer  [)art  of 
Walton's  Cove  clearly  corresponds  to  the  gap  between  the 
islands  and  the  ledges  parallel  with  their  southern  shores  ;  but 
the  inner  part,  influenced  no  doubt  by  the  diagonal  jointing  of 
the  conglomerate  already  referred  to,  is  oblique  to  the  stratifi- 
cation, and  the  same  bed  of  conglomerate  forms  both  shores. 
South  of  this  conglomerate,  and  in  line  with  the  outer  part  of 
the  cove,  there  are  outcrops,  as  the  map  shows,  of  a  purple, 
banded  slate  and  gray  sandstone.  The  slate  is  contorted,  and 
is,  doubtless,  underlain  as  well  as  overlain  by  sandstone.  The 
dips  of  the  slate  are,  of  course,  unreliable,  but  the  sandstone 
shows  that  the  dip  observed  north  of  the  conglomerate  still 
continues.  South  of  these  soft  beds,  and  forming  the  south 
shore  of  the  cove,   east  of  the   bridge,  is  a  third  bed  of  con- 

I  liy  mistiikc  the  dip  is  iiuidu  25°  on  the  map. 


237 

glomerate.  This  is  small  pebbled,  and  is  probably  the  bed 
forming  the  ledges  south  of  the  ishuids,  althougii  dislinetly 
smaller  pebbled.  South  of  this  eomglomerate  there  are  no 
outcrops  east  of  Downer  Avenue. 

On  the  west  side  of  the  avenue,  we  have  first  the  con- 
glomerate already  referred  to  east  of  Whiton  Avenue,  fol- 
lowed by  a  blank  space  wide  enough  for  the  sandstone  north 
of  Walton's  Cove  ;  and  then  we  come,  in  the  western  part  of 
Melville  Garden,  to  a  broad  exposure  of  the  main  conglom- 
erate. In  passing  westward  the  strike  changes  gradually  from 
east- west  to  northwest,  and  there  is,  apparently,  a  marked 
flattening  of  the  dip,  intercalated  layers  of  slate  and  sandstone 
showing  a  southwesterly  dip  of  only  5° — 10°.  South  of  this 
conglomerate  are  ledges  of  sandstone  followed  by  a  blank 
space  which  probably  conceals  the  bed  of  purple  slate  already 
noticed  east  of  the  avenue ;  and  then  come,  in  the  western 
part  of  the  gjirden,  near  Crescent  Avenue,  30  feet  in  breadth 
of  the  sandstone  seen  south  of  the  purple  slate,  and  after  a 
blank  of  50  feet,  100  feet  in  breadth  of  massive  gray  sand- 
stone with  a  southwesterly  dip  of  20°.  Following  the  strike  of 
this  heavy  bed  of  sandstone  east  across  the  small  pond  and  the 
avenue,  we  have,  apparently,  no  alternative  but  to  connect  it 
with  the  third  or  most  southerly  conglomerate  in  that  part  of 
the  garden.  If  this  correlation  is  correct,  we  have  here  a 
remarkable  instance  of  rapid  lateral  change  in  the  character  of 
the  coarser  sediments,  in  view  of  which  we  may  well  hesitate  in 
correlating  this  section  with  those  south  and  west  of  the  o-ranite 
axis.  Although  this  explanation  has  the  merit  of  simplicity, 
and  has  been  followed  in  the  construction  of  the  map,  it 
appears  best,  on  the  whole,  as  will  be  more  clearly  shown 
later,  to  introduce  a  third  transverse  or  north-south  fault  here, 
the  fault  cutting  obliquely  across  Downer  Avenue  between  the 
two  ponds. 

The  beds  of  slate  are  commonly  marked  by  an  absence  of 
outcrops,    forming    smooth     open    lanes     or    narrow    valleys 


238 

between  the  conglomerate  ridges  ;  and  this  peculiarity  is  the 
only  fact  that  can  be  cited  in  support  of  the  bed  of  slate 
shown  on  the  map  south  of  this  sandstone  bed.  That  is,  the 
topography  is  suggestive  of  slate,  but  no  outcrops  have  been 
observed.  Following  this  blank,  comes  a  belt  of  fine  con- 
glomerate a  good  hundred  feet  in  breadth.  It  outcrops  boldly 
on  the  southwest  border  of  the  garden,  northwest  of  the 
larger  pond  ;  and  can  be  satisfactorily  traced  westward  along 
the  east  side  of  Grove  Avenue  to  a  point  750  feet  northwest 
of  Crescent  Avenue,  where  it  passes  beneath  the  Pleasant  Hill 
drumlin.  The  outcrops  of  this  bed,  in  connection  with  the 
preceding,  first  fully  outline  the  great  curve  in  the  strike  ;  and 
it  will  be  noticed  that  while  the  lower  part  of  this  general 
section  is  based  upon  outcrops  conforming  with  the  eastern  area 
of  the  curve  and  dipping  south,  the  numerous  outcrops  of  the 
upper  half,  yet  to  be  described,  follow  the  northern  area  and 
dip  west.  Northwest  of  the  pond.  Grove  Avenue  utilizes  onie 
of  the  narrow  slate  lanes,  and  the  slate,  which  is  brown  and 
dips  west  about  30°,  comes  to  the  surface  in  the  northwestern 
extension  of  the  avenue.  The  breadth  of  this  slate  is  scarcely 
30  feet ;  and  then  follow  in  continuous  outcrops  30  feet  in 
breadth  of  gray  sandstone,  40  feet  of  fine  conglomerate,  30 
feet  of  brownish  slate,  90  feet  of  conglomerate  with 
streaks  of  sandstone,  and  40  feet  of  gray  sandstone.  These 
beds  are  clearly  traceable  along  the  strike  for  nearly  one  third 
of  a  mile,  as  shown  on  the  map  ;  and  the  dip,  throughout, 
varies  but  little  from  30°,  although  constantly  changing  in 
direction . 

We  have  now  reached  a  slate  valley  of  unusual  breadth. 
At  the  southeast  end,  near  Grove  Avenue,  there  is  an  obscure 
outcrop  of  brown  slate ;  and  greenish  gray  slate,  changing  west 
to  brown  or  reddish,  is  well  developed  on  the  west  side  of  the 
artificial  pond.  But  the  only  complete  section  is  in  the 
northern  part  of  the  vaHcy.  Here  the  outcrops,  across  the 
strike,  are    almost    continuous,  and    the   breadth    of  the  slate. 


239 

from  the  sandstone  on  the  east  to  tliat  on  the  west,  is  about 
350  feet.  The  shite  is  somewhat  variable  in  charaeter  ;  l>ein"- 
mainly  brownish  or  chocolate-colored  near  the  upper  and  lower 
borders,  and  gray  or  greenish  gray,  banded  with  dull  red,  in  tlie 
middle  of  the  belt.  It  is,  throughout,  thin-bedded,  banded  or 
shaly,  and  finely  jointed.  The  dip  is  constantly  to  the  west 
and  southwest,  but  inconstant  in  amount.  The  ano-le  "-iven 
on  the  map — 60° — is  the  highest  observed,  the  normal  varia- 
tion being  from  30°  to  40'^. 

The  slate  valley  is  bounded  on  the  west  by  60  feet  or  more 
of  coarse  gray  sandstone,  about  30  feet  of  brown  slate,  and 
100  feet  of  sandstone  ;  and  these  beds  can  be  traced  south  by 
frequent  outcrops  to  the  end  of  Planter's  Fields  Lane.  The 
last  sandstone  is  here  ovei'lain  on  the  west  by  a  bed  of  brown 
slate,  which  can  not  be  traced  far  to  the  north.  Then  comes 
a  heavy  bed  of  conglomerate,  which  at  the  northern  end  is 
actually  exposed  for  a  hundred  feet  in  breadth  and  seems  to 
broaden  southward,  possibly  enclosing,  as  the  map  shows, 
another  bed  of  brown  slate.  At  the  extreme  north  this  conglom- 
erate is,  apparently,  overlain  by  still  another  bed  of  sandstone. 
That  all  of  these  beds  are  cut  off  on  the  south  by  the  great 
dike  scarcely  admits  of  doubt ;  but  whether  we  have  reached 
the  end  of  the  series  toward  the  west,  and  how  it  terminates  in 
that  direction,  we  can  only  conjecture,  as  there  is  an  absolute 
blank  of  about  400  feet  between  the  last  of  the  sedimentary 
outcrops  and  the  first  appearance  of  the  Huit's  Cove  mela- 
phyr,  except  at  the  extreme  northwest  corner  of  the  sedimentary 
area.  lam  now  satisfied  that  the  small  outcrops  of  sandstone 
shown  on  the  map  near  the  border  of  the  melaphyr  really  are 
melaphyr  ;  and  the  boundary  line  should  be  carried  far  enough 
to  the  east  to  include  them.  The  actual  gap  between  the  two 
rocks  at  this  point,  measured  across  the  strike,  is  probably  not 
more  than  150  feet. 

Although  there  are  no  reversed  dips,  a  general  view  of  this 
series  of  strata  sujjo^ests  that  the  main  band  of  slate  marks  a 


240 

synclinal  axis,  the  beds  on  the  west  appearing  to  be,  so  far  as 
they  go,  a  repetition  in  reverse  order  of  those  on  the  east. 
This  is  the  explanation  which  I  proposed  twelve  years  ago,  in 
my  "Contributions  to  the  geology  of  eastern  Massachusetts," 
and  it  is  expressed  in  the  section  accompanying  the  special 
map.  This  section  shows  an  inverted  syncline,  on  the  east  side 
of  which  the  beds,  as  they  recede  from  the  axis,  round  over  to 
a  neai'ly  horizontal  position,  as  indicated  by  the  observations  in 
the  northern  part  of  Melville  Garden,  Avest  of  the  avenue  ; 
while  on  the  west  they  maintain  a  high  inclination  until  cut  off 
by  a  fault  against  the  melaphyr.  But  few  dips  have  been 
observed  west  of  the  slate,  and  none  so  high  as  the  section 
represents.  Hence  the  syncline,  if  it  really  exists,  is 
probably  more  completely  inverted  than  it  has  been  drawn. 
Then,  again,  the  repetition  of  the  strata,  as  the  map  shows,  is 
by  no  means  perfect ;  and  we  note  especially  that  it  is  impos- 
sible to  find  on  the  west  side  as  many  beds  of  slate  as  are 
clearly  exposed  on  the  east.  Of  the  three  beds  which  the  map 
shows  on  the  west  side,  the  second  is  based  upon  a  single 
outcrop  and  the  thii'd  upon  none  ;  while  the  conglomerate  and 
sandstone  prevent  the  extension  of  either  of  these  far  to  tlie 
north.  In  the  construction  of  the  map,  it  will  be  noticed,  that 
the  interpretation  was  preferred  in  each  case  which,  witliout 
doing  violence  to  the  actual  observations,  is  most  favorable  to 
the  synclinal  theory.  It  can  not  be  denied,  however,  that  an 
equally  strong  or  stronger  case  can  be  made  out  for  a 
monoclinal  structure  ;  and  the  latter  appears  to  me  now  the 
more  probable  view. 

Although  this  section,  as  a  whole,  bears  a  general  re- 
semblance to  the  Vilhige  and  Bcal's  Cove  sections,  tlie  pre- 
cise correlation  of  fhe  beds  is  a  puzzling  problem.  The 
much  gentler  dips  and  consequent  broader  outcrops  must  be 
borne  in  mind  ;  for  it  may  very  well  be  that  what  in  the  other 
sections  is  reckoned  as  a  single  bed  appears  here,  tlirough  tlic 
expansion  of  its  outcrop,  as  two  or  more  distinct  beds.      Great 


241 

allowance  must  also  be  made  for  the  undoubted  lateral 
changes  in  the  character  of  the  strata.  In  fact,  this  considera- 
tion renders  the  independent  correlation  of  any  of  the  coarser 
beds  extremely  hazardous  ;  and  I  am  fully  persuaded  that  the 
only  reasonably  safe  clue  is  afforded  by  the  main  belt  of  slate. 
This  bed,  if  not  folded  upon  itself,  is  about  150  feet  thick  : 
and  if  it  is  the  equivalent  of  anything  in  the  Village  section, 
it  must  be  the  bed  of  red  and  gray  slate  near  the  middle  of  the 
section  (6)  which  passes  downward  (south)  into  sandstone 
and  upward  (north)  into  conglomerate  and  has  a  thickness  on 
Hei'sey  Street  of  130  feet.  Assuming  this  correlation  to  be 
substantially  correct,  the  importance  of  the  lateral  changes  in 
the  strata  becomes  apparent  when  we  turn  to  the  coarser  and 
more  variable  parts  of  the  section,  and  especially  when  we 
attempt  to  find  in  beds  3,  4,  and  5  beneath  the  slate  in  the 
Village  section  the  extended  series  of  sti^ata  north  and  east  of, 
i.  e.,  below,  the  slate  in  the  Melville  Garden  section.  This 
point  of  view  makes  it  almost  necessary  to  postulate  the 
transverse  fault  previously  referred  to  as  possibly  crossing 
Downer  Avenue  obliquely  between  the  two  ponds.  Sup- 
posing the  downthrow  to  be  to  the  east,  as  with  the  faults  east 
and  west  of  Ragged  Island,  the  series  of  outcrops  in  the 
center  and  eastern  part  of  Melvill-e  Garden  become  the 
equivalent  of  those  in  the  extreme  western  part  of  the 
garden  and  extending  from  Downer  Avenue  along  the 
north  side  of  the  main  slate.  Even  then  a  satisfactory 
detailed  correlation  is  scarcely  possible,  perhaps  on  ac- 
count of  imperfect  outcrops,  and  especially  are  we  at  a  loss 
to  assign  a  place  to  the  beds  of  Langlee  Island,  without  the 
further  assumption  of  a  profound  east-west  or  strike-fault,  with 
the  downthrow  to  the  north,  between  Langlee  and  Sarah 
Islands,  the  beds  of  Langlee  Island  being,  approximately,  or  in 
part,  a  repetition  of  those  of  Sarah  Island.  This  hypothetical 
strike-fault  may  or  may  not  be  supposed  to  cross  the  north- 
south  fault  between  Rao-o-ed  and  Sarah  Islands.     In  the  former 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  16. 


242 

case,  Crow  Point  and  the  extreme  northern  part  of  Hingham 
Harbor  are  probably  underlain  by  the  conglomerate  series. 
While  in  the  absence  of  strike-faults  this  concealed  area  may 
be  regarded,  with  much  probability,  as  melaphyr,  an  oblong 
and  approximately  horizontal  block  with  the  conglomerate 
series  dipping  gently  away  from  it  on  the  south  and  west. 
It  is  important  to  notice,  however,  that  if  the  very  low  dip 
west  of  Walton's  Cove  be  regarded  as  normal,  the  conglom- 
erate series  would  naturally  reach  across  Crow  Point  and 
cover  the  melaphyr ;  and  since  this  view  is  most  distinctly 
indicated  by  the  actual  observations,  it  has  been  followed  in  the 
construction  of  the  map  and  sections.  The  eastward  extension 
of  the  beds  of  Langlee  and  Sarah  Islands  would  carry  them 
beneath  Pine  and  Planter's  Hills ;  and  in  the  absence  of 
evidence  to  the  contrary  the  conglomerate  series  is  represented 
on  the  general  map  as  ending  in  this  direction  against  the 
o-reat  fault  separating  the  Hingham  and  Nantasket  areas. 

The   Huit's   Cove  Area. 

This  area  embraces  all  that  remains  of  the  third  special  map 
(Plate  9),  including  the  large  quadrangular  body  of  melaphyr 
east  of  Huit's  Cove  and  the  sedimentary  rocks  which  border 
the  melaphyr  on  the  north  and  west  and  form  the  immediate 
shores  of  the  cove.  It  is  a  moderately  elevated  and  ledgy 
tract,  with  comparatively  little  swampy  ground.  The  east  and 
southeast  borders  of  the  melaphyr,  as  previously  noticed,  are 
not  exposed  at  any  point ;  and  it  is  possible  to  determine 
neither  the  exact  position  nor  the  nature  of  the  contact  with 
the  sedimentary  rocks.  But  on  the  north  and  west  the  con- 
ditions are  much  more  favorable,  and  the  boundary  is  drawn 
with  substantial  accuracy  at  most  points.  This  melaphyr  is, 
for  the  most  part,  a  compact  to  imperfectly  crystalline  dark 
o-ray  rock.  The  color  varies,  according  to  the  condition  of  tlie 
iron  oxide,  from  gray  or  greenish  to  reddish  or  purplish   tints. 


243 

In  texture  it  most  resembles  the  melaphyi-  of  the  Village  area, 
but  is  more  slaty  in  some  parts  and,  apparently,  more  crystal- 
line in  others,  the  crystallization  being,  perhaps,  most  marked 
in  the  least  altered  dark  or  gray  variety.  A  finely  porphyritic 
variety  resembling  porphyrite  has  also  been  observed.  It  is 
especially  contrasted  with  the  melaphyr  bordering  the  granite 
axis  by  the  comparative  absence  of  the  amygdaloidal  and  scoria- 
ceous  characters.  Amygdules  are  often  present,  usually  in 
small  patches,  but  are  mainly  small  and  indistinct,  althouo-h 
occasionally  very  large  and  conspicuous.  They  consist  partly, 
as  elsewhere,  of  epidote  or  epidote  and  quartz,  but  chiefly  of  a 
pure,  dark  green  chlorite ;  more  rarelv  of  cleavable  calcite. 
The  calcite  amygdules  have  been,  superficially,  very  generally 
removed  through  solution,  leaving  the  original  steam-holes 
essentially  intact.  Chlorite  is  decidedly  the  most  abundant 
and  characteristic  secondary  mineral,  and  epidote,  whether  dif- 
fused or  segregated  is  of  subordinate  importance. 

The  melaphyr  area,  as  a  whole,  may  be  fairly  described  as 
quite  uniform  in  character  ;  and  no  reasons  are  apparent  for 
drawing  geological  boundaries  through  it.  No  intrusive  con- 
tacts have  been  observed ;  but  this  point  will  be  referred  to 
again  in  describing  the  slate.  Neither  are  there  any  known 
facts  requiring  us  to  refer  the  melaphyr  to  diflPerent  periods  of 
eruption  ;  although  it  is,  perhaps,  doubtful  whether  so  large  a 
body  should  be  regarded  as  belonging  to  a  single  flow.  In 
attempting  to  settle  this  point,  I  have  searched  without  success 
for  some  development  of  the  flow  structure  suflficiently  marked 
to  show  whether  the  melaphyr  still  retains  in  the  main,  as  I 
suppose  it  does,  an  approximately  horizontal  position.  Of 
course,  if  still  horizontal,  the  thickness  is  not  necessarily  great. 
The  only  facts  which  I  have  observed  that  seem  to  have  any 
bearing  upon  this  point  are  as  follows:  (1)  At  the  extreme 
northeast  corner  of  the  melaphyr,  not  only  are  the  ledges 
marked  as  sandstone  just  east  of  the  boundary,  on  the  map, 
all  melaphyr,   but  the  ledge  immediately  within  the  boundary 


244 

marked  as  partly  sandstone  is  also  all  melaphyr  ;  so  that  there 
is  no  evidence,  as  once  supposed,  of  sandstone  overlying  the 
melaphyr.  (2)  Directly  north  of  these  ledges,  about  half 
way  between  there  and  the  shore,  in  the  area  colored  as  slate, 
is  a  ledge  which  appears  on  the  map  as  sandstone  and  slate  ; 
but  I  am  now  satisfied  that  it  is  really  fine-grained  granite  and 
felsite  overlain  by  compact,  slaty,  and  chloritic  melaphyr. 
The  boundary  of  the  melaphyr  at  this  point  should  thus  be 
carried  a  hundred  feet  farther  north  as  well  as  east.  But  the 
special  significance  of  this  outcrop  seems  to  be  that  it  shows  us 
the  actual  base  of  the  melaphyr,  and  that  the  northeast  corner 
of  this  great  block  of  melaphyr  is  tilted  up  a  little  higher,  at 
least,  than  any  other  part. 

That  the  contacts  between  the  melaphyr  and  the  sedimentary 
rocks  bounding  it  on  the  west  and  north  are  lines  of  profound 
displacement  is  unquestionable,  unless  we  are  prepared  to  regard 
the  melaphyr  as  intrusive  in  the  slate  and  conglomerate,  that 
is,  as  forming  a  vast  dike  or  laccolite  ;  a  view  which,  it  may 
be  stated  once  more,  finds  no  support  whatever  in  the 
petrographic  characters  of  the  melaphyr,  nor  in  any  facts  now 
exposed  to  our  observation.  The  slate  forming  the  shores  of 
Huit's  Cove  and  extending  around  the  northern  end  of  the 
melaphyr  is  undoubtedly,  as  previously  explained,  the  great 
slate  (14)of  the  Beal's  Cove  section,  and  in  its  normal  position, 
or  stratigraphically,  it  must  be  separated  from  the  melaphyr  by 
more  than  a  thousand  feet  in  thickness  of  the  conglomerate 
series  ;  yet  here  it  lies  directly  against  the  melaphyr  or  separated 
from  it  only  by  the  single  bed  of  conglomerate,  which,  as  the 
detailed  observations  will  show,  is  clearly  the  highest  member 
of  the  conglomerate  series.  We  thus  see  that,  simply  as  a 
measure  of  erosion,  the  melaphyr  is  impressive,  since  there 
must  have  been  removed  from  its  surface,  not  only  the  eiitire 
conglomerate  series  of  Hingham,  but  also  the  still  greater  volume 
of  the  overlying  slate  series  ;  and  the  bordering  displacements, 
although  they  have  barely  sufficed  to  bring  the   bottom  of  the 


Occas, Papers,  Boston  Soc.Nat.  Hist.,  Vol.  IV, 


Plate. 


Cong  I  om  era  ie 


AAA 
A  A  A  A 


Slate 


Mehiphyr 


Dikes 


SCALE:IIKCH    =330   FEET. 


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I 


mt 


fi 


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V       V 


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^      ^        V 


Huit's    Gove 


245 

slate  clown  to  the  top  of  the  melaphyr,  can,  taking  the  high 
inclination  of  the  beds  into  account,  scarcely  measure  less  than 
2,000  feet.  The  main  strike-fault  is  broken,  as  the  map 
shows,  by  several  transverse  faults,  which  add  much  to  the 
complexity  of  the  contact. i 

At  the  southeast  angle  of  the  cove,  the  slate  is  exposed 
almost  continuously  for  a  distance,  measured  at  right  angles  to 
the  strike,  of  about  90  feet.  The  contact  with  the  melaphyr 
can  not  be  observed,  the  nearest  outcrops  of  the  two  rocks  at 
this  point  being  30  feet  apart.  It  is  probable,  however,  that 
the  real  contact  is  near  the  most  easterly  exposure  of  slate,  since 
a  hundred  feet  to  the  north  the  outcrops  of  melaphyr  advance 
nearly  to  this  line.  The  strike  of  the  slate  is  about  N.  20°  E.  ; 
and  it  dips  away  from  the  melaphyr,  the  inclination  diminishing 
westward  from  65"  or  70°  to  60°.  For  the  first  20  feet  it  is 
quite  massive  and  of  a  very  dark  gray  color,  although  weathering 
whitish  ;  but  beyond  this,  or  upwards,  it  becomes  gradually 
much  lighter  gray  and  distinctly  banded  or  laminated.  It  is 
well  jointed  throughout,  but  shows  no  distinct  cleavage.  The 
lower,  massive  portion  of  the  slate,  which,  fortunately,  has 
been  quarried  to  a  limited  extent  and  thus  affords  a  clean,  fresh 
exposure,  exhibits  within  the  first  five  to  eight  feet  of  the  base, 
several  distinct  zones  coinciding  in  direction  with  the  bedding 
of  what  appear  at  first  glance  to  be  simply  black  spots 
or  blotches  in  the  rock.  More  careful  inspection,  however, 
shows  that  they  are  really  sharply  outlined  inclusions  of  a  nearly 
compact  and  evidently  igneous  rock,  in  other  words,  pebbles. 
They  are,  as  a  rule,  somewhat  rounded  or  water- worn.  They 
vary  from  a  small  fraction  of  an  inch  to  several  inches  in 
diameter;    and  their  longer    axes    usually   coincide,   after    the 

'  During  the  three  years  since  the  colored  map  of  tliis  district  was  drawn,  the 
discovery  of  additional  outcrops  of  slate  along  the  east  shore  of  Huit'sCove  has  led 
me  to  a  somewhat  different  view  of  the  relations  of  the  slate  and  conglomerate,  the 
conglomerate  appeai-ing  now  as  a  limited  bed  in  the  slate.  Consequently  a  new  map 
of  this  interesting  shore  has  been  drawn  (PI.  11),  which  will  be  found  to  agree  much 
better  than  the  colored  map  with  the  following  description  of  the  ledges. 


246 

manner  of  pebbles,  with  the  stratification.  The  most  I'emark- 
able  feature  of  these  inclusions  is  that  they  are  all  of  the  same 
lithological  character  :  very  dark  gray  to  nearly  black  in  color, 
finely  and  imperfectly  crystalline  to  almost  compact  in  texture, 
and  entirely  massive  in  structure.  The  resemblance  to  the 
dark  gray  melaphyr  of  the  Huit's  Cove  area  is  very  marked, 
the  main  point  of  difference  being  that  the  inclusions  or  pebbles 
are  blacker.  They  are  seen,  however,  on  close  examination,  to 
contain  threads  and  amygdules  of  chlorite  and  calcite  ;  while 
the  fact,  which  has  been  observed  repeatedly,  that  steam-holes 
on  the  original  surface  of  a  pebble  are  now  filled,  not  with 
chlorite,  etc.,  but  w^ith  the  fine  greenish  gray  slate  and  minute 
fragments  of  the  melaphyr  itself,  points  to  the  conclusion  that 
these  secondary  minerals  have  been  developed  in  the  pebbles 
subsequently  to  their  inclusion  in  the  slate,  the  melaphyr  having 
been  comparatively  fresh  and  unaltered  at  the  time  of  its 
enclosure.  The  pebbles,  although  scattered  to  some  extent 
through  the  slate,  are  distributed  chiefly,  as  stated,  in  several 
zones  or  layers  from  three  to  six  or  eight  inches  in  thickness. 
Under  the  lens,  however,  the  fragments  of  melaphyr  are  seen 
to  be  of  all  sizes  down  to  the  finest  sand  and  dust ;  and  this 
almost  impalpable  debris  of  the  melaphyr,  which  really  forms  a 
considerable  fraction  of  the  whole,  is  not  limited  to  the  pebbl}' 
layers,  but  pervades  the  entire  thickness  of  the  dark  gray  slate 
and  is,  obviously,  a  suflficient  explanation  of  its  darker  color. 
Following  the  shore  southwest  from  this  little  quarry,  we 
find  from  40  to  50  feet  higher  up  in  the  slate  several  other  pebbly 
layers  from  a  few  inches  to  a  foot  in  thickness.  These,  however, 
are  strongly  contrasted  with  the  preceding,  in  being  made 
up  of  the  normal  variety  of  pebbles,  different  kinds  of  granite 
and  felsite  chiefly ;  but  including,  also,  an  occasional  pebble 
of  precisely  the  same  black  melaphyr.  This  circumstance 
alone  proves  almost  beyond  the  possibility  of  a  doubt  that  the 
enclosed  fragments  of  the  lower  layers  are  genuine  pebbles,  and 
not  due  in  any  way  to  the  alteration  of  the  slate.     Again,  these 


247 

melaphyr  pebbles,  although  they  liave  not  been  exactly  identi- 
fied Avitli  any  mass  of  melaphyr  now  exposed  to  observation, 
lend  some  support  to  the  view  that  there  is  or  has  been  in  this 
vicinity  a  body  of  melaphyr  of  more  recent  date  tlian  the  great 
bed  which,  as  we  have  seen,  belongs  near  the  base  of  the 
conglomerate  series  ;  for  certainly  it  is  not  easy  to  see  how 
melaphyr  covered  by  a  thousand  feet  of  conglomerate,  sand- 
stone, and  shale,  could  have  yielded  these  pebbles  to  a  still  higher 
part  of  the  same  conformable  series  of  strata  ;  but  a  limited 
eruption  —  dike,  laccolite,  or  surface  flow  —  at  the  close  of  the 
conglomerate  series  would  explain  the  phenomena.  It  should 
be  added  that  the  outcrops  of  melaphyr  nearest  to  the  pebbly 
slate  and  upon  or  against  which  it  seems  to  rest,  are  of  wholly 
different  character  from  the  enclosed  frao-ments,  beino-  OTeenish 
and  profusely  amygdaloidal. 

The  outcrop  of  slate  is  almost  continuous  northward  across 
the  outlet  of  the  swamp  to  the  ledge  of  beautifully  jointed 
slate  which  projects  somewhat  into  the  cove.  The  breadth 
of  the  slate  here  is  fully  100  feet ;  but  the  dip  is  lower, 
varying  from  60°  near  the  melaphyr  to  55°  at  the  top  of  the 
bed.  The  lower  half  of  the  bed  is  very  imperfectly  exposed  ; 
but  it  is  clear  now  that  the  conglomerate  layers  near  the  middle 
of  the  bed  are  as  limited  in  this  section  as  in  the  first,  and  that 
there  is  no  appreciable  break  or  displacement,  and  hence  that 
the  compensating  faults  bounding  this  ledge  on  the  colored 
map  are  not  needed.  The  jointing  of  the  slate  in  this  ledge  is 
exceptionally  perfect,  yielding  many  flat-surfaced,  sharp-edged, 
crystal-like  blocks  ;  and  certain  layers  contain  scattering  cubic 
crystals  of  pyrite. 

A  short  beach  now  interrupts  the  slate,  but  the  background 
of  melaphyr  continues  without  deviation ;  and  toward  the 
north  end  of  the  beach  the  slate,  reappearing,  presents  an 
interesting  contact  with  what  appears  to  be  a  north-south  dike. 
The  melaphyr  along  this  beach,  which  has  been  well  exposed 
by  quarrying,  is  the  dark  gray,    semi-crystalline,  and  sparingly 


248 

but  very  coarsely  amygdaloidal  variety,  many  of  the  larger 
steam-holes,  especially,  being  filled  with  cleavable  calcite,  and 
others  with  chlorite.  The  eruptive  rock  in  contact  with  the 
slate,  although  bearing  some  resemblance  to  this  melaphyr,  is 
probably  a  true  diabase,  or,  at  least,  intrusive  in  the  melaphyr 
as  well  as  in  the  slate.  The  contact,  which  is  exposed  for 
about  twelve  feet,  is  somewhat  undulating  or  wavy  ;  but  the 
plainly  marked  lamination  of  the  slate  is  quite  closely  con- 
formable with  the  curving  contact,  so  that  where  two  convex 
curves  meet  a  very  sharp  fold  of  the  slate  projects  a  foot  or 
more  into  the  trap.  Occasionally,  however,  the  contact  breaks 
across  the  bedding  ;  and,  although  it  is  a  conceivable  fault- 
fracture,  it  appears  best,  for  the  reasons  stated,  to  regard  the 
strike-fault  between  the  slate  and  melaphyr  as  followed,  in  this 
instance,  by  a  dike  younger  than  either  but  lithologically 
somewhat  similar  to  the  melaphyr. 

On  attempting  to  follow  the  slate  northwai-d  along  this 
contact  we  encounter  an  undoubted  transverse  fault,  for  we 
pass  at  once  to  melaphyr,  and  the  contact  is  shifted  abruptly 
to  the  westward  or  toward  the  outer  edge  of  the  beach  about  40 
feet,  equal  to  a  vertical  slip  of  nearly  60  feet,  with  the  down- 
throw, evidently,  to  the  south.  North  of  the  displacement  the 
contact-dike  appears  to  be  wanting  ;  and  the  slate  and  melaphyr 
are  seen  in  actual  contact.  The  exposure,  however,  is  unsatis- 
factory ;  and  the  evidence  is  still  inconclusive  as  to  the  true 
relations  of  these  two  rocks.  The  displacement  is  proved 
also  by  the  conglomerate  layers  in  the  slate,  previously 
mentioned ;  for  these  reappear,  essentially  unchanged,  on  the 
point  at  the  end  of  the  beach.  The  westerly  dip  of  the  slate 
is  very  constant,  being  55°  on  this  part  of  the  shore. 

About  120  feet  north  of  this  beach  and  the  small  quarry, 
the  transverse  fault  is  repeated  in  the  reverse  direction,  the 
two  displacements  being  approximately  compensating.  The 
contact  between  the  slate  and  mclajihyr,  unchanged  in  direction, 
is  now  in  the  woods  from  130  to  170  feet  back  from  the  shore  ; 


249 

and,  although  tlic  ledges  on  either  side  locate  it  closely  enoiigli  fbr 
mapping,  it  is  nowhere  exactly  or  satisfactorily  exposed.  We 
now  learn  for  the  first  time  that  the  full  breadth  of  this  bed  of 
slate  does  not  exceed  100  feet,  equal  to  a  thickness  of  about 
80  feet,  and  also  that  it  is  conformably  overlain  on  the  west  by 
a  heavy  bed  of  conglomerate,  which  forms  the  shore  for  a 
distance  of  400  feet.  The  exposed  breadth  of  the  conglomerate 
is,  at  first,  only  30  feet ;  but  it  increases  northward  to  a 
maximum  of  70  feet,  equal  to  a  thickness  of  nearly  60  feet. 
This  is  probably  not  the  entire  thickness  of  the  conglomerate  ; 
at  any  rate  there  are  no  indications  on  this  part  of  the  shore 
of  its  being  overlain  by  slate.  It  is,  for  the  most  part,  a 
rusty  and  readily  disintegrating  rock.  At  some  points  it  is  quite 
ochery  in  appearance  ;  and  it  is,  doubtless,  owing  to  the  lack  of 
an  efficient  cement  and  its  consequent  friable  or  crumbling 
character  that  it  fails  to  outcrop  farther  south  on  the  shore. 
Although  mainly  of  medium  and  uniform  texture,  a  portion  of 
the  bed,  as  exposed  on  the  shore,  is  extremely  coarse  and 
irregular.  The  pebbles  are  of  all  sizes  up  to  a  yard  or  more  in 
diameter,  the  largest  observed  being  a  bowlder  of  coarse  granite 
over  five  feet  in  leno;th.  Furthermore,  the  various  sizes  are 
jumbled  together  promiscuously,  without  evident  assorting  or 
stratification,  looking  not  unlike  an  indurated  till  or  bowlder 
clay. 

On  account  of  its  relations  to  the  slate,  the  composition  of 
this  conglomerate  possesses  unusual  interest.  The  conglom- 
erate is  not  only  underlain,  but,  as  will  be  seen  later,  it  is 
overlain  by  slate,  the  overlying  bed  embracing  a  thickness 
of  a  thousand  feet  or  more.  It  is  clear  that,  in  the  absence 
of  fossils  in  the  slate,  the  key  to  its  geological  age  is  to  be 
sought  in  this  intercalated  conglomerate,  proceeding  on  the 
principle  that  every  rock, of  this  region  represented  among  the 
pebbles  of  the  conglomerate  must  be  older  than  both  the  con- 
glomerate and  the  great  overlying  slate.  We  are  able  to  prove 
in  this  way  that  the  slate  is  newer  than  most,  at  least,  of  the 


250 

granites  and  felsites,  as  well  as  some  of  the  melapliyrs  and  por- 
phyrites  ;  and  since  some  of  these  rocks,  and  especially  the 
granite,  are  clearly  eruptive  at  many  points  through  the  Lower 
and  Middle  Cambrian  slates  of  the  Boston  Basin,  it  follows 
that  the  Hingham  slate  should  be  referred  to  a  higher  horizon. 
I  have  also  observed  in  this  conglomerate  frequent  pebbles  of 
slate,  a  further  confirmation  of  the  view  that  there  is  an  older 
slate  in  this  region.  Of  especial  interest  in  this  connection, 
however,  are  the  calcareous  portions  of  the  conglomerate. 
The  most  casual  observer  would  be  likely  to  notice  that  limited 
portions  of  the  coarse  conglomerate  have  an  etched  appearance, 
weathering  in  the  smooth  and  cavernous  manner  peculiar  to 
calcareous  rocks,  and  the  test  with  acid  at  these  points  shows 
an  abundant  calcareous  cement,  the  solution  of  which  allows 
the  pebbles  to  fall  apart.  This  calcareous  conglomerate  is 
chiefly  in  the  form  of  irregular  but  rounded  and  sharply  defined 
masses  or  patches  from  a  foot  to  a  yard  or  so  across  :  and  else- 
where in  the  bed  the  calcareous  cement  is  wholly  wanting.  This 
is  analogous  to  what  may  sometimes  be  observed  in  the  modified 
drift  of  limestone  districts,  such  as  western  New  England,  beds 
of  loose  sand  and  gravel  enclosing  irregularly  rounded  but  often 
extremely  graceful  masses  of  firm  sandstone  and  conglomerate 
due  to  the  solution  and  segregation  of  limestone  debris. 
The  source  of  the  calcareous  material  in  the  Hingham  conglom- 
erate is  an  interesting  question.  Does  it  indicate  contempora- 
neous shells  and  corals — fossils  belonging  to  the  same  geological 
age  as  the  conglomerate,  or  is  it,  as  in  the  more  modern  exam- 
ples just  cited,  limestone  debris  from  some  older  formation, 
which,  after  its  enclosure  in  the  cono-lomerate,  has  undero-one 
segregation — solution  and  deposition-^so  as  to  assume  a  concen- 
trated form  in  limited  portions  of  the  rock?  The  latter  is 
undoubtedly  the  true  view  ;  for  there  still  remain  undissolved 
fragments  of  the  limestone,  well-rounded  distinct  pebbles  of  all 
sizes  up  to  a  foot  or  more  in  diameter,  some  of  thcni  beautifully 
stratified  in  directions  entirely  at  variance  with  the  bedding  of 


251 

the  conglomerate.  The  pebbles  of  limestone  are  of  (|iiil('  uni- 
form character;  and  are  to  be  recognized  especially  by  the 
mode  of  weathering  and  the  behavior  with  acid.  The  rock  is 
dull  gray,  finely  and  imperfectly  crystalline,  evidently  impure, 
and  leaves  a  lai-ge  insoluble  residue  when  treated  with  acid. 
Where  the  segregation  of  the  calcareous  debris  has  occurred  in 
the  finer — sandy  or  slaty — portions  of  the  conglomerate,  the 
resultino;  forms  are  sometimes  difficult  to  distinguish  from  the 
original  pebbles.  The  further  consideration  of  the  source  of 
these  fragments  and  of  the  light  which  they  throw  upon  the 
age  of  the  enclosing  rocks  may  most  profitably  be  reserved  for 
the  general  discussion  of  the  age  and  relations  of  the  Hingham 
strata. 

On  following  the  conglomerate  north  it  is  fovmd  to  end 
abruptly,  as  shown  on  the  map,  against  the  slate.  The 
contact,  which  is  clearly  exposed  and  can  be  traced  over  the 
cliff,  is  transverse  to  the  strike  of  both  rocks  and  a  very  obvious 
fault.  The  fault-plane  strikes  N.  80°  E.  and  hades  to  the  north 
10°,  and  parallel  with  it  the  slate  shows  a  very  marked  cleavage. 
On  account  of  the  cleavage,  and  the  consequent  shaly  character- 
of  the  slate  near  the  surfixce,  the  true  bedding  is  difficult  to 
make  out.  It  can  be  proved,  however,  that  the  dip  has  the 
normal  westerly  direction  for  this  shore,  but  is  somewhat 
steeper  than  usual.  Along  the  fault-line,  for  a  thickness  of  a 
foot  or  so,  the  slate  is  distinctly  comminuted,  having,  evidently, 
experienced  unusual  compression  and  friction  ;  and  in  the  con- 
glomerate, parallel  with  the  fault,  n^ay  be  observed  several  more 
or  less  distinct  planes  of  either  cleavage  or  shearing.  The 
marked  cleavage  of  the  slate  is  evidently  a  local  feature,  for 
along  the  north  side  of  the  cove,  400  feet  distant,  it  is  scarcely 
noticeable,  being  quite  subordinate  to  the  joint-structure.  We 
may,  therefore,  find  a  sufficient  explanation  of  the  cleavage  in 
the  fact  that  the  slate,  as  the  result  of  the  faulting,  has  been 
dropped  down  along  side  of  the  massive  conglomerate  and  then 
squeezed  up^against  it. 


252 

This  slate  is  not  that  which  we  have  previously  observed 
underlying  the  conglomerate  ;  for  on  ascending  the  slope  north 
of  the  fault  it  is  found,  as  shown  on  the  map,  to  rest  upon  the 
conglomerate,  which  has  been  shifted  laterally  a  little  more  than 
its  own  breadth,  equivalent  to  a  vertical  slip  of  nearly  200  feet. 
From  this  point  the  conglomerate  can  be  traced  across  the  fields 
in  frequent  outcrops,  approximately  as  mapped,  for  nearly  a 
quarter  of  a  mile,  closely  bordered  by  the  outcrops  of  slate  on 
the  west  and  of  melaphyr  on  the  east,  the  termination  of  the 
conglomerate  being  marked  by  a  large  bowlder  of  coarse  con- 
glomerate resting  upon  a  ledge  of  melaphyr.  We  look  in  vain 
here  for  any  trace  of  the  slate  which  we  have  found  along  the 
eastern  shore  of  the  cove  separating  the  melaphyr  and  con- 
glomerate. Apparently,  the  throw  of  the  transverse  fault, 
added  to  the  strike-fault  bounding  the  melaphyr,  has  been 
sufficient  to  conceal  the  lower  slate  ;  and  we  thus  reach  the 
conclusion  that,  as  shown  on  the  new  map,  the  transverse  fault 
probably  breaks  but  does  not  necessarily  cross  the  strike-fault. 
The  transverse  fault  marked  as  crossing  the  conglomerate 
northeast  of  the  cove  is  clearly  indicated  by  the  jogging  of  both 
the  upper  and  lower  contacts  of  the  conglomerate ;  and  the 
change  of  strike  on  crossing  this  line  is  obvious.  At  the  point 
where  the  conglomerate  is  mapped  as  coming  to  an  abrupt 
termination  it  evidently  shares  the  fate  of  the  lower  slate.  A 
transverse  fault  is  almost  a  necessity  here  ;  and  its  relations  to 
the  strike-fault  and  the  strata  are  the  same  as  before.  The 
contact  between  the  conglomei'ate  and  melaphyr  is  exposed  at 
several  points,  but  not  sufficiently  to  reveal  clearly  their  true 
relations.  The  contact  is,  apparently,  an  irregular  surface  and 
is  broken  by  minor  faults  as  well  as  by  the  principal  displace- 
ments already  described. 

Undoubtedly  the  best  section  of  the  slate  series  of  Hingham 
is  that  along  the  north  side  of  Huit's  Cove.  From  the 
conglomerate  west  across  the  strike  the  slate  is  exposed 
almost     continuously     for     about     1,100    feet.      The    dip     is 


253 

constantly  to  the  west,  varying  from  45°  to  G0°  or  more 
as  we  approach  the  conglomerate,  the  actual  exposed  thick- 
ness of  the  slate  being,  probably,  about  900  feet.  In  one 
ledge  only  on  the  northeast  shore  of  the  cove,  a  few  scatter- 
ing pebbles  have  been  observed  in  the  slate.  It  resembles 
the  lower  slate  in  the  fine  and  perfect  jointing,  which  is 
particularly  well  developed  on  the  point,  yielding  prismatic 
blocks  which  are  sometimes  remarkably  slender,  less  than  a 
square  inch  in  section,  and  several  feet  in  length  —  monolithic 
columns  in  miniature.  For  the  most  part,  at  least,  it  is  of  a 
darker  gray  color  than  the  lower  slate,  and  somewhat  darker, 
also,  than  the  slate  of  the  Beal's  Cove  section.  In  following 
the  slate  north  from  Huit's  Cove,  the  strike  seems  to  change 
gradually  with  the  conglomerate,  and  the  dip  becomes  more 
variable ;  but  on  crossing  the  fault  terminating  the  conglom- 
erate the  strike  changes  abruptly  to  N.  65°-70°  E.,  and  holds 
that  general  direction  across  the  north  end  of  the  melaphyr ; 
while  the  dip  between  the  melaphyr  and  the  north  shore  is 
extremely  inconstant.  This  mass  of  slate  is,  in  fact,  highly 
contorted,  being  contrasted  in  that  respect  with  all  the  other 
outcrops  of  Hingham.  The  slate  shows  no  special  alteration 
in  the  vicinity  of  either  the  melaphyr  or  the  great  masses  of 
diabase  which  break  through  it.  In  general  it  is  a  soft,  thin- 
bedded,  dark  gray  slate,  showing  in  some  of  the  ledges  a  good 
lamination  cleavage  ;  although  the  true  slaty  cleavage,  which 
holds  a  nearly  constant  attitude  throughout  this  district,  is 
usually  transverse  to  the  bedding.  In  spite  of  the  fact  that 
the  dip  of  the  slate  is  extremely  variable,  it  is  easily  seen  that, 
as  the  map  shows,  the  prevailing  or  true  dip  is  north  near  the 
melaphyr  and  south  near  the  shore,  indicating  an  irregular 
synclinal  fold  east  of  the  transverse  fault,  the  axis  of  which 
is  roughly  marked  by  the  great  dike.  It  does  not  appear, 
however,  that  the  dike  has  materially  influenced  the  character 
of  the  folding.  The  contortions  are,  of  course,  a  normal 
feature  of  an  open  syncline  ;  but  whether    either  the  fold  or 


254 

the  contortions  are  connected  genetically  with  the  great 
strike-fault  supposed  to  separate  the  slate  and  melaphyr, 
we  can  only  conjecture. 

The  extreme  northwest  corner  of  this  area  is  low  and  devoid 
of  outcrops  ;  but  the  ledges  on  the  south  and  east  are  evidently 
sufficient  to  justify  mapping  the  whole  as  slate.  Eastward,  on 
the  contrary,  the  ledges,  alike  of  melaphyr,  slate,  and  diabase, 
end  at  the  western  edge  of  the  meadow  and  its  barrier  beach  ; 
and  the  band  of  slate  represented  as  crossing  the  meadow  and 
the  northern  end  of  Pleasant  Hill  is  based  largely  upon  theoretic 
considerations.  In  the  way  of  direct  evidence  we  have  the 
important  fact  that  this  north  shore,  including  the  beach  and 
the  section  of  till  above  it,  is  made  up  almost  wholly  of  debris 
of  the  gray  slate,  many  of  the  fragments  being  large  and 
angular.  There  are,  of  course,  occasional  bowlders  of  granite 
and  felsite,  glacial  erratics  from  the  hills  north  of  Boston 
Harbor;  and  more  rarely,  also,  a  stray  block  of  conglomerate 
or  sandstone.  But  it  is  perfectly  obvious  that  we  find  here  no 
adequate  representation  of  the  great  development  of  the  con- 
glomerate series — conglomerate,  sandstone,  and  red  slate  — 
outcropping  immediately  south  of  this  shore  and  striking 
directly  toward  it.  Considering  the  extremely  local  origin  of 
the  main  part  of  the  drift,  we  have  no  alternative  but  to 
conclude  that  this  shore  is  underlain  by  the  gray  slate  and  not 
by  either  the  conglomerate  series  or  the  melaj)hyr.  This  pro- 
longation of  the  slate  series  directly  across  the  strike  of  tlie 
conglomerate  series  implies,  of  course,  an  eastward  extension 
of  the  fault  between  the  slate  and  melaphyr.  At  the  northeast 
corner  of  the  melaphyr,  as  already  explained,  this  fault  should 
be  carried  a  little  nearer  the  shore  than  tlie  map  shows  it ;  but 
there  is,  on  the  whole,  no  theoretical  part  of  the  map  which  I 
am  able  to  regard  with  greater  satisfaction.  According  to  this 
view,  the  Melville  Garden  and  Planter's  Fields  area  of  the 
conglomerate  scries  is  bounded  on  the  west  by  a  profound 
uptlirovv  fault,  and  on  the  north  by  an  equally  profound 
downthrow  fault. 


255 


The  Islands  North  of  Ilingham. 

Slate  Island  and  Grape  Island,  although  belonging  politically 
the  one  to  Hull  and  the  other  to  Weymouth,  are  geologically 
a  part  of  Hinghani ;  and  at  extreme  low  tide  they  are 
topographically  almost  joined  to  the  main  land.  These  two 
islands  are  contrasted  in  their  topographic  features.  Slate 
Island,  the  name  of  which  is  unusually  appropriate,  is  almost 
bare  of  drift ;  and  with  the  exception  of  some  unimportant 
beach  deposits  on  its  southern  shore,  the  ledges  of  slate  are 
exposed  over,  virtually,  its  entire  surface,  or  would  be  but  for 
the  exceedingly  dense  growth  of  sumach,  raspbei'ry,  and  other 
shrubs.  The  island  rises  abruptly  on  the  north  shore  in  a  low 
cliff  from  15  to  25  feet  high,  and.  descends  thence  in  one  long 
gentle  slope  to  the  low  southern  shore.  Grape  Island,  on  the 
other  hand,  although  clearly  underlain  by  slate,  is  heavily 
drift-clad,  bearing  two  distinct  drumlins,  with  a  broad  depres- 
sion holding  a  small  pond  between  them. 

Slate  Island,  especially,  is,  geologically,  essentially  a 
continuation  of  the  northwest  shore  of  Ilingham.  With  the 
exception  of  the  numerous  rusty  gray  dikes  of  diabase,  the  rock 
is  all  the  same  soft,  dark  gray,  thin-bedded  slate  with  which  we 
have  become  familiar  north  and  northeast  of  Huit's  Cove, 
the  chief  difference  being  that  on  the  island  the  slate  is  less 
contorted,  and  has  a  higher  and  more  constant  dip  ;  and  the 
cleavage  and  bedding  are  in  constant  instead  of  only  occasional 
agreement.  The  strike  is  usually  N.  65°— 70°  E.,  the  extreme 
range,  however,  being  N.  55°— 75°.  The  dip  is  almost 
vertical  over  the  whole  surface  of  the  island,  varying  usually 
between  S.  85°  and  90°,  although  at  one  point,  where 
influenced  by  a  large  dike,  it  is  N.  85°.  The  stratification  is 
everywhere  very  thin,  even,  and  regular;  and  many  of  the 
exposures  are  exceedingly  beautiful  examples  of  the  upturned 
leaves  of  the  geological  record.  Smooth,  glaciated  surfaces 
present  a  remarkably  perfect  striping  or  lining ;  while  on  the 


256 

beach,  where  the  Avaves  and  frost  have  fully  developed  the 
cleavage,  the  appearance  is  suggestive  of  gigantic  crystals  of 
mica  set  on  edge  and  undergoing  exfoliation.  The  cleavage, 
which,  as  stated,  is  everywhere  sensibly  parallel  with  the 
bedding  planes,  is,  perhaps,  the  most  perfect  to  be  observed 
anywhere  in  the  Boston  Basin  ;  and  it  is  a  noteworthy  illustra- 
tion of  the  principle  that  when  the  bedding  planes  nearly  coin- 
cide with  the  normal  direction  of  the  cleavage,  the  latter  is 
deflected  so  as  to  make  the  agreement  perfect,  the  normal 
inclination  of  the  cleavage  in  this  part  of  the  Boston  Basin 
being  N.  70°— 80°.  The  fissile  character  of  the  slate  is  seen 
also  in  the  fact  that  with  few  exceptions  the  numerous  dikes 
follow  the  bedding  planes  and  the  cleavage. 

Although  we  do  not  observe  on  Slate  Island  the  minor  pli- 
cations or  contortions,  the  strongly  marked  wrinklings  of  the 
slate  which  characterize  the  north  shore  of  Hingham,  there  is 
abundant  evidence  of  local  stresses  and  deformation.  The 
indications  are  that  during  the  powerful  compression  of  the  slate 
evidenced  by  its  high  dip  and  perfect  cleavage  it  experienced 
intense  but  very  local  torsional  and  shearing  strains,  resulting 
in  the  development  of  (1)  numerous  small  monoclinalbendings, 
along  the  axes  of  which  the  slate  is  often  pinched  to  half  its 
normal  thickness  ;  (2.)  innumerable  parallel  and  overlapping  or 
en  echelon,  oblique  rifts  or  cracks  from  an  inch  to  a  foot  or 
more  in  length.  The  rifts  are  invariably  attended  by  slight 
displacements  of  the  layers,  thus  aflfbrding  most  complete  and 
instructive  examples  of  normal  faults  ;  the  slate,  without  excep- 
tion, being  bent  or  compressed  in  such  a  Avay  that  the  hanging- 
wall  of  each  rift  or  fracture  is  depressed  Avith  reference  to  the 
foot-Avall.  The  displacement  dies  out  gradually  from  the 
middle  toward  the  ends  of  each  fault ;  and  the  cases  are  frequent 
where  monoclines  can  be  seen  passing  into  normal  faults,  a 
simple  plication  changing  Avithin  a  few  inches  to  an  actual 
break  and  slip.  In  many  of  the  fault  cracks,  also,  calcite  has 
been  deposited  from  solution,  forming  miniature  veins,  Avhich 


257 

have  thus  faulted  walls  and  many  of  the  normal  features  of 
more  important  examples.  Slate  Island  is,  indeed,  a  fine  field 
for  the  student  of  structural  geology  :  and  it  is  interesting  to 
observe  in  this  connection  that  the  conditions  are  unusually 
favorable  for  obtaining  cabinet  specimens  of  the  slate  exhibiting 
its  various  structural  features ;  for  besides  the  debris  which 
naturally  encumbers  the  shore,  we  have  that  afforded  by  the 
numerous  small  quarries  scattered  over  the  island,  the  slate 
having  been  extensively  quarried  in  the  past  for  ballast. 

The  half-tide  ledge  southwest  of  Slate  Island  is  composed 
of  slate  precisely  similar  to  that  on  the  island,  with  a  nearly 
vertical  southerly  dip.  The  bed  rock  of  Grape  Island  is 
exposed  at  only  two  points,  on  the  north  and  south  shores, 
near  the  eastern  end  of  the  island.  The  southern  ledge,  which 
is  in  the  line  of  strike  of  the  north  shore  of  Slate  Island,  is  a 
dark  gray  to  black,  thin-bedded,  and  fissile  slate.  The  cleavage 
conforms  perfectly  with  the  bedding,  and  the  slate  is  well  jointed. 
The  strike  is  N.  75°  E.  and  the  dip  S.  80°-90°.  The  slate  is 
somewhat  contorted  in  places,  and  it  is  traversed  by  several  small 
irregular  gray  dikes  similar  to  those  of  Slate  Island.  The 
northern  ledge,  which  is  on  the  extreme  northeast  corner  of 
the  island,  is  quite  extensive  at  low  tide.  It,  also,  is  all  slate  ; 
but  mostly  a  coarse,  gritty,  gray  variety,  and  not,  as  a  rule, 
very  distinctly  bedded.  Toward  the  northern  end  of  the  ledge, 
however,  it  is  darker  and  softer,  and  more  like  the  slate  on  the 
south  shore.     The  strike  is  N.  70°  E.  ;  and  the  dip  S.  70°-75°. 

Beyond  these  islands  there  are  no  outcrops  of  any  kind 
nearer  than  Raccoon  Island  on  the  west,  the  village  of  Hull 
on  the  north,  and  Rocky  Neck  on  the  east.  Hence  we  can 
only  speculate  as  to  the  extension  and  probable  relations  of 
the  newer  slates  in  those  directions  ;  but  southward  there  can 
be  no  reasonable  doubt  that  they  are  coaitinuous  with  the 
similar  slates  forming'  the  north  shore  of  Hinoham.  Unfortu- 
nately,  however,  the  vertical  position  is  a  very  equivocal  one  ; 
and   it  is   quite  impossible   to   say   with    certainty  whether  in 

OCCAS.    PAPERS   B.    S.    N.    H.  IV.  17 


258 

looking  south  from  Slate  Island  we  are  facing  a  syncline  or  an 
anticline.  The  former  is,  of  course,  indicated  by  the  beds  on 
the  island,  and  the  latter  by  those  on  the  shore,  and  it  may 
very  well  be  that  several  axes  intervene. 

THE    DIKES    or    HINGHAM. 

It  is  intended,  of  course,  to  include  here  only  those  intru- 
sive masses  which  are  either  contemporaneous  with  or  newer 
than  the  sedimentary  rocks  ;  and  the  granitic  rocks  — diorite, 
o-ranite,  and  felsite  —  are  thus  wholly  excluded.  No  dikes  of 
porphyrite  have  been  certainly  identified  in  Hingham  ;  but,  as 
previously  stated,  the  dikes  associa.ted  with  the  slate  and  con- 
o-lomerate  are  all  of  more  basic  character  and  are  certainly  nearly 
all  true  diabase.  It  is  possible,  however,  that  several  of  the  dikes 
which  have  been  observed  should  be  classed  as  melaphyr.  One 
of  these,  Avhich  is  colored  as  melaphyr  on  the  map,  crosses  the 
small  field  in  the  angle  between  Downer  and  Grove  Avenues, 
south  of  the  pond.  The  rock  has  the  texture  and  general  appear- 
ance of  melaphyr  ;  but  the  outcrops  are  insuflicientto  show  clearly 
its  width,  exact  trend,  or  relations  to  other  rocks,  except  that  it 
appears  to  be  cut,  as  mapped,  by  a  dike  of  normal  diabase.  It  is, 
however,  probably  intrusive  in  the  conglomerate  ;  and  hence 
sugo;ests  a  flow  of  melaphyr  mucli  higher  up  in  the  stratigraphic 
scale  than  any  that  we  liave  recognized.  A  dike  of  similar  litho- 
logical  character,  and  possibly  a  continuation  of  this,  although  it 
has  been  inadvertently  colored  as  diabase,  breaks  through  the 
conglomerate  near  Otis  Street,  in  the  south  part  of  Melville 
Giu'den.  It  is  about  15  feet  wide,  irregular  in  outline,  and 
liades  to  the  north. 

A  glance  at  the  maps  suffices  to  show  that  the  diabase  dikes, 
which  we  may  suppose  are  wholly  subsequent  to  the  sedimen- 
tary deposits,  dating  fi'om  the  period  of  disturbance  when 
the  strata  were  folded  and  faulted,  have,  with  few  exceptions, 
a  general  east-west  trend  ;  and  it   is   highly  probable   that   the 


259 

two  east-west  systems  of  the  Nantasket  area  arc  represented 
here,  but  there  are  no  clearly  exposed  intersections  fully  to 
prove  it.  The  dikes  of  the  south-of-east  or  newer  system 
evidently  predominate  ;  but  the  maps  render  it  unnecessary  to 
specify  more  particularly  those  belonging  to  the  different 
systems.  The  newest  or  north-south  system  of  the  Nantasket 
area  is,  possibly,  wholly  unrepresented.  Certainly  no  typical 
examples  of  it  have  been  observed.  No  attention  has  been 
sciven  to  the  numerous  dikes  of  diabase  traversino-  the  granitic 
areas,  further  than  to  observe  that  they  also  show  a  general 
agreement  in  trend  with  the  east-west  systems  of  Nantasket ; 
and  hence  are  probably  mainly  if  not  wholly  of  post-Cambrian 
age.  In  other  words,  on  account  of  the  great  difficulty  of 
tracing  and  correlating  the  dikes,  no  attempt  has  been  made  to 
map  them  outside  of  the  sedimentary  areas  ;  and  within  these 
areas  it  is  probable  that  a  large  proportion  of  the  dikes  are 
wholly  concealed  by  drift,  while  many  others  are  so  imperfectly 
exposed  as  to  preclude  their  delineation .  The  maps  show  the 
trend  and,  in  most  cases,  the  width  in  feet  of  each  dike.  The 
prevailing  hade,  as  at  Nantasket,  is  to  the  north. 

Dikes  of  the    Village    Area. 

Beginning  at  the  east,  we  have  first  the  short  dike,  from 
50  to  70  feet  in  width,  in  the  granite  between  Elm  Street  and 
Lafayette  Avenue.  This  is  of  special  interest  as  helping 
to  pi-ove  the  faults  which  limit  it  on  the  east  and  west,  and  as 
showing  that  in  this  case  at  least  the  dike  probably  antedates 
the  faults.  The  large  masses  of  trap  in  the  granite  on  and 
near  Hersey  Street  present  no  points  of  special  interest. 
They  are,  possibly,  connected  with  each  other  and  also  with 
the  large  dike  in  the  conglomerate  series  which  crosses  Hersey 
Street  at  the  junction  with  the  lane.  The  southward  extension 
of  this  dike  is,  however,  quite  problematical ;  but  the  east-west 
branching  portion  has  been  quite  clearly  traced  out,  varying  in 


260 

width  from  less  than  three  feet  to  more  than  thirty  feet  on 
Hersey  Street.  In  the  end  of  the  lane  the  outcrop  of  diabase 
measures  100  feet  north  and  south  :  and  the  southern  extension 
appears  the  best  explanation  of  this  fact.  The  east-west 
branches  are,  possibly,  independent  and  intersecting  dikes. 
Certainly  the  main  or  more  southerly  branch  east  of  Hersey 
Street  presents  striking  characters  not  observed  in  the  others. 
Its  eastern  outcrops  in  the  sandstone,  where  it  is  from  6  to  7 
feet  wide,  are  very  distinctly  and  quite  coarsely  porphyritic, 
enclosing  feldspars  one  fourth  of  an  inch  long.  It  is  also  very 
highly  magnetic,  containing  a  large  amount  of  magnetite  in 
grains  up  to  half  an  inch  or  more  in  diameter.  Farther  north 
Hersey  Street  is  crossed  by  two  parallel  dikes  3  feet  and  8 
feet  in  width ;  but  east  of  the  street  they  are  smaller  and 
appear  to  unite  and  then  divide  again  ;  while  west  of  the  street 
they  seem  to  be  offshoots  from  a  large  mass  of  trap  of 
undetermined  form.  The  other  dikes  call  for  no  special 
comment.  Where  the  width  is  not  given  on  the  map,  it 
means  that  the  outcrop  is  unsatisfactory ;  and  several  dikes 
have  been  omitted  for  this  reason.  Evidently,  the  dikes  of 
this  area  are  especially  characterized  by  their  irregular, 
branching  forms,  which  makes  it  unsafe  to  map  them  beyond 
their  outcrops. 

Dikes  of  the  Seal's    Cove  Area. 

This  part  of  Ilingham  appears  to  be  comparatively  free  from 
dikes  ;  the  only  one  distinctly  represented  on  the  map  being  the 
irregular  ten-foot  dike  at  tlie  mouth  of  Beal's  Cove.  This 
a[)pears  to  belong  to  the  south-of-east  system  ;  but  its  trend  may 
mean  nothing  more  than  a  natural  tendency  to  follow  the  bedding 
of  the  slate.  About  1,000  feet  directly  east  of  the  northern 
arm  of  the  cove,  on  the  south  side  of  Tucker's  Swamp,  a  large 
mass  of  trap  breaks  through  the  conglomerate  and  sandstone, 
which  was   inadvertently  omitted  from  the  map.       It  is  impos- 


261 

sible  to  be  sure  of  its  size  or  trend,  hut  it  is  probably  a  dike  at 
least  fifty  feet  wide,  having  a  general  north-south  direction. 
Obscure  outcrops  of  several  large  dikes  have  also  been 
observed  on  the  line  of  Beal  Street,  in  the  granite  and 
melaphyr. 

Dikes  in  the  Area  North  of  Lincoln  Street. 

This  is  the  area  of  the  third  special  map  (PL  9),  embracing 
also  the  islands  of  the  harbor.  We  naturally  notice  first  the 
great  masses  of  coarsely  crystalline  diabase  which  are  such 
conspicuous  features  of  the  map.  The  actual  outcrops,  as 
may  be  seen  on  referring  to  the  map,  are  quite  insufficient  for 
the  accurate  delineation  of  these  immense  dikes.  But,  although 
the  outlines,  as  engraved,  are  largely  hypothetical,  they  serve 
to  show  their  approximate  extent  and  probable  relations  to  the 
enclosing  strata.  The  long  branch  extending  southward  from 
the  western  end  of  the  broad  dike  which  crosses  Planter's  Fields 
Lane  has  certainly  a  very  slender  basis  of  facts  ;  but  still  it 
correlates  the  scattering  outcrops  and  is  inconsistent  with  no 
actual  observation.  The  same  is  true  of  the  western  part  of 
the  large  dike  north  of  Huit's  Cove.  If  it  has  not  exactly^ 
that  form,  it  probably  has  some  similar  form.  The  boundaries 
of  all  the  dikes  are  represented  by  continuous  lines  where  they 
are  reasonably  well  determined,  and  broken  lines  where  they 
are  hypothetical.  The  slate  separating  the  two  branches  of 
the  large  dike  between  the  melaphyr  and  the  north  shore 
extends  at  least  150  feet  farther  east  than  represented  ;  and  it 
is  by  no  means  certain  that  these  two  are  really  united  on  the 
land.  The  clearest  exposure  of  the  contact  with  the  stratified 
rocks  is  on  the  north  side  of  the  southern  dike,  where  it  breaks 
through  a  bed  of  reddish  brown  slate.  The  slate  is  somewhat 
warped,  the  dip  and  strike  being  appreciably  aflTected  ;  but 
there  is  no  marked  lithological  (^hange  ;  and  this  is  the  general 
fact,  the  slate  and  sandstone    appearing  rarely  to  be  sensibly 


262 

altered  by  the  proximity  of  the  trap.  No  clear  intersections  of 
these  large  masses  of  diabase  by  the  smaller  dikes  of  the 
district  have  been  observed ;  and  perhaps  the  most  probable 
view  is,  that  some  of  the  smaller  and  more  typical  dikes 
are  offshoots  from  these  ffreat  intrusive  masses.  One  thino'  is 
certainly  very  clear;  viz.,  that  the  latter  occur  where  the 
stratified  rocks  show  exceptional  disturbance,  and  hence 
probably  date,  like  the  smaller  dikes,  from  the  period  when 
the  strata  were  folded  and  faulted. 

The  three  dikes  on  Langlee  Island  appear  to  agree  in  hading 
N.  10°— 15°,  but  while  the  north  dike  and  the  south  dike  are 
parallel  Avith  the  strike  of  the  conglomerate,  trending  a  little 
north  of  east,  the  middle  dike  is  oblique  to  the  strike, 
trending  a  little  south  of  east.  The  latter  dike  is  highly 
altered,  appearing  to  be  made  up  very  largely  of  secondary 
minerals  —  epidote  and  quartz.  These  dikes  can  not  be 
identified  on  the  main  land,  passing,  probably,  beneath  the 
drift  of  Crow  Point.  The  numerous  dikes  in  Melville  Garden 
and  Planter's  Field  and  on  the  shores  of  Huit's  Cove  evidently 
belong  mainly  to  the  south-of-east  system,  but  the  outcrops  are 
not  easily  connected,  with  the  single  exception  of  the  large 
dike  which  follows  the  general  course  of  Grove  Avenue,  andean 
be-  traced  in  frequent  outcrops  for  1 ,800  feet.  The  very  unusual 
trend  of  this  and  one  or  two  other  dikes  may,  perhaps,  be 
due  in  some  way  to  the  northerly  strike  of  the  strata.  The 
nearly  north -south  dike  in  the  main  belt  of  slate  appears,  at 
first  glance,  to  belong  to  the  third  or  newest  system  of  Nan- 
tasket.  Lithologically,  however,  it  does  not  agree  with  that 
system ;  and  it  seems  best  to  regard  it  as,  normally,  an 
east-west  dike  which  has  been  influenced  in  its  trend  by  the 
bedding  of  the  slate. 

Dikes  of  Slate  and   Grape  Islands. 

About  twenty  dikes  from  2  to  8  feet  wide  have  been 
observed    on     Slate   Island.     Fully  two   thirds    of  these    are 


263 

exactly  panillel  with  the  slate  ;  and  the  remainder,  which  are 
extremely  irregular  in  form,  break  across  the  slate  approxi- 
mately at  right  angles.  Although  two  systems  of  dikes 
may  thus  be  recognized,  as  regards  mere  direction,  they  are 
undoubtedly  all  of  the  same  age  ;  for  while  they  are  somewhat 
peculiar  in  litliological  aspect,  they  are  all  extremely  alike  in 
this  respect.  The  trap  is,  without  an  important  exception,  a 
finely  and  indistinctly  crystalline,  almost  compact,  light 
brownish  gray  variety,  of  a  dull,  ashen  luster.  Trap  of  this 
general  character  and  evidently  highly  altered,  has  been 
observed  repeatedly  in  the  thin-bedded  slates  of  this  district, 
and  is,  perhaps,  somewhat  peculiar  to  them.  On  the  north 
shore  some  very  small  dikes  (from  1  to  6  inches)  not  included  in 
the  previous  enumeration  have  been  altered  to  impure  epidote. 
The  small  gray  dikes  in  the  slate  on  the  south  side  of  Grape 
Island  are  precisely  similar  to  those  on  Slate  Island. 

Dikes  in  the   Granitic  Rocks. 

Mr.  Bouve  has  carefully  traced  out  the  dikes  of  the  granitic 
area,  so  far  as  they  are  exposed  on  or  near  the  streets,  and  the 
following  notes  are  condensed  from  his  paper. 

On  Meeting  House  Hill,  Main  Street,  South  Hingham,  a 
few  steps  north  from  the  church,  a  dike  from  5  to  6  feet  wide 
is  exposed  in  the  granite  for  about  70  feet,  trending  west- 
northwest.  The  bold  leda-e  of  2:ranite  in  the  ano-le  between- 
Leavitt  and  Jones  Streets  is  divided  by  three  prominent  east- 
west  dikes.  They  are  readily  found  by  proceeding  700  feet 
along  Leavitt  Street  on  the  left  side  from  Weir  River  to 
the  house  of  Mr.  Alanson  Crosby,  and  then  passing  to  the  rear 
of  the  house  about  300  feet  from  the  road.  The  most  northerly 
of  the  three  dikes  is  from  2  to  3  feet  in  width  ;  the  second, 
18  feet  from  the  first,  is  10  feet  wide,  and  exposed  for  7.5  feet ; 
and  the  third,  40  feet  from  the  second,  is  from  3  to  4  feet  wide. 
On  Leavitt  Street  about    a   mile    and    a    half  from    Leavitt's 


264 

Bridge,  going  east,  and  less  than  a  quarter  of  a  mile  before 
reaching  the  town  line,  a  porphyritic  dike  6  feet  wide  crosses  the 
road  diagonally,  trending  about  east-west.  On  the  east  side 
of  Lazell  Street,  740  feet  south  of  Free  Street,  the  high  ledge 
of  granite  is  cut  by  a  very  similar  east-west  porphyritic  dike  6 
feet  wide.  About  50  feet  south  of  it  is  a  parallel  dike  32 
inches  wide.  At  the  granite  quarry  on  Long  Bridge  Lane 
may  be  seen  two  east-west  dikes  about  30  feet  apart,  one  about 
a  foot  and  the  other  22  inches  in  width. 

On  Friend  Street,  near  Main  Street,  two  east-west  dikes 
cut  through  the  granite  of  the  roadway.  The  first  is  330  feet 
from  Main  Street,  from  4  to  6  feet  wide,  aiid  has  been  traced 
from  the  road  east  about  100  feet  and  west  across  the  meadows 
about  1,000  feet.  The  second  is  about  40  feet  beyond  the 
first,  2  feet  wide,  and  has  been  traced  120  feet  or  more.  In  a 
ledge  of  granite  on  Union  Street,  about  360  feet  from  Lazell 
Street,  an  east-west  dike  can  be  traced  about  100  feet.  It 
varies  in  width  from  15  inches  to  2  feet,  and  is  quite  irregular. 
About  2,000  feet  beyond  this,  going  from  Lazell  Street, 
another  east- west  dike,  from  3  to  4  feet  wide,  crosses  the  street 
diagonally,  and  has  been  traced  75  feet  into  the  field  on  the 
left". 

A  dike  at  least  12  feet  in  maximum  width  crosses  the  junction 
of  Rockland  and  Summer  Streets,  on  Old  Colony  Hill,  and 
has  been  traced  east  across  the  adjoining  field  a  total  distance 
of  815  feet.  On  the  east  side  of  the  harbor,  about  275  feet 
north  of  the  steamboat  landing,  is  an  east-west  dike  9  feet 
wide,  with  veinlets  of  epidote.  About  80  feet  farther  north  is 
a  more  irregular  but  similar  and  parallel  dike  about  2  feet 
wide.  Beyond  this,  about  150  feet,  a  third  dike  encloses  a 
large  mass  of  granite.  A  fourth  dike  of  the  system,  6  feet 
wide,  is  exposed,  125  feet  farther  along  the  shore,  at  the 
entrance  of  Mansfield's  Cove.  At  the  north  end  of  Martin's 
Lane,  an  east-west  dike,  6  feet  wide,  can  be  traced  100  feet  in 
the  granite ;  and  in  the  outcrops  of  granite  about  500  feet  in 


265 

length  along  the  east  bank  of  Weymouth  Back  River,  from 
800  to  1,300  feet  south  of  Beal's  Cove,  are  three  dikes  from 
5  to  10  feet  or  more  in  width  and  trending  approximately  east- 
west. 


AGE    OF    THE    IIINGHAM    STRATA. 

The  principal  facts  bearing  upon  this  problem  have  been 
jjresented  in  the  preceding  pages,  and  it  remains  now  simply 
to  marshal  the  scanty  evidence  and  note  its  collective  value. 
Paleontological  evidence  is,  at  present,  wholly  wanting ; 
although  we  may  reasonably  entertain  the  hope  that  fossils 
will  yet  be  found  in  the  slates  or  sandstones  of  Hingham.  The 
lithological  evidence,  although  it  might  be  said  to  point  to 
the  correlation  of  the  Hingham  slates  with  those  of  Weymouth 
and  Braintree,  is  certainly  very  unreliable  in  a  case  like  this  ; 
and,  furthermore,  it  is  entirely  at  variance  with  the  plain 
indications  of  stratigraphy.  But  the  stratigraphic  evidence, 
again,  is  far  from  direct  or  satisfactory,  since  the  deposits  of 
Hingham  are  completely  isolated  by  the  drift  formations  and 
the  sea, — cut  off,  alike  from  the  stratified  rocks  of  Nantasket 
on  the  east  and  those  of  Weymouth  and  the  Blue  Hills  on  the 
west.  Notwithstanding  these  difficulties,  however,  we  have 
two  clues  which  are  satisfactory,  so  far  as  they  go,  although 
they  are,  unfortunately,  not  of  such  a  nature  as  to  lead  to  a 
definite  determination    of  the  geological  horizon.      These   are 

( 1 )  the  relations  to  the  older  eruptives — the  granitic    rocks  ; 

(2)  the  composition  of  the  conglomerate. 

The  Paradoxides  beds  of  Braintree  and  the  Blue  Hills, 
which  Walcott  now  regards  as  of  Middle  Cambrian  age,  are 
clearly  intersected  by,  and  therefore  older  than,  the  different 
varieties  of  granite  and  felsite  of  that  district.  We  have  no 
reason  to  doubt  that  these  granitic  rocks  are  the  same  for  the 
entire  South  Shore,  from  Scituate  and  Cohasset  westward  ; 
and  therefore   it  follows  that  the   conglomerates  of  Nantasket 


266 

and  Hingham,  which  are  so  largely  composed  of  the  debris  of 
these  eruptives  and  are  seen  in  several  sections  to  rest  directly 
upon  them,  must  represent  an  horizon  above  the  Paradoxides 
beds.  Tlie  conglomerate  series  is  overlain  conformably  by  the 
gi'eat  slate  series  of  Hingham,  with  some  interstratification  or 
blendino;  of  the  two  series.  We  are  thus  oblis^ed  to  recoo-nizc 
in  the  Boston  Basin  a  thousand  feet  or  more  of  argillaceous 
strata  above  the  Paradoxides  or  Middle  Cambrian  zone  and 
separated  from  it  by  a  corresponding  or  greater  thickness  of 
coarser  sediments  and  lavas — the  conglomerate  series — with  a 
probable  unconformity  at  the  base  of  the  latter.  This  uncon- 
formity between  the  Paradoxides  beds  and  the  conglomerate 
series  is  proved,  not  alone  by  the  extensive  erosion  of  the 
granitic  rocks,  but  we  also  find  in  the  conglomerate,  at  Pluit's 
Cove  and  elsewhere,  pebbles  of  slate  similar  to  that  of  the 
Paradoxides  beds.  Of  special  interest  in  this  connection,  as 
already  explained,  are  the  pebbles  of  limestone  in  the  conglom- 
erate of  Huit's  Cove.  Limestone  is  a  rare  rock  in  eastern 
Massachusetts ;  and  the  only  beds  now  known  that  can  be 
regarded  as  a  probable  source  of  these  pebbles  are  the  limited 
and  impure  layers  in  the  Cambrian  slates  of  Nahant  and 
Weymouth,  and,  possibly  of  Stoneham  and  other  points  outside 
of  the  Boston  Basin . 

It  is  apparent  from  the  foregoing  that,  although  we  may 
fairly  regard  the  stratified  rocks  of  Hingham  as  forming  one 
conformable  series  from  the  lowest  cono'lomerate  to  the  hisrhest 
slate,  and  this  series,  which  is  quite  certainly  2,000  and 
probably,  including  the  Nantasket  beds,  3,000  feet  in  thickness, 
is  newer  than  the  Middle  Cambrian  beds  and  separated  from 
them  by  an  important  unconformity,  we  are  still  wholly  at  sea 
as  regards  the  precise  horizon  of  the  Hingham  and  Nantasket 
strata.  We  might,  consistently  v.dth  the  facts  so  far  examined, 
refer  them  to  any  horizon  between  the  Middle  Cambrian  and 
the  top  of  the  Carboniferous.  It  remains  to  be  noted,  however, 
that  but  for    the  contemporaneous  lavas  and    tlic   intersecting 


267 

dikes,  the  congloiuenite  series  of"  Nautusket  :in<l  IliiigliiUM, 
including  the  conglomerate  proper  and  the  red  and  green  sand- 
stones and  shales,  bears  a  marked  resemblance  to  the  undoubted 
Carboniferous  strata  of  the  Norfolk  Basin,  extending  from 
Braintree  south  of  the  Blue  Hills  to  Wrentham,  where  it  joins 
the  Narragansett  Basin.  The  conglomerate  series  of  the 
Norfolk  and  Narragansett  Basins  underlies  the  true  Coal 
Measures,  and  has  been  referred  with  much  probability  to  the 
horizon  of  the  Millstone  Grit.  Now,  since  it  is  the  nature  of 
volcanic  phenomena  to  be  localized,  there  appears  to  be  no 
serious  obstacle  in  the  way  of  referring  the  conglomerate  series 
of  the  Boston  Basin,  with  the  associated  igneous  rocks,  to  the 
same  horizon  ;  and  we  may  reasonably  suppose  that,  in  conse- 
quence of  the  more  yielding  nature  of  the  crust  indicated  by  the 
igneous  phenomena,  while  the  formation  of  the  conglomerate 
series  was  followed  in  the  southern  basins  by  conditions  favorable 
to  the  formation  of  beds  of  coal  and  the  enclosing  shales  and 
sandstones,  a  marked  local  depression  made  the  site  of  the 
Boston  Basin  a  deep-water  area  over  which  were  deposited 
the  barren  slates  which  now  overlie  the  conglomerate  series. 
It  is  desired  to  simply  suggest  this  correlation  here,  and  the 
various  facts  which  support  it  will  be  more  fully  set  forth  in 
Part  III. 


THE   SURFACE   GEOLOGY  OF   HINGHAM. 

For  the  general  outlines  of  this  last  chapter  in  the  geology 
of  Hingham  the  reader  is  referred  to  the  corresponding  section 
of  Part  I.  These  two  areas  (Nantasket  and  Cohasset,  and 
Hingham)  are  not  only  contiguous,  but,  so  far  as  the  surface 
geology  is  concerned,  they  are  actually  continuous  ;  and  the 
only  notable  contrast  which  they  present  is  this  :  the  modified 
drift  (sandplains  and  eskers),  which  is  so  scantily  developed 
in  Nantasket  and  Cohasset,  is  a  very  pi"ominent  feature  of 
Hingham. 

DEUMLINS,    GLACIAL    STRIAE,    AND    BOWLDERS. 

As  in  Nantasket  and  Cohasset,  the  unmodified  drift  or  till, 
so  far  as  it  is  exposed  at  the  surface,  exists  almost  wholly  in 
the  form  of  drumlins  ;  and,  relatively  to  the  area,  they  are 
quite  as  numerous  and  important  in  this  district  as  in  the  other. 
The  latter  statement  would,  perhaps,  be  more  than  justified, 
but  for  the  vast  accumulations  of  modified  drift  from  which  the 
drumlins  south  of  the  railroad  immediately  arise  and  in  which, 
no  doubt,  some  of  the  smaller  ones  are  wholly  concealed.  All 
the  drumliuwS  observed  in  Hingham  are  shown  on  the  map  in 
precisely  the  same  way  as  those  of  Nantasket  and  Cohasset, 
with  the  exception  of  Prospect  Hill  (218  feet),  which  lies 
just  beyond  the  southern  limit  of  the  map,  on  the  east  side  of 
Beechwood  River.  Prospect  Hill  is  probably  about  equal  in 
size  to  the  main  Turkey  Hill,  and  it  is  easily  one  of  the  largest 
and  most  imposing  drumlins  in  the  Boston  Basin.  The  eleva- 
tions refer  to  sea  level  (mean  tide).  This  is  the  toj^ographic 
height ;  and  in  the  case  of  the  inland  drumlins  it  is  considerably 
in  excess  of  the  true  geologic  height ;  for  the  heiirht  of  a 
druinlin,  regarded  simply  as  a  mound  of  bowlder-clay,  should, 


269 

evidently,  be  measured  from  the  rocky  base  on  which  it  stands. 
This  is  a  partial  explanation  of  the  fact  that  so  many  inland 
drumlins  have  elevations  incommensurate  with  their  limited 
areas  and  overlooking  the  large  drumlins  near  the  shore.  But 
it  must  be  borne  in  mind,  also,  that  some  of  the  inland 
drumlins  are  much  larger  than  they  appear,  only  their  summits 
protruding  from  the  sandplains. 

As  the  map  shows,  there  is  a  noticeable  difference  in  trend  of 
the  drumlins  in  the  northern  and  southern  parts  of  the  town, 
the  contrast  being  almost  as  marked  here  as  in  Nantasket  and 
Cohasset.  North  of  the  railroad  the  prevailing  trend  is  south- 
east or  between  that  and  east-southeast,  while  south  of  this 
line  it  is  between  southeast  and  south-southeast.  The  Turkey 
Hills  belong  in  the  northern  division.  Undoubtedly  the 
correct  explanation  of  this  contrast  is  that  proposed  in 
Part  I;  viz.,  that  the  ice-sheet,  at  least  during  the  period 
■when  the  drumlins  were  forming,  was  deflected  to  the  east- 
ward by  Boston  Harbor,  but  the  portion  of  the  ice  which 
overcame  the  southern  wall  of  the  harbor  regained  very  nearly 
its  south-southeast  trend.  Although  it  can  be  said  that 
a  general  agreement  exists  between  the  trends  of  the  drumlins 
and  the  glacial  striae  on  the  adjacent  ledges,  yet  this  agreement 
is  by  no  means  perfect  in  all  cases.  The  striae,  as  the  following 
table  shows,  depart,  as  a  rule,  less  widely  than  the  drumlins 
from  the  normal  direction  of  glacial  movement ;  indicating, 
apparently,  that  they  were  formed  chiefly  during  the  period  of 
maximum  glaciation,  when  the  ice-sheet  was  less  influenced  by 
the  subjacent  topography  and  had  power  to  move  the  entire 
thickness  of  the  ground  moraine  and  abrade  the  solid  ledges  ; 
while  the  drumlins,  it  is  generally  conceded,  must  have  been 
formed,  or  at  least  finally  shaped,  during  the  waning  of  the  ice- 
sheet,  when  it  was  easily  turned  from  its  course  and  no  longer 
an  efficient  agent  of  erosion,  but,  partly  slipping  over  the 
ground  moraine  and  partly  dragging  it  along,  banked  it  up 
against  the  ledges.      This  change  in   the    direction  of  the  ice- 


270 

movement  over  the  harbor  area  is  proved,  also,  by  a 
corresponding  variation  in  the  striae  observable  sometimes 
on  the  same  ledge  and  often  on  adjacent  ledges  ;  and  the  fact 
that  the  later  striae  have  been  superimposed  upon  the  earlier, 
without  entirely  obliterating  the  latter,  testifies  to  the  feebly 
erosive  character  of  the  later  movement. 

Directions  of  Glacial  Striae. 

Grape  Island,  ou  slate ,     .     .     .     .  S.  60°  E. 

Huit's  Cove,  onaclike S.  40°  E. 

Planter's  Fields,  on  slate    . .  S.  25°  E. 

Beal  Street,  on  coarse  conglomerate S.  23°-25°  E. 

"  melaphyr S.  32°-35°  E. 

Beal's  Cove,  on  slate ' S.  30°-35°  E. 

West  Hingham  (Fort  Hill),  on  diorite S.  25°-35°  E. 

(near  the  Station),  on  melaphyr  .    .  S.  23°-25°  E. 

Weir  River  Street,  on  granite S.  230-26°  E. 

Corner  Union  Street  and  Long  Bridge  Lane,  on  granite  S.  22°-25°  E. 

Lazell  Street,  1,000  feet  south  of  Free  Street      .     .  S.  23°-250  E. 

Rockland  Street,  corner  of  Summer  Street      .     .     .  S.  25°-30°  E. 

Granite  quarry  near  Abington  Street S.  25°  E. 

The  fragments  of  marine  shells  forming  an  integral  part  of 
the  till  in  several  of  the  Nantasket  drumlins  and  testifying  to 
the  preglacial  existence  of  Boston  Harbor,  appear,  so  far,  to 
be  wliolly  wanting  in  the  Hingham  drumlins.  This  negative 
evidence  is  probably,  however,  of  little  value  ;  for  the  discovery 
of  the  shells  has  been  found  to  require,  in  eveiy  instance,  a 
deep  fresh  section  of  the  till,  the  shells  having  been  removed 
by  solution  from  the  superficial  portion  of  the  till.  Such  a 
section  is  afforded  by  only  one  drumlin  in  Hingham  —  Crow 
Point  Hill.  On  the  north  side,  this  drumlin  is  exposed  to 
several  miles  of  open  water ;  and  the  waves  have  cut  away, 
as  the  map  shows,  at  least  one  fourth  of  the  whole  mass, 
forming  a  fresh  scarp  from  30  to  40  feet  in  height.  A 
careful  examinalion  of  this  section  on  several  difiercnt  occasions 
has  failed  to  reveal  the  slightest  trace  of  enclosed  or  preglacial 


271 

shells.  It  should  be  stated,  however,  tliat  the  lower  part  of 
this  section  is,  unfortunately,  concealed  by  the  roadway  and 
the  retaining  wall,  and  the  greatest  accessible  depth  below  the 
original  surface  is  probably  not  over  25  feet.  Pleasant  Hill 
presents  on  the  same  shore  a  long  low  section  which,  although  not 
so  satisfactory  as  the  other,  might,  perhaps,  be  expected  to 
disclose  the  shells  if  they  were  at  all  abundant  in  the  hill.  But 
this  can  not  be  said  of  the  very  low  and  imperfect  sections 
afforded  by  the  shores  of  the  World's  End,  Planter's  Hill,  and 
Pine  Hill.  Turkey  Hill,  Otis  Hill,  and  perhaps  one  or  two 
other  druralins,  present  shallow  roadside  sections  ;  but  on  none 
of  the  inland  drumlins  are  there  clear  sections,  natural  or 
artificial,  extending  below  the  superficial  and  highly  oxidized 
till.  The  insufficiency  of  this  purely  negative  evidence  is 
obvious,  especially  in  view  of  the  fact  that  shells  have  been 
found  in  well-sections  in  the  inland  drumlins  of  Cohasset 
(p.  143)  and  Braintree  ;  and  it  may  be  safely  predicted  that 
fossil  shells  would  be  found  in  some  of  the  drumlins  of 
Hingham  if  the  blue  or  unoxidized  till  were  exposed  to  our 
observation. 

The  extremely  local  origin  of  the  main  part  of  the  till 
becomes  very  obvious  immediately  on  passing  southward  from 
the  sedimentary  rocks  of  Hingham  to  the  broad  area  of 
granite.  For  the  first  mile  or  two  fragments  of  conglomerate, 
slate,  and  melaphyr  are  fairly  abundant ;  but  beyond  a  distance 
of  three  or  four  miles  from  the  boundary  they  are  rather  rarely 
met  with.  I  have  elsewhere^  pointed  out  that  the  ice-sheet 
probably  slid  over  the  subglacial  till  or  ground  moraine,  the 
latter  moving  very  much  less  rapidly  than  the  ice  itself;  just  as 
the  stones  in  the  bed  of  a  river  move  less  rapidly  than  the 
water  flowing  over  them.  The  englacial  fragments  only,  like 
floating  objects  in  a  river,  are  far-travelled  or  measure  the 
actual  movement  of  the  ice  ;  and  these  form  but  a  very  small 
fraction  of  the  till. 

iProc.  B.  S.N.  H.,  V.  25,134. 


272 

There  is  one  rock  in  Hingham  which  is  so  unique  and 
striking^  in  its  litholoo^ical  character  and  so  restricted  in  its 
distribution  in  situ  as  to  give  its  distribution  in  bowlders  or 
glacial  erratics  a  special  interest.  This  is  the  red  felsite  in  the 
vicinity  of  Bradley  Hill,  on  Lincoln  and  Thaxter  Streets. 
The  accompanying  map  (PI.  12)  shows  the  distribution  of 
this  beautiful  rock  in  the  drift,  so  far  as  it  has  been  traced  out. 
The  bowlders  of  red  felsite  are  of  frequent  occurrence  north  of 
the  railroad  :  but  beyond  a  mile  from  Bradley  Hill  they  are 
few  and  far  between,  as  indicated  by  a  careful  scrutiny  of  the 
stonewalls,  and  every  observed  example  has  been  noted  on  the 
map.  The  bowlders  undoubtedly  extend  farther  to  the  south- 
east than  I  have  traced  them  ;  but  their  lateral  limits  are  indi- 
cated at  least  approximately  by  the  two  lines  on  the  map 
diverging  southward  from  Bradley  Hill.  The  directions  of  these 
lines  are  approximately  S.  23°  E.  and  S,  67°  E.  The  initial 
breadth  of  the  included  area  measures  the  probable  east-west 
extent  of  the  red  felsite  in  situ;  but  the  constantly  increas- 
ing breadth  southward  must  be  attributed  to  the  natural  fanning- 
out  or  radial  dispersion  which  has  been  noted  in  other  cases 
of  the  derivation  of  glacial  erratics  from  a  limited  area.  The 
mean  of  the  two  lateral  limits  stated  above  (S.  45°  E.)  may 
be  regarded  as  an  approximation  to  the  normal  direction  of 
glacial  movement  in  Hingham  ;  or,  perhaps  it  would  be  better 
to  say  the  average  direction,  since  it  is  probable  that  the  trend 
of  the  ice-sheet  was  not  at  any  point  constant  in  direction. 

The  fact,  that,  although  the  red  felsite  erratics  have  been  found 
near  the  southern  end  of  Main  Street,  none  could  be  found  on 
tlie  line  of  or  west  of  this  street,  is  an  indication  that  the 
movement  of  the  ice  was  not  rectilinear  ;  but,  as  shown  by  the 
drumlins  and  glacial  striae,  it  was  first  east-southeast,  changing 
gradually  southward  to  south-southeast,  and  describing  a  curve 
of  which  Main  Street  is  the  chord.  Probably,  also,  the 
movement  Avas,  as  previously  explained,  more  easterly  during 
the  earlier  and  later  stages  of  glaciation,  when  the  ice-sheet  was 


Occas.  papers  Bost.  Soc.  Nat.  Hist.  IV. 


Plate  12. 


■^  \  Felsite Bowlders.  C^^  Outlines  of  BrumliTis .  0SS?ff^^Eskers. 

Map  showing  the  distribution  of   the   Bowlders   of   Red   Felsite  in    Hingham, 

COHASSET,    and    NoRWELL.       ScALE,    ONE    INCH  =  480O   FEET. 


273 

comparatively  thin  and  easily   diverted,    and    more    southerly 
during  the  long  intervening  period  of  maximum  glaciation. 

Of  the  local  origin  of  the  larger  fragments,  at  least,  in  the 
till,  we  have  an  impressive  illustration  in  the  numerous 
bowlders  on  Prospect  Hill.  This  large  drumlin  is  something 
like  Booth  Hill  in  Scituate  in  being  well  sprinkled  with 
bowlders  over  a  large  part  of  its  surface  ;  but  owing,  perhaps, 
to  aqueous  erosion  which  has  swept  away  the  finer  material, 
they  are  especially  abundant  on  the  southwest  and  south  sides 
of  the  hill,  these  slopes  being  the  most  remarkable  bowlder- 
fields  observed  in  the  South  Shore  district  and  recalling  the 
bowlder-clad  areas  of  Cape  Ann.  But  among  these  thousands 
of  bowlders,  of  all  sizes  from  six  inches  to  six  feet  or  more  in 
diameter,  Mr.  Bouve  and  I  have  observed  very  few  that  are 
not  granite  ;  and  certainly  the  sedimentary  and  volcanic  rocks 
of  northern  Hingham  are  very  sparingly  represented.  Still,  in 
a  walk  along  the  shore  between  Crow  Point  and  Huit's  Cove, 
where  the  detritus  is  nearly  all  slate  or  slate  and  conglomerate, 
one  meets  quite  frequent  bowlders  of  granite  and  felsite,  which 
must  have  crossed  Boston  Harbor.  The  most  impressive 
example  of  this  sort  which  I  have  observed  is  afforded  by  the 
two  large  bowlders  of  granite  lying  on  the  north  side  of  Otis 
Hill.  They  are  coarsely  crystalline,  subangular,  and  from  10 
to  12  feet  in  maximum  diameter.  Underlying  this  end  of  Otis 
Hill  and  extending  thence  to  the  north  shore  of  Hingham  we  have 
quite  certainly  only  the  sedimentary  rocks  and  intersecting 
dikes  of  diabase ;  and  the  nearest  visible  source  of  these 
bowlders  is  the  o-ranite  ledo-es  of  Sauo-us,  16  miles  distant. 

The  most  notable  bowlder  in  Hingham,  so  far  as  mere  size 
is  concerned,  is  the  large  mass  of  granite  on  the  north  side  of 
Rockland  Street,  near  the  base  of  Old  Colony  Hill.  It  lies  on 
top  of  the  ground  and  quite  close  to  the  road,  but  is  partly 
concealed  by  the  young  trees  growing  about  it.  It  is  an 
irregularly  angular  block  of  granite  19  feet  long,  16  feet 
wide,  and   17  feet  high  ;  and  was  probably  derived   from  one 

OCCAS.  PAPERS  I?.  S.  N.  II.  IV.  18. 


274 

of  the  ledges  in  the  vicinity  of  Martin's  Lane  or  Planter's  Hill. 
Another  large  bowlder  of  granite,  almost  concealed  by  young 
trees  and  vines,  is  partly  buried  in  the  swamp,  close  to  the 
north  side  of  Pleasant  Street  and  immediately  west  of  Beech- 
wood  River. 

Two  bowlders  of  conglomerate  are  large  enough  to  merit 
special  mention.  One  of  these  lies  on  the  shore  at  the  north 
end  of  Crow  Point  Hill  and  is  partly  submerged  at  high  tide. 
It  was  originally  from  12  to  15  feet  in  diameter,  but  has  been 
ruthlessly  broken  by  blasting  into  three  unequal  masses.  It  is 
a  very  firm,  distinct,  and  coarse  conglomerate,  with  many 
pebbles  of  reddish  granite  one  foot  or  more  in  diameter  ;  quite 
distinct  from  any  conglomerate  observed  in  Hingham,  but 
resembling  the  coarse  conglomerate  of  the  Green  Hill  ledge  at 
Nantasket.  The  other  cono-lomerate  bowlder  is  in  the  northwest 
part  of  Planter's  Fields,  northeast  of  Huit's  Cove  and  about 
midway  between  the  nearest  water  of  the  cove  and  the  north 
shore.  It  rests  on  a  ledge  of  nielaphyr  sloping  northward, 
within  a  few  feet  of  the  northwestern  extremity  of  the  belt  of 
confylonierate  shown  on  the  map,  and  measures  15  feet  in 
length,  8  feet  in  width,  and  9  feet  in  height.  It  is  also  a  firm 
and  rather  coarse  rock,  the  pebbles  ranging  mostly  from  2  to  G 
inches  in  diameter. 


MODIFIED    DRIFT    AND    TERRACES. 

The  modified  drift,  in  its  various  forms,  is  a  very  prominent 
feature  of  Hingham  geology  ;  and  it  is  liere  that  we  find  the 
strongest  contrast  between  the  geology  of  Hingham  and  that  of 
Cohasset  and  Nantasket.  The  sandplains,  except  the  low 
plains  immediately  bordering  the  streams,  are,  to  a  large 
extent  at  least,  indicative  of  standing  water ;  and,  except 
possibly  for  the  very  lowest  plain,  we  must,  as  explained  in 
Part  I,  postulate  fresh  ratlier  than  salt  Avater.  It  a[)pea.rs 
j)robablc,  tlicrefore,  that  the  slight   development  of   sandplains 


275 

in  Cohasset  and  their  almost  complete  absence  in  the  Nantasket 
peninsula  is  due  to  the  general  absence  of  barriers  capable, 
especially  at  the  higher  levels,  of  retaining  the  water  from  the 
melting  ice-sheet.  The  temporary  lakes  indicated  by  the  sand- 
plains  of  the  South  Shore  must,  however,  have  owed  their  exist- 
ence in  every  instance  to  a  wall  of  ice  on  the  north  —  the  front  of 
the  retreating  ice-sheet.  During  the  recession  of  the  ice  the 
imprisoned  waters  north  of  the  water-parting  found  outlets  at 
successively  lower  levels,  and  it  is  undoubtedly  to  the 
somewhat  abrupt  changes  of  level  in  the  lakes  thus  determined 
that  we  owe  the  broadly  terrace  or  step-like  arrangement  of  the 
sandplains  which  is  observed  on  passing  up  from  the  Coastal 
Plain  across  the  Lower  and  Glad  Tidings  Plains  to  Liberty 
Plain.  It  follows  from  this  that,  as  in  the  case  of  ordinary 
terraces,  the  upper  plains  are  the  oldest  and  the  lowest  were 
formed  last.  At  each  level  the  glacial  lakes  received  the 
natural  drainage  of  the  low  water-shed  on  the  south  and  of 
the  ice-sheet  itself,  with  its  superficial  rivers,  on  the  north. 
The  detritus  deposited  in  the  lakes,  like  the  water,  must  have 
had  a  double  origin,  coming  partly  from  the  washing  of  ordi- 
nary till  from  which  the  ice  had  retreated  and  partly  from  the 
eno;lacial  till  of  the  ice-sheet.  The  glacial  streams  had 
undoubtedly,  in  many  cases,  deeply  grooved  and  divided  the 
marginal  portion  of  the  ice,  and  in  these  grooves  or  channels 
was  accumulated,  after  the  manner  of  rivers,  the  narrow 
deposit  of  sand  and  gravel,  which,  when  the  ice  melted  away, 
became  the  steep,  winding  ridges  which  we  call  kames  and 
eskers.  At  the  mouths  of  these  glacial  rivers,  delta-plains 
were  formed  in  the  lakes,  adjacent  deltas  often  becoming  con- 
fluent. The  plains  are  thus  naturally  coincident  in  height  with 
the  eskers,  and  often  appear  as  expansions  of  the  latter.  The 
remarkably  steep  marginal  slopes  presented  by  the  plains  at 
some  points  are  believed  not  to  be  the  free,  growing  front  — 
the  foresets  —  of  the  deltas,  but  to  indicate  supporting  walls  of 
ice,  the  melting  of  which,   as    with    the    kames,    allowed    the 


276 

material  to  assume  the  maximum  angle  of  stability.  In  a 
similar  manner,  isolated  masses  of  ice  gave  rise  to  the  kettles 
and  larger  depressions  or  basins  of  the  plains  ;  and  the  valleys 
occupied  by  the  modern  drainage,  although,  doubtless,  the 
product,  in  part,  of  erosion,  especially  of  erosion  during  the 
formation  of  the  lower  plains,  were  probably  outlined,  at  least 
in  their  wider  parts,  by  stationary  masses  of  ice,  and  since  such 
remnants  of  the  ice-sheet  would  naturally  linger  longest  in  the 
valleys,  we  have  here  a  cause  determining  an  approximate 
coincidence  of  the  ancient  and  modern  —  the  preglacial  and 
postglacial  —  drainage  lines. 

Extending  south  from  the  railroad  along  the  boundary 
between  Hingham  and  Cohasset,  and  forming  the  water- 
parting  between  Weir  River  and  Bound  Brook,  is  a  broad 
and  nearly  continuous  remnant  of  the  ancient  peneplain,  bearing 
several  accumulations  of  till,  among  which  the  Turkey  Hills 
and  Prospect  Hill  are  most  prominent.  South  of  Prospect  Hill 
and  Accord  Pond  this  high  land  separates  the  basins  of  Weir 
River  and  Bound  Brook  from  the  much  larger  basin  of  North 
River.  This  broad  divide  is  diversified  in  its  eastern  half  by 
several  large  drumlins — Mt.  Blue,  Black  Pond  Hill,  Otis  Hill, 
Simons  Hill,  etc. — and  many  smaller  ones,  and  a  remarkable 
series  of  elevated  swamps  ;  but  west  of  the  meridian  of  Prospect 
Hill  the  surface  deposits  are  almost  wholly  modified  drift ; 
and  southwest  of  Union  Street  in  Hingham  there  is  no  pass  out 
of  the  Weir  River  basin  below  the  120  feet  contour-line. 

When,  during  the  retreat  of  the  ice-sheet,  its  front  rested 
ao-ainst  the  hio:h  land  to  the  eastward,  the  water  from  the 
ablation  of  a  large  area  of  ice  must  have  flowed  across  the 
divide  west  of  Prospect  Hill,  forming  the  broadly  extended 
sandplains  which  enclose  Accord  Pond  (Accord  Plain),  with  a 
height  of  from  160  to  180  feet,  and  slope  gently  southward  for 
several  miles  ;  and  when  the  ice-front  had  finall}^  withdrawn 
from  the  summit  of  this  divide,  the  northward  drainage  was 
obstructed  by  the  ice  itself  and  the  lake  formed  in  which  were 


277 

deposited  the  beds  of  sand  and  gravel  forming  Liberty  Plain. 
During  the  maxinuini  development  of  this  lake  the  ice-front 
mnst  have  joined  the  divide  near  the  junction  of  Union  and 
Pleasant  Streets  and  extended  thence  southwest  and  west  along 
the  northern  edge  of  Liberty  Plain  across  Main,  Gushing,  and 
Whiting  Streets  into  Weymouth,  as  far  at  least  as  Old  Swamp 
River,  the  valley  of  which  may  have  been  occupied  by  a  long 
lobe  of  ice.  This  embayment  of  the  ice-sheet  holding  Liberty 
Lake  was  thus  from  3  to  4  miles  in  length  and  from  1  to  2 
miles  in  breadth  ;  and  the  waters  probably  found  an  outlet  in 
the  direct  line  of  Beech  wood  River,  along  the  west  side  of 
Prospect  Hill,  through  Valley  Swamp,  and  so  over  the  divide 
into  the  basin  of  North  River. 

The  valleys  occupied  by  Beechwood  River,  the  small  streams 
draining  into  Fulling  Mill  and  Gushing  Ponds,  and  the  branches 
of  Plymouth  River,  dividing  Liberty  Plain  or  indenting  its 
northern  edge,  probably  correspond  in  a  large  degree  to  irregu- 
larities of  the  ice-front ;  for  where  this  lobate  character  of  the 
plain  is  best  developed,  the  abruptness  and  steepness  of  the 
marginal  slopes  clearly  indicate  that  they  are  not  the  free  and 
natural  forms  or  frontal  slopes  of  deltas,  and  their  relations 
to  the  streams  and  to  the  internal  structure  of  the  plain  forbid 
us  to  regard  them  as  the  product  of  erosion.  The  hypothesis 
that  these  lobes  are  deltas  pure  and  simple,  unmodified  by 
erosion  or  retaining  walls  of  ice,  requires  us  to  postulate 
streams  flowing  from  the  south  of  improbable  magnitude. 
Breakneck  Hill,  extending  north  from  Whiting  §treet  between 
two  branches  of  Plymouth  River,  is  a  remarkably  perfect  lobe 
of  Liberty  Plain.  It  rises  abruptly  from  the  bordering  streams, 
and  its  surface,  save  where  dimpled  with  kettles,  is  approxi- 
mately level  for  a  breadth  of  1,000  feet,  being  much  too  broad 
and  plateau-like  to  be  classed  as  a  kame.  Beyond  Whiting- 
Street,  Liberty  and  Accord  Plains,  with  occasional  ledges  of 
granite  protruding  through  them,  embrace  almost  the  entire 
southwest    corner    of    Hingham     and     the    adjacent    parts    of 


278 

Weymouth  and  Rockland,  the  vai'ious  parts  being  known 
locally  as  Huckleberry  Plain,  Mosquito  Plain,  Farm  Hills, 
Smooth  Hills,  etc. 

Some  of  the  finest  and  most  extensive  eskers  in  Hingham 
are  connected  with  Liberty  and  Accord  Plains.  On  the  east 
side  of  Accord  Pond,  in  the  vicinity  of  Pond  Street  in  Nor  well, 
Accord  Plain  has  a  very  perfect  development  with  a  height  of 
about  160  feet;  and  branching  off  from  it  is  a  typical  esker 
which  runs  north  and  northwest  to  the  outlet  of  the  pond, 
where  it  is  interrupted  by  an  erosion-gap  which  has  been 
widened  by  artificial  excavation.  West  of  the  stream  (Beech- 
wood  River)  it  follows  a  somewhat  serpentine  course  towards 
and  across  Whiting  Street,  being  traceable  nearly  500  feet 
beyond  the  street.  Accord  Pond  is  due  to  the  obstruction  of 
the  valley  of  Beechwood  River  by  the  plain  and  especially  by 
this  esker.  What  is  virtually  a  somewhat  detached  branch  of 
this  esker  runs  north  from  Whiting  Street  east  of  the  river. 
Continuing  in  a  ojeneral  north-northwest  direction  from  these 
eskers  for  nearly  half  a  mile  to  and  across  Gardner  Street,  we 
reach  the  southern  end  of  the  remarkable  esker  which  extends 
thence  for  nearly  a  mile  in  the  same  direction  to  and  across 
Gushing  Street,  showing  a  general  agreement  in  height  with 
Liberty  Plain.  It  is  throughout  a  prominent  and  sharply 
defined  ridge  of  gravel;  and  can  be  traced  about  1,100  feet 
west  of  Gushing  Street.  About  200  feet  south  of  this  esker 
Gushing  Street  cuts  through  a  second  and  similar  esker  which 
has  in  its  western  half  a  curving  course  convex  to  the  south, 
joins  the  first  esker  600  feet  east  of  the  street,  and  then 
branches  off"  and  runs  approximately  parallel  with  it,  as  a  high 
sharp  ridge  (Mullein  Hill),  nearly  halfway  to  Gardner  Street. 

Although  the  eskers  of  Gushing  Street,  which  are  known 
together  as  the  High  Hills,  are  such  prominent  and  important 
features  in  the  surface  geology  of  Hingham,  they  are  not  more 
typical  or  beautiful  than  those  next  to  be  described.  These 
are  on  the  west  side  of  the  valley  which  is  the  direct  continua- 


279 

tion  southward  j)ast  the  west  side  of  Prospect  Hill  of"  the 
valley  of  Beech  wood  River,  and  through  which,  it  seems 
probable,  the  waters  of  Liberty  Lake  found  an  outlet.  The 
eskers  form  two  adjacent  and  approximately  parallel  ridges 
from  20  to  40  feet  high  immediately  west  of  and  parallel  with 
Prospect  Street,  commencing  at  the  bend  in  the  street  south  of 
Beechwood  River  and  extending  in  a  south-southeast  direction 
across  the  town-line  into  Norwell  as  far  as  Valley  Swamp,  the 
exti'eme  length  being  nearly  half  a  mile.  The  western  ridge 
reaches  a  little  farther  north  than  the  other,  and  both  are  some- 
what serpentine,  being  separated  by  shallow  kettles,  and  uniting 
at  one  point  to  form  for  a  few  rods  a  single  ridge. 

That  these  eskers  are  connected  in  origin  with  a  glacial  river 
flowing  southward  through  this  valley  there  can  be  but  little 
doubt.  The  whole  appearance  of  the  valley,  east  of  Prospect 
Street,  indicates  that  it  has  been  swept  by  such  a  current. 
The  very  broad,  low  hill  of  till  extending  from  Prospect  Hill 
southwest  to  Valley  Swamp  is  thickly  strewn  with  bowlders, 
the  finer  portion  of  the  till  having  been  washed  away  ;  and  on 
the  lower  western  and  southwestern  slopes  of  Prospect  Hill,  as 
previously  noticed,  the  ground  is  literally  covered  with  bowlders, 
especially  on  the  erosion  terraces,  which  are  a  conspicuous 
feature  of  this  drumlin.  The  most  prominent  bench  or  terrace 
is  from  45  to  50  feet  below  the  summit.  It  i^s  strongly  marked 
and  continuous  along  the  entire  southwest  side  of  the  hill.  About 
30  feet  below  this  is  a  second  terrace  ;  and  20  feet  below  that 
a  third,  the  latter  agreeing  approximately  in  elevation  with  the 
kames  and  Liberty  Plain.  The  upper  terraces,  at  least,  must 
antedate  Liberty  Plain,  requiring  the  same  explanation  as  the 
similar  terraces  on  the  drumlins  of  Cohasset  and  Nantasket ;  and 
thus  dating  from  a  time  when  the  ice-sheet  filled  this  valley, 
with  the  top  of  the  drumlin  protruding  and  a  glacial  river 
running  between  the  slope  of  till  and  the  edge  of  the  ice. 

The  distinctly  double  or  duplicate  character  of  the  Prospect 
Street   eskers  is   very  suggestive  of  the  material   having  been 


280 

deposited  in  a  channel  having  a  bottom  as  well  as  walls  of  ice. 
When  the  bordering  ice  melted  away,  it  allowed  the  sand  and 
gravel  to  fall  down  on  either  side  of  a  ridge  of  ice  ;  and  the 
subsequent  melting  of  this  core  or  nucleus  of  ice  caused  the 
crest  of  the  ridge  to  fall  in,  dividing  the  single  ridge  into  two, 
with  kettles  between.  In  the  same  direct  line  with  the  Accord 
Pond  and  Gushing  Street  eskers  is  the  splendid  series  on  the 
west  side  of  Weymouth  Back  River,  in  Weymouth  ;  and  it  is 
in  the  highest  degree  probable  that  they  were  formed  in 
succession  by  the  same  great  glacial  river. 

Between  Union  Street  and  Leavitt  Street  there  is  a  trans- 
verse depression  or  gap  in  the  water-parting  separating  Weir 
River  and  Bound  Brook  which  does  not,  apparently,  rise  above 
the  80  feet  contour-line:  and  we  may  suppose  that,  when  the 
ice-front  had  retreated  from  the  high  land  at  this  point,  the 
imprisoned  waters  found  an  outlet  here,  and  Liberty  Lake  was 
drained,  the  overflow  being  now  into  Bound  Brook  instead  of 
into  North  River.  The  continued  recession  and  embayment  of 
the  ice,  while  the  waters  discharged  at  this  level,  formed  the 
temporary  lake  in  which  were  deposited  the  sands  and  gravels 
of  Glad  Tidings  Plain,  the  normal  height  of  which  is  from  65 
to  70  feet.  This  plain  has  a  very  perfect  development  in  the 
vicinity  of  Glad  Tidings  Rock,  between  Free  and  Pleasant 
Streets,  and  also  west  of  Gushing  Pond  and  Plymouth  River, 
between  High  and  Ward  Streets,  and  the  Weymouth  boundary, 
and  north  of  High  Street,  between  Hemlock  Swamp  and 
French  Street,  surrounding  and  partially  burying  the  drumlin 
of  Nutty  Hill,  and  also  west  of  French  Street.  The  little 
plateau  known  as  Pigeon  Plain,  soutii  of  Hobart  Street  and 
southwest  of  Great  Hill,  is  an  isolated  but  singularly  perfect 
portion  of  this  plain.  Liberty  Plain  does  not  rise  directly  from 
Glad  Tidings  Plain,  but  a  marked  depression  separates  them  at 
nearly  all  points.  The  basins  of  Fulling  Mill  and  Gushing 
Ponds  arc  a  part  of  this  depression  ;  but  even  on  the  line  of 
Main  Street,  where  there  is  neither   pond  nor  stream,  it  is  very 


281 

noticeable.  The  northwestern  part  especially  of  Glad  Ticlinnjs 
Plain  is  indented  by  many  beautiful  kettles  and  basins,  some  of 
which  are  ponds  ;  but  the  only  well-developed  or  typical  esker, 
so  far  as  I  have  observed,  connected  with  this  plain,  is  that 
skirtino;  the  east  side  of  Fullino;  Mill  Pond.  This  esker 
formerly  terminated  on  the  line  of  Pleasant  Street,  but  it  has 
been  dug  away  for  50  or  60  feet.  Its  maximum  height  is 
above  40  feet,  and  it  is  nearly  2,000  feet  long.  It  is  some- 
what winding,  but  the  general  course,  as  shown  on  the  map, 
is  approximately  north -south.  Somewhat  less  than  1,500  feet 
south  from  its  northerly  termination  it  divides,  the  western 
branch  following  the  shore  of  the  pond  westward  750  feet 
farther,  with  an  elevation  of  25  feet. 

South  of  Turkey  Hill,  the  basins  of  Weir  River  and  Bound 
Brook  are  connected  by  one  or  more  gaps  less  than  60  feet  in 
height  through  which  the  water  probably  passed  during  the 
formation  of  the  principal  and  higher  part  of  Lower  Plain 
(from  50  to  55  feet).  But  later  the  overflow  appears  to  have 
gained  the  still  lower  passage  north  of  Turkey  Hill,  following 
the  present  course  of  the  railroad  (Weir  River  Bay  being, 
presumably,  occupied  by  ice)  to  Cohasset  Harbor ;  thus 
explaining  the  sand  deposits  along  this  line  and  the  very  typical 
sandplain  of  Cohasset  Village,  Little  Harbor  being  still  occu- 
pied by  ice.  as  indicated  by  the  abrupt  slopes  of  the  plain  on 
that  side. 

Although  Lower  Plain  is  only  from  10  to  20  feet  lower  than 
Glad  Tidings  Plain,  they  are  easily  distinguished  at  nearly  all 
points  on  account  of  the  line  of  depression  which  separates 
them.  Lower  Plain  has  no  important  development  east  of 
Weir  River,  but  its  inland  boundary  is  approximately  defined 
by  the  valley  of  this  stream  and  the  small  tributary  stream 
crossing'  Main  Street  north  of  Free  Street  and  draining 
Hemlock  Swamp.  From  the  north  end  of  this  swamp  the  line 
passes  along  Hobart  Street  between  Great  Hill  and  Pigeon 
Plain,  which  we  have  already  recognized  as  an  outlier  of  Glad 


282 

Tidings  Plain,  to  French  Street  and  the  Weymouth  line. 
This  southern  and  highest  part  of  the  plain  has  its  best  devel- 
opment along  the  lines  of  Main,  East,  and  Central  Streets,  its 
elevation  here  varying  but  slightly  from  50  feet.  West  of 
Hersey  and  Main  Streets  it  is  more  fragmentary,  divided  by 
swamps  and  meadows  and  the  branches  of  Town  Brook  and 
Fresh  River.  It  closely  invests  Great  Hill,  greatly  dimin- 
ishing the  apparent  height  of  this  drumlin  ;  and  some  of  these 
isolated  portions  are  very  typical  in  form,  though  more  com- 
monly presenting  the  forms  of  rounded  hills  and  hillocks  or 
kame-like  spurs  and  ridges. 

Undoubtedly  the  most  interesting  feature  of  the  Lower 
Plain,  south  of  the  railroad,  is  the  tongue  or  lobe  of  it  which 
extends  northeast  along  East  Street  from  the  vicinity  of  Main 
Street  to  Andrew  Heights.  This  ridge  of  gravel  and  sand 
rises  abruptly  50  feet  from  the  salt  marsh  (Home  Meadows) 
on  the  north  and  slopes  gently  down  to  Weir  River  on  the 
south.  It  was  evidently  formed  at  a  time  when  the  basin  of 
the  Home  Meadows  was  occupied  by  a  lobe  of  the  ice-sheet,  the 
sand  drifting,  probably,  from  the  main  part  of  the  plain  on  the 
west  and  accumulating  against  the  edge  of  the  ice,  the  subse- 
quent melting  of  which  gave  the  bank  its  steep  northern  slope- 
The  special  interest  of  this  deposit  is,  of  course,  that  it  forms, 
as  previously  stated,  a  natural  dam  directly  across  the  original 
course  of  Weir  River,  causing  it  to  turn  at  a  right  angle,  flow 
up  the  valley  of  what  was  once  a  small  tributary,  over  the 
water-parting  north  of  Turkey  Hill,  and  thence  down  through 
Foundry  Pond  and  a  straight  narrow  valley  to  Weir  River 
Bay,  the  drainage  of  almost  the  entire  area  of  Hingham  south 
of  the  railroad  becoming,  in  consequence  of  this  geological  acci- 
dent, tributary  to  Nantasket  Harbor  instead  of  Hingham 
Harbor.  South  of  the  glacial  barrier,  the  valley  of  the  main 
stream  and  its  principal  tributaries  is  broad,  open,  smooth,  and 
unbroken  by  ledges,  the  streams  meandering  through  extensive 
meadows  and  swamps  ;  and  north  of  the  barrier  the  valley  is 


283 


continued  with 
tlie  same  char- 
acter in  Home 
Meadows  ;  while 
the  modern  char- 
acter of  the 
present  valley 
of  Weir  River 
below  this  point 
is  seen  in  its 
contracted  form, 
its  rapid  descent 
north  of  the  rail- 
road, and  the 
fact  that  the 
actual  channel 
of  the  river  is 
frequently  bor- 
dered by  the 
massive  granite 
ledges. 

When  the  ice 
had  retreated 
some  distance, 
north  of  the  rail- 
road, but  still 
filled  the  valleys 
of  Weymouth 
Back  River  and 
Hingham  Har- 
bor, and  rested 
against  the  his^h 
rocky  land  ex- 
tending from  Old 
Colony  Hill  east- 


284 

ward  to  Little  Harbor,  the  water  findino;  an  outlet  alous; 
the  line  of  the  railroad  into  Cohasset  Harbor,  the  part  of  the 
Lower  Plain  north  of  the  railroad,  and  separated  by  tlie 
valleys  of  Town  Brook  and  Fresh  Kiver  from  the  higher 
southern  part  of  the  plain,  was  formed.  It  is  most  perfectly 
developed,  with  a  normal  elevation  of  from  40  to  45  feet,  in 
the  area  south  of  Broad  Cove  and  across  the  southern  end 
of  the  harbor,  this  eastern  extension  almost  repeating  the 
barrier  along;  East  Street  which  has  diverted  the  drainajye  of 
the  Weir  River  Basin.  Its  most  interesting  development, 
however,  is  west  of  the  drumlins,  in  the  Hockley  District, 
which  embraces  the  triangular  area  between  Fort  Hill  Street, 
Hockley  Lane,  and  Weymouth  Back  River.  The  typical 
plain  is  almost  wholly  wanting  here  ;  but  the  deposit  is  of  a 
distinctly  tumultuous  character,  gracefully  undulating,  with 
rounded  knolls,  short  ridges  or  kames,  and  kettles.  It  is  to 
the  smoothly  hummocky  character  of  this  district  that  it  owes  its 
name.  The  highest  part  is  a  ridge  from  60  to  80  feet  high 
which  should,  perhaps,  be  referred  to  the  Glad  Tidings  Plain. 
Several  groups  of  eskers  maybe  referred  to  the  Lower  Plain, 
although  only  one  of  these  is  actually  included  within  its  area. 
This  is  the  beautiful  double  esker  at  the  north  end  of  Great 
Hill.  As  shown  on  the  map,  these  ridges  trend  east- west,  the 
southern  one  just  touching  the  extreme  north  end  of  Great  Hill, 
from  the  slope  of  which  the  view  (Fig.  24)  is  taken,  the 
large  drumlin  which  looms  up  in  the  distance  being  Baker's 
Hill.  The  ridges  are  approximately  straight,  parallel,  of  nearly 
equal  height  —  from  30  to  50  feet — and  so  near  together  as 
to  make  it  reasonable  to  suppose  that  they  are  the  product  of 
one  glacial  stream,  as  explained  on  page  279.  The 
northern  ridge  is  about  1,800  feet  long,  extending  nearly  600 
feet  east  of  New  Bridge  Street ;  while  the  southern  ridge  begins 
at  the  street ;  and  about  900  feet  west  of  the  street  and  350 
feet  from  their  western  terminations  they  are  connected  by 
a  cross  ridge,  tlie  beautiful,   elongated  basin   between  this  and 


285 

the  street  being  known  as  the  "dry  dock."  Tlic  unusual 
trend  of  these  eskers  is  not  readily  accounted  for,  but  we  risk 
little  in  saying  that  their  relation  to  Great  Hill  was  probably 
a  determining  cause. 

Passing  the  Hockley  district,  which  is  largely  kame-like  in 
character,  the  next  group  embraces  three  parallel  ridges  north 
of  Seal's  Cove,  from  400  to  (iOO  feet  long  and  trending 
about  west-northwest.  Still  farther  northwest,  at  the 
re-entrant  angle  in  the  east  shore  of  Weymouth  Back  River, 
about  1,600  feet  south  of  Lincoln  Street,  we  have  the 
beginning  of  one  of  the  finest  and  most  extensive  groups  of 
eskers  in  Hingham.  It  skirts  the  shore  without  an  appre- 
ciable break  across  Lincoln  Street  to  the  northern  end  of 
Stoddard's  Neck,  a  total  distance  of  fully  two  thirds  of  a  mile, 
the  general  trend  being  quite  normal,  south-southeast.  In 
detail,  however,  it  is  curving,  winding,  and  branching,  some- 
times single  and  sometimes  double,  enclosing  kettles  and  a 
pond,  as  shown  on  the  map.  The  height  varies  from  25  to  50 
feet,  except  on  the  west  side  of  Stoddard's  Neck,  where  it 
exceeds  60  feet. 

Taking  a  general  view,  it  is  readily  seen  that  all  the  eskers 
of  Hingham  may  be  referred  to  two  slightly  diverging  lines  or 
systems.  The  western  system  has  already  been  traced,  begin- 
ning in  the  magnificent  series  on  the  west  side  of  Weymouth 
Back  River,  in  Weymouth,  and  including  the  eskers  of  Gushing 
Street  and  Accord  Pond.  The  eastern  system  begins  on 
Stoddard's  Neck,  and  includes  the  kames  and  eskers  of  Beal's 
Gove,  Great  Hill,  Fulling  Mill  Pond,  and  Prospect  Street. 

On  both  sides  of  Great  Hill  are  distinct  erosion  terraces 
corresponding  in  height  to  Glad  Tidings  Plain.  Similar 
benches,  but  agreeing  in  elevation  with  the  Lower  Plain,  are 
strongly  marked  on  the  southwest  sides  of  Otis  and  Baker's 
Hills,  prominent  features  of  these  great  drumlins  and  the  work, 
probably,  of  glacial  streams  tributary  to  the  lake  in  which  the 
Lower  Plain  was  formed. 


286 

When  the  ice-sheet  had  retreated  from  the  northwest  corner 
of  Hingham,  but  yet  lingered  in  Weymouth  Back  River,  the 
prominent  line  of  ledges  extending  from  Huit's  Cove  across 
Lincoln  and  Beal  Streets  to  Beal's  Cove,  with  the  drumlins 
back  of  it,  formed  a  barrier  northwest  of  which  was  spread  a 
very  perfect  sandplain  having  a  normal  elevation  of  from  25  to 
30  feet  and  designated  in  the  preceding  pages  as  the  Coastal 
Plain.  Traces  of  it  may  be  observed  all  through  the  northern 
part  of  Hingham,  and  corresponding  to  it  are  more  or  less 
distinct  erosion  terraces  on  the  southwestern  side  of  Pleasant 
Hill  and  the  northeastern  side  of  Squirrel  Hill. 

So  far  as  known,  there  are  no  deposits  of  stratified  clay  in 
Hingham,  above  sea-level.  This  finest  part  of  the  modified 
drift  was  probably  carried  far  beyond  the  limits  of  the  town  by 
the  overflow  waters  from  the  glacial  lakes.  We  may  reasonably 
suppose,  however,  that  beneath  the  argillaceous  silt  now 
depositing  in  Hingham  Harbor  and  exposed  to  view  at  low-tide, 
there  is  a  considerable  thickness  of  true  glacial  clay. 

POSTGLACIAL     DEPOSITS     AND    CHANGES. 

The  evidences  of  postglacial  changes  of  level  observed  in 
Nantasket  are  not  repeated  with  equal  distinctness,  if  at  all,  in 
the  Hingham  area  ;  but  above  the  present  level  of  the  sea  there 
are,  so  far  as  observed,  no  traces  of  marine  erosion  or  marine 
deposits,  or  any  proofs  of  a  postglacial  elevation  of  the  land. 
The  lowest  or  coastal  sandplain,  like  the  higher  plains,  is,  as 
already  stated,  best  explained  as  the  product  of  glacial  lakes 
and  streams  during  the  departure  of  the  ice-sheet.  Beneath 
some  of  the  lower  bogs  and  marshes  there  are,  possibly,  layers 
of  marine  silt  and  shells,  but  nothing  of  the  kind  appears  to 
have  been  observed.  On  the  other  hand,  it  is  in  the  highest 
degree  probable  that  sections  of  the  salt  marshes  would  present, 
in  the  form  of  buried  peat  beds,  if  not  otherwise,  evidence  of  a 
moderate  postglacial  subsidence  of  the  land.      But  even  if  such 


287 

facts  were  shown  to  be  wanting,  we  need  not  doubt  that 
Hingham  has  participated  in  the  slight  downward  movement  of 
which  we  have  satisfactory  evidence  in  other  parts  of  the 
Boston  Basin.  All  the  evidence  now  at  hand  indicates  that 
the  land  has  been  nearly  if  not  quite  stationary  for  a  long 
period ;  and  if,  as  has  been  supposed,  a  subsidence  is  still  in 
progress,  it  must  be  extremely  slow. 

The  erosive  action  of  the  streams  of  Hingham  during  post- 
glacial time  has  been  insignificant.  The  almost  entire  absence 
of  rapids  indicates  that  the  streams  have  to  a  very  large  extent 
regained  the  preglacial  drainage  lines  ;  and  in  consequence  we 
rarely  find  them  now  eroding  the  hard  rocks.  Weir  River, 
below  Leavitt  Street,  is,  of  course,  a  notable  instance  of 
diverted  drainao:e,  and  the  artificial  fall  where  the  river  issues 
from  Foundry  Pond  is  almost  the  only  suggestion  of  a  cascade 
chai'acter  observed  in  any  of  the  streams  of  Hingham.  The 
general  absence  of  ledges  in  the  valleys  is,  obviously^  explained 
by  the  strong  preglacial  elevation  of  the  land,  during  which 
the  valleys  were,  in  many  cases,  cut  down  below  the  present 
level  of  the  sea.  Marine  erosion  in  postglacial  times  is  practi- 
cally limited  to  the  drumlins  about  the  harbor  and  the  sand- 
plains  and  kames  bordering  Weymouth  Back  River. 

The  principal  salt  marshes  of  Hingham  are  the  Home 
Meadows,  and  those  bordering  Broad  Cove  and  the  tidal  portion 
of  Weymouth  Back  River  between  the  railroad  and  the  barrier 
due  to  the  jutting  out  from  either  shore  of  a  kame-like  ridge 
of  modified  drift.  Several  of  the  smaller  marshes,  as  previously 
stated,  were  reclaimed  during  the  early  settlement  of  the 
country  by  means  of  dikes.  A  large  part  of  Hingham  Harbor 
has  advanced,  in  the  process  of  silting  up,  to  the  eel-grass  stage. 
Weymouth  Back  River  is  much  less  advanced  simply  because 
it  is  a  long  narrow  basin  and  the  tidal-scour  is  more  concen- 
trated and  efficient. 

Besides  the  growth  of  peat  on  the  salt  marshes,  there  are 
extensive  deposits  in  the  fresh-water  swamps  and    meadows  ; 


288 

and  in  the  past  some  of  these  have  been  dug  for  fuel.  This  is 
especially  true  of  Tucker's  Swamp,  west  of  Beal  Street  and 
Hockley  Lane,  where  the  digging  of  peat  was  at  one  time  an 
important  industry.  In  connection  with  the  peat-bogs  I  have 
learned  of  no  deposits  of  diatomaceous  earth  ;  but  it  is  very 
probable  that  such  exist.  Bog-iron  ore  is  forming  in  many 
places,  the  most  important  occurrence  that  I  have  happened  to 
notice  being  on  the  farm  of  David  Breen,  in  the  meadows  west 
of  Downer  Avenue  and  north  of  Lincoln  Street.  It  forms  a 
layer  from  one  to  two  feet  thick  beneath  a  similar  thickness  of 
peat.  A  beautiful  orange  sand,  that  is,  a  sand  strongly  impreg- 
nated with  limonite,  was  exposed  in  a  ditch  dug  to  drain  the 
swamp  on  Lincoln  Street  west  of  Squirrel  Hill. 


OF    THE 

§0ston  Snrielji  0f  |liitxiriil  ^istorg. 

IV. 


GEOLOGY  OF  THE  BOSTON  BASIN 


WILLIAM  0.  CROSBY. 


VOL.  I. 


PART  III.— THE  BLUE  HILLS  COMPLEX. 


BOSTON : 

BOSTON  SOCIETY  OF  NATURAL  HISTORY. 

1900. 


THE  BLUE  HILLS  COMPLEX. 

INTRODUCTION. 


The  Blue  Hills  Complex  is  the  area  of  granitic  rocks  and 
associated  vJarab'"ian  strata  which  includes  the  Blue  Hills 
proper  .aid  extends  thence  eastward  across  Quincy  and  the 
northern  parts  of  Braintree  and  Weymouth.  The  Complex 
is  thus  a  geologic  rather  than  a  topographic  unit,  embracing, 
besides  the  Blue  Hills  themselves,  which  may  be  said  to  finally 
die  out  eastward  at  Weymouth  Fore  River,  the  relatively  low 
and  level  area  of  granite  and  Cambrian  strata  in  northern 
Weymouth  ;  the  true  eastern  and  western  limits  of  the  Com- 
plex being,  respectively,  W^eymouth  Back  River  and  the  Ne- 
ponset  River.  These  boundaries,  it  will  be  seen  later,  are  as 
strongly  marked  geologically  as  topographically,  coinciding 
with  two  of  the  most  important  displacements  of  the  region. 
In  other  words,  the  Blue  Hills  Complex  carries  the  southern 
rim  of  the  Boston  Basin  westward  twelve  miles  from  Wey- 
mouth Back  River  and  the  border  of  Hingham  to  its  topo- 
graphic culmination  in  Great  Blue  Hill  and  its  abrupt  termina- 
tion at  the  Neponset  River.  The  Complex  is  bounded  on  the 
north  by  the  great  conglomerate  series  and  newer  slates,  prob- 
ably of  Carboniferous  age,  which,  in  a  series  of  broken  and 
denuded  anticlines  and  synclines,  stretch  northward  across  the 
Boston  Basin  ;  and  its  southern  border  is  also  very  sharply 
defined  for  nearly  half  its  length,  or  as  far  east  as  Great  Pond 
in  Brainti-ee,  by  the  northeastern  extension  of  the  Norfolk 
Basin, — a  long,  narrow  trough  of  Lower  Carboniferous 
strata  (conglomerate  and  sandstone)  which  branches  off  from 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  19. 


290 

the  broad  Narragansett  Basin  in  Wrentham  ;  the  Boston  and 
Norfolk  Basins  being,  as  Professor  Shaler  long  since  pointed 
out,  overlapping  troughs.  Our  study  will,  therefore,  include 
the  relations  of  the  Complex  to  these  newer  formations.  Be- 
yond the  eastern  end  of  the  Noi'folk  Basin  the  southern  border 
of  the  Complex  is  lost,  since  it  merges  here  with  the  great 
granitic  area  lying  between  the  Boston  and  Narragansett 
Basins.  The  eastward  prolongation  to  Weymouth  Back 
River  of  the  line  between  the  Blue  Hills  and  the  Norfolk  Basin 
affords,  however,  a  convenient  arbitrary  boundary,  since  it  lies 
to  the  southward  of  all  the  known  Cambrian  outcrops.  Owing 
to  the  extensive  drift  deposits  north  of  the  Blue  Hills,  in 
Quincy  and  Milton,  the  natural  northern  boundary  of  this 
geologic  area  is  not  easily  traced  ;  but  it  will  be  regarded  as 
passing  along  the  northwest  sides  of  Peddock's  Island  and 
Hough's  Neck,  and  thence  west  and  southwest  by  Wollaston 
Heights,  East  Milton,  and  Milton  Center  to  the  Neponset  at 
Paul's  Bridge.  This  line,  it  will  be  observed,  divides  some- 
what equally,  but  diagonally,  the  second  sheet  of  the  general 
map. 

Although  this  area  is  several  times  as  large  as  that  described 
in  the  second  part  (Hingham),  it  is  quite  as  distinctly  a  geo- 
logic unit ;  and  it  scarcely  admits  of  natural  subdivision. 
East  of  Weymouth  Back  River,  in  Hingham  and  Nantasket, 
we  have,  besides  the  granitic  I'ocks,  so  far  as  is  now  known, 
only  the  great  conglomerate  series  and  the  newer  slates  ;  but 
west  of  the  line  indicated  we  appear  to  pass  at  once  to  the 
older  slates  that  normally  underlie  unconformably  the  con- 
glomerate series,  their  high  antiquity  being  shown  by  their 
relations  to  the  granite,  which  is  everywhere  clearly  intrusive 
in  the  slate.  This  Complex  of  the  granitic  rocks  and  the  older 
slate  series,  interrupted  only  by  drift  deposits,  can  be  traced 
westward  from  the  eastern  part  of  Weymouth  across  Braintree, 
Quincy,  and  Milton  almost  to  the  Neponset.  The  slate,  being 
at  most  points  the  softer  rock,  has,  in  general,  less  relief  than 


291 

the  granite  ;  and  is,  in  consequence,  relatively  inconspicuous. 
But  the  map  shows  that  it  forms,  in  the  aggregate,  an  impor- 
tant fraction  of  the  total  area  of  the  Complex. 

These  older  slates  are  less  sharply  limited  than  the  conglom- 
erate series  and  the  newer  slates  to  the  topographic  area  com- 
monly known  as  the  Boston  Basin,  forming  numerous  outliers 
among  the  granitic  rocks  that  enclose  the  Basin  ;  and  hence  the 
boundary  of  the  Basin  is,  in  the  geologic  sense,  less  definite 
and  distinct  in  the  Blue  Hills  region  than  elsewhere,  as  in  Hing- 
ham  and  Cohasset.  That  is,  the  margin  or  rim  of  the  Basin  is 
here,  as  farther  east,  simple  and  well  defined,  and  wholly 
north  of  the  granite,  as  regards  the  conglomerate  series  and 
the  newer  slates,  but  intricate  and  vague  as  regards  the  older 
slates  ;  and  it  may  be  noted  in  passing  that  this  fact  strongly 
corroborates  the  view  that  a  profound  stratigraphic  break 
exists  between  the  older  slates  and  the  conglomerate  series. 
The  position  of  the  Blue  Hills  Complex  between  the  Boston 
Basin  and  the  two  connected  basins  known  to  be  Carboniferous 
and  which  may  be  regarded,  in  a  general  view,  as  one  basin  ; 
and  the  fact  that  it  thus  embi'aces,  in  addition  to  the  Cambrian 
slates  and  limestones  and  the  associated  granitic  series,  typical 
developments  of  the  conglomerate  and  volcanic  series  and  the 
newer  slates  of  the  Boston  Basin  on  the  north,  and  the  basal 
Carboniferous  conglomerate  on  the  south,  make  it  virtually 
the  key  to  the  stratigraphy  of  the  entire  Cambro-Carboniferous 
area  of  eastern  Massachusetts.  The  study  of  any  other  equal 
area  could  be  more  safely  neglected  than  of  this.  Again,  the 
Blue  Hills,  which  might  well  be  called  the  Boston  Mountains, 
are  the  most  prominent  topographic  feature  not  only  of  the 
Cambro-Cai'boniferous  area,  but,  considering  their  proximity 
to  the  coast,  of  the  entire  Atlantic  sea-board  of  the  United 
States  south  of  New  Hampshire.  They  are,  therefore,  an 
erosion-monument  of  exceptional  importance,  an  isolated  rem- 
nant or  outlier  of  an  ancient  peneplain  ;  and  one  of  the  most 
interesting  of  the  minor  problems  of  the  district  is  found  in  the 


292 

fact  that  this  narrow  granitic  wall  or  partition  between  the 
Boston  and  Norfolk  Basins  has  such  an  exceptional  elevation, 
dominating,  as  it  does,  the  much  larger  granitic  areas  of  the 
immediate  environment  and  of  the  entire  region. 

Among  the  more  general  and  important  problems  presented 
by  the  area  here  outlined  are  :  first,  the  age  of  the  older  slate 
and  its  relations  to  the  granite  ;  second,  the  structural  relations 
of  this  Complex  to  the  sedimentary  deposits  of  both  the  Boston 
and  Norfolk  Basins  ;  and  third,  the  true  stratigraphic  positions 
of  these  newer  formations.  Fortunately,  the  first  problem, 
the  age  of  the  older  slates,  finds  an  easy  and  satisfactory  solu- 
tion in  the  occurrence  in  it,  at  several  points,  of  a  Cambrian 
fauna.  There  seems  to  be  no  reason  to  doubt  that  this  older 
or  Cambrian  slate  is  the  oldest  formation  now  exposed  in  the 
entire  Blue  Hills  region  ;  and  we  thus  have  the  satisfaction, 
which  was  denied  us  in  the  Hingham  and  Nantasket  areas,  of 
a  well-determined  and  definite  base  or  starting  point  for  our 
stratigraphic  study. 

The  exposures  of  the  conglomerate  series  and  the  newer 
slates,  on  the  other  hand,  are  so  limited  as  to  make  the  deter- 
mination of  the  second  and  third  problems,  in  the  absence  of 
fossils,  far  less  conclusive,  and  almost  to  preclude  satisfactory 
comparison  or  close  correlation  with  the  strata  of  Hingham 
and  Nantasket.  It  is  a  reasonable  assumption,  however,  as 
will  be  shown  in  the  following  pages,  that  the  conglomerate, 
melaphyr,  and  newer  slate  are  the  general  equivalents  of  the 
corresponding  Hingham  terranes  ;  all  of  which  lends  additional 
emphasis  to  the  fact  that  in  this  new  field  the  older  slates  are 
the  feature  of  paramount  interest.  Perhaps  the  most  puzzling 
and  difficult  determination  which  the  Blue  Hills  area  presents 
is  the  distinction  of  the  older  and  newer  slates.  The  wide- 
spread and  almost  continuous  drift  deposits  make  it  practically 
impossible  to  locate  the  contacts  or  to  prove  by  direct  evidence 
the  unconformity  that  is  believed  to  exist  between  them  ;  and 
our  conclusions  are  of  necessity  based  very  largely  upon  litho- 
loofical  contrasts. 


293 

For  important  aid  in  the  prosecution  and  publication  of  this 
work  acknowledgments  are  due  and  gratefully  rendered  first 
of  all  to  Mr.  Thomas  A.  Watson  and  Dr.  Geo.  G.  Kennedy. 
At  my  suggestion,  Dr.  Theodore  G.  White,  of  Columbia 
University,  New  York,  has  kindly  undertaken  and  completed 
a  general  lithologic  study  of  the  igneous  rocks  (both  plutonic 
and  volcanic)  of  the  Blue  Hills  Complex.  His  work,  which 
corroborates  and  strengthens  my  own  conclusions  based  upon 
a  structural  study  of  these  rocks  has  been  published  in  full 
in  the  Proceedings  of  the  Society  (vol.  28,  p.  117-156).  Mr. 
Amadeus  W.  Grabau  has  worked  out  in  a  most  thorough  and 
painstaking  manner,  sparing  neither  time  nor  labor,  the  details 
of  Lake  Bouve,  a  glacial  lake  which,  at  the  close  of  the  Ice  Age, 
occupied  a  large  part  of  the  South  Shore.  He  has  also  kindly 
prepared  the  comprehensive  and  detailed  account  of  the  Cam- 
brian fauna  of  the  Boston  Basin.  Both  of  his  contributions 
form  separate  sections  under  his  name,  the  latter,  because  of 
its  length  and  its  relations  to  other  parts  of  the  Boston  Basin, 
appearing  as  an  appendix.  Assistance  in  field-work  has  also 
been  rendered  by  Messrs.  M.  L.  Fuller,  W.  S.  Rhodes,  and 
Benjamin  Hodge,  while  students  in  the  Massachusetts  Insti- 
tute of  Technology,  and  by  Miss  Hetty  O.  Ballard  in  both 
field-work  and  the  drawing  of  various  maps  and  sections. 
Miss  Elvira  Wood  has  also  rendered  valuable  assistance  in 
drawing. 

TOPOGRAPHY. 

The  chief  topographic  elements  to  be  noted  in  the  Blue  Hills 
Complex  are  the  elevated  preglacial  peneplain,  the  main  range 
of  the  Blue  Hills  rising  above  it,  the  intricate  system  of  valleys 
by  which  it  is  divided,  and  the  drift  deposits — drumlins,  plains, 
and  eskers  —  which  mask  and  obscure  it.  The  peneplain  is  the 
westward  extension  of  that  which  we  have  traced  (Parts  I  and 
II)  from  the  shore  of  Massachusetts  Bay,  with  gradually  increas- 


294 

ing  elevation,  across  Cohasset  and  Hingham.  Constituting  as 
this  does  the  chief  topographic  expression  of  the  entire  coastal 
area  of  Massachusetts  and  New  England,  it  may  be  con- 
veniently designated  the  coastal  peneplain.  It  is  not  the 
smoothness  of  the  interstream  surfaces,  for  that  may  be  wholly 
wanting,  but  the  a.pproximate  uniformity  of  the  rock  elevations, 
that  proves  the  peneplain  ;  and  this  we  recognize  in  the  level 
horizon  lines  all  over  the  coastal  area,  where  the  drift  hills  or 
drumlins  are  not  too  thickly  planted.  The  general  map 
(PI.  13)  locates  the  summits  and  gives  the  approximate  heights 
of  all  the  principal  or  more  distinct  rock  hills  ;  and  it  can  be 
seen  at  a  glance  that,  except  for  the  main  range  of  the  Blue 
Hills,  the  elevations  vary  chiefly  from  100  to  160  feet.  But  it 
is  obvious  that  the  maximum  elevation  represented  by  a  goodly 
number  of  points  probably  corresponds  most  closely  to  the 
normal  surface  of  the  peneplain  ;  and  hence  we  may  conclude 
that  the  normal  height  of  the  dissected  coastal  peneplain  or 
ancient  base  level  in  this  region  is  now  not  less  than  150  feet, 
though  rapidly  diminishing  near  the  shore. 

In  a  study  of  the  topography  of  the  Blue  Hills,  the  most 
important  distinction  to  be  recognized  is  that  between  the 
coastal  peneplain  and  the  main  range  rising  above  it.  The 
geological  significance  of  this  distinction  will  be  fully  discussed 
in  a  later  section,  but  it  may  be  noted  in  passing  that  the  main 
range  is  a  widely  isolated  and  degraded  remnant  of  an  older 
peneplain,  to  be  designated  hereafter  the  submountainous  or 
piedmont  peneplain,  and  that  it  owes  its  present  superior  eleva- 
tion to  a  lithological  rather  than  a  structural  contrast ;  for  the 
peneplain  portion  of  the  hills  presents  a  surface  of  distinctly 
and  coarsely  gi^anular  granite,  while  all  the  summits  of  the 
main  range  are  capped,  or  recently  have  been,  with  compact 
and  intensely  resistant  quartz  porphyry.  The  main  range  is 
an  irregular  and  somewhat  curving  linear  group  of  hills,  gradu- 
ally declining  in  height  eastward,  rather  than  a  well-defined 
ridge.     It    embraces    all    the  summits  exceeding^  200  feet    in 


height  between  Great  Blue  Hill  and  Rattlesnake  Hill.  Beyond 
Rattlesnake  Hill  the  range  is  continued  northeasterly  by  the 
higher  quarry  hills  of  West  Quincy  and  eastward  by  Pine  Hill, 
Payne  Hill,  Eldridge  Hill,  and  Wyman  Hill  to  Weymouth 
Fore  River.  All  the  summits  east  of  the  Granite  Branch  R.  R. 
may  be  referred  to  the  coastal  peneplain,  and  also  the  entire 
northern  range  or  belt  of  the  hills,  north  of  the  east-west  valley 
or  intervale  of  Pine  Tree  Brook.  Except  to  a  limited  extent 
south  of  the  Blue  Hills,  in  the  Norfolk  Basin,  the  sedimentary 
rocks  rarely  rise  to  the  full  height  of  the  coastal  peneplain,  but 
this  plain  now  finds  expression  almost  exclusively  in  the  gra- 
nitic rocks. 

The  exceptional  elevation  and  semi-mountainous  character 
of  the  main  range  of  the  Blue  Hills  make  this  easily  the  first 
topographic  feature  of  eastern  Massachusetts ;  and  it  might 
well  receive  here  a  more  detailed  and  elaborate  description. 
But,  fortunately,  the  necessity  for  this  is,  in  a  measure,  obvi- 
ated by  the  very  complete  and  perfect  topographic  map  of  the 
Blue  Hills  Reservation  (PI.  14),  which  shows  in  all  desirable 
detail  the  entire  range  from  Great  Blue  to  Rattlesnake  Hill 
and  Willard  Street,  and  will  repay  careful  study.  The  contour 
lines  express  clearly  the  rounded  or  dome-shaped  forms  of  the 
hills,  and  also,  in  connection  with  the  shading  for  the  ledges, 
the  roche  moutonnee  profiles  or  stoss  and  lee  slopes  due  to 
glaciation.  This  is  especially  well  shown  in  Great  Blue, 
Tucker,  Buck,  Chickatawbut,  and  Catamount  Hills,  but  is 
more  or  less  marked  in  nearly  all.  These  roche  moutonnee 
profiles  are  due  in  part  to  drift  banked  upon  the  stoss  slopes, 
but  in  all  cases  the  summits  are  bare  ledges,  the  accumulations 
of  drift  never  assuming  a  distinctly  drumloid  character.  Some 
of  the  cols  or  passes  between  the  hills  are  deep  and  narrow, 
veritable  notches,  recalling  White  Mountain  scenery,  and  tes- 
tifying, apparently,  to  the  inefficiency  of  glacial  erosion  on  this 
flinty  quartz  porphyry. 

A  glance  at  the  map  shows  that,  even  in  a  more  marked 


296 

degree  than  in  Hingham,  the  di'umlins  are  especially  character- 
istic of  the  lowlands,  occurring  chiefly  on  the  islands  and 
shores  of  Boston  Harbor  and  along  the  northern  base  of  the 
Blue  Hills.  The  map  exhibits  also  a  tendency  of  the  drunilins 
to  form  linear  groups  with  trends  normal  to  the  movement  of 
the  ice-sheet.  One  of  these  groups  embraces  the  drumlins  of 
North  Weymouth,  including  Eastward  Neck,  and  Gi'ape 
Island  ;  and  is  continued  in  Bumkin  Island  and  Strawberry 
Hill.  A  second  series  includes  Quincy  Great  Hill  on  Hough's 
Xeck,  Xut  Island,  and  the  four  drumlins  of  Peddock's  Island, 
and  is  continued  in  the  drumlins  of  Hull  and  Point  AUerton. 
Several  shorter  groups  may  be  detected  along  the  north  side  of 
the  Blue  Hills. 

Of  far  greater  topographic  value  than  the  drumlins  are  the 
sandplains  and  associated  eskers.  The  sandplains  are  chiefly 
deltas  formed  during  successive  stages  of  Lake  Bouve  and 
other  glacial  lakes  :  and  the  eskers  mark  the  courses  of  some 
of  the  glacial  streams  which  built  the  deltas.  South  of  the 
Blue  Hills  and  west  of  Randolph  Avenue,  the  plains  rise  to 
heights  of  160  and  200  feet,  being'  referable  to  a  glacial  lake 
in  the  upper  valley  of  the  Neponset  River.  But  elsewhere  in 
the  district,  from  the  Neponset  River  along  the  north  side  of 
the  Blue  Hills  and  over  Quincy,  Braintree,  and  Weymouth, 
the  plains  agree  closely  in  elevation  with  and  are  essentially 
continuous  with  those  of  Hingham,  and  may  conveniently  be 
designated  in  general  by  the  same  names.  Mr.  Grabau  has 
worked  out  the  following  classification  of  the  plains  of  Lake 
Bouve  :  Randolph  Plain,  170  feet ;  Liberty  and  Monatiquot 
Plains,  180  to  140  feet;  Whitman  Plain,  90  feet;  Glad  Tid- 
ings Plain,  60  to  70  feet;  Hingham  Plain,  50  feet;  Quincy 
Plain,  40  feet ;   Coastal  Plain,  20  to  30  feet. 

Randol[)h  Plain  lies  wholly  and  Monatiquot  Plain  lies  cliiefly 
beyond  the  southern  limit  of  the  maj) ;  but  the  latter  has  a 
notable  development  in  the  valley  of  Blue  Hill  Stream  and  in 
the  northwest  corner  of  Braintree,   between  Great  Pond   and 


297 

Pine  Hill.  The  full  development  of  these  and  the  other  plains 
of  the  South  Shore  is  shown  on  the  special  map  of  Lake 
Bouv6  (PI.  25 )r  Whitman  Plain  embraces  AVhitman  Pond;  ''''  j^^ 
and  jointly  to  this  level  and  Glad  Tidings  Plain  may  be  referred 
the  magnificent  series  of  eskers  along  the  w'cst  side  of  Wey- 
mouth Back  River.  The  beautiful  plateau  northeast  of  King 
Oak  Hill  in  Weymouth  is  correlated  with  the  Hingham  Plain. 
North  of  the  Blue  Hills,  in  West  Quinc}'  and  Milton,  the 
Glad  Tidings  level  is  well  developed  at  intervals,  and  it  forms 
the  floor  of  the  Neponset  Valley  from  Hyde  Park  southward 
beyond  the  limit  of  the  map.  Quincy  Plain  is  broadly  and 
typically  developed  in  North  Weymouth,  in  Quincy  south  of 
Town  River  Bay,  and  thence  westward  through  East  Milton 
to  the  valley  of  Pine  Tree  Brook.  To  the  coastal  plain,  pos- 
sibly of  marine  origin,  may  be  referred  Hough's  Neck  and  a 
large  part  of  North  Quincy. 

.  The  hydrography  of  the  Blue  Hills  Complex  is  far  more 
diversified  than  that  of  Hingham.  This  contrast  is  attribu- 
table in  part  to  the  greater  area  and  the  influence  of  the  Blue 
Hills,  but  mainly  to  the  greater  extent  and  continuity  of  the 
sandplains,  since  it  is  to  these  chieflv  that  we  owe  the  obstruc- 
tion of  the  drainage  and  the  formation  of  the  ponds.  Whit- 
man Pond  in  Weymouth  and  Little  Pond  in  Braintree  may 
thus  be  referred  to  Glad  Tidings  Plain,  while  the  Great  Pond 
of  each  town  belongs  to  Liberty  Plain,  and  Hoosickwhissick 
Pond  to  the  200  foot  plain  of  the  Neponset  Basin.  The  prac- 
tical absence  of  natural  ponds,  other  than  small  kettle  ponds, 
in  the  lower  plains  is  characteristic  of  the  entire  southern  mar- 
gin of  the  Boston  Basin  ;  and  the  most  natural  explanation 
appears  to  be  that  the  greater  volume  and  erosive  power  of 
the  streams  in  their  lower  courses  enabled  them  to  cut  down 
the  drift  barriers  and  drain  areas  that  might  otherwise  have 
been  flooded. 

The  streams  evidently  follow  quite  faithfully  for  the  most 
part  the  preglacial  and  early  glacial  rock  valleys  ;   but  several 


298 

instances  of  diverted  drainage  have  been  recognized  which  will 
be  described  in  the  section  on  the  surface  geology.  Wey- 
mouth Back  River  and  Weymouth  Fore  River  are  conspicuous 
examples  of  drowned  valleys  or  fiords,  which  are  prevented 
from  silting  up  by  the  efficient  scour  of  the  tides.  The  shores 
of  Weymouth,  Braintree,  and  Quincy  present  no  phenomena 
of  erosion  or  deposition  essentially  different  from  what  have 
been  noted  for  Hino-ham. 


MAPS. 

The  second  sheet  of  the  general  map  is  constructed  on  essen- 
tially the  same  plan  as  the  first,  and  requires  no  special 
description.  It  is,  topographically,  a  compilation  of  the  best 
obtainable  data,  including  the  new  Coast  Survey  chart  of  Bos- 
ton Harbor,  the  latest  town  surveys,  the  State  map,  the  new 
map  of  the  Metropolitan  District,  and  the  recent  and  elaborate 
topographic  survey  of  the  Blue  Hills  Reservation.  The  details 
of  the  hydrography,  including  the  streams,  ponds,  and  especially 
the  swamps,  are  based  to  a  considerable  extent  upon  original 
observations.  Among  the  special  maps  may  be  noted  par- 
ticularly the  map  of  Lake  Bouve,  based  upon  the  State  map, 
without  the  contour  lines,  which  would  have  interfered  in  some 
cases  with  the  geologic  representation,  and  the  topographic 
map  of  the  Blue  Hills  Reservation.  The  latter  has  been 
redrawn  from  the  original  sheets  on  a  scale  of  y^ViT'  with  5  foot 
contour  intervals,  and  reduced  to  x^ixrTy^  with  10  foot  contour 
intervals.  For  permission  to  use  this  valuable  work,  1  am 
indebted  to  the  courtesy  of  the  Metropolitan  Park  Commission, 
and  the  Commission  has  also  shared  the  expense  of  the  reduc- 
tion from  the  original  sheets. 


299 
THE   CAMBRIAN   STRATA. 

DISTRIBUTION. 

In  consequence  of  the  relative  softness  of  the  sedimentary 
rocks  of  the  Complex  and  their  consequent  topographic  inferior- 
ity to  the  granitic  I'ocks,  they  are  concealed  by  an  almost  con- 
tinuous mantle  of  drift  ;  and  therefore  their  distribution  as 
shown  on  the  general  map  (PI.  13)  is  necessarily  somewhat 
hypothetical  and  incomplete.  Every  area  indicated  is  based, 
in  part  at  least,  upon  actual  outcrops,  but  the  lack  of  detail  in 
the  boundaries  testifies  to  the  infrequency  of  the  outcrops  ;  and 
more  especially  must  we  suppose  that  some  of  the  smaller  areas 
have  wholly  escaped  observation  and  are  unrepresented. 
Where  the  boundaries  are  not  defined  by  outcrops,  they  must 
of  necessity,  if  they  are  to  be  represented  at  all,  reflect  the 
writer's  oreneral  views  of  the  sfeolog'ical  structure  of  the  regfion. 
This  is  seen  more  particularly  in  the  fault  lines  bounding  the 
Cambrian  strata  along  the  north  and  south  sides  of  the  Com- 
plex east  of  the  Blue  Hills,  but  also  in  the  minor  fault  bounda- 
ries, both  longitudinal  and  transverse.  The  most  important 
feature  of  the  Cambrian  strata  expressed  by  the  map  is  their 
occurrence  in  approximately  parallel  east-west  belts.  The  two 
principal  belts  are,  however,  distinctly  converging  eastward. 
These  form  respectively  the  north  and  south  boundaries  of  the 
eastern  half  of  the  Complex.  The  northern  boundary  belt 
traverses  the  drift  plain  of  North  Weymouth  and  Quincy  Point 
and  is  here  necessarily  largely  hypothetical.  It  embraces, 
however,  east  of  the  main  line  of  the  Old  Colony  Railroad,  the 
fine  section  of  red  calcareous  slate  and  inclosed  limestone  on 
the  north  side  of  Mill  Cove,  Weymouth  (Pl.-ifi),  and  the 
essentially  similar  outcrops  in  the  vicinity  of  Washington, 
Union,  and  Main  Streets  in  Quincy.  West  of  the  railroad  it 
is  continued  in  the  outcrops  of  massive  gray  slate  on  the  north- 


?/, 


300 

ern  slopes  of  President's  Hill  and  North  Common  Hill  (PL 
17),  and,  beyond  the  Granite  Branch  Raih'oad  (Pl.^=^,  in  -<'. 
the  ledges  of  the  same  character  in  the  valley  of  Furnace 
Brook  and  along  Pleasant  Street  to  within  half  a  mile  of 
Randolph  Avenue.  In  its  western  half,  or  west  of  the  main 
line  of  the  Old  Colony  Railroad,  the  breadth  of  this  belt  is 
determined  with  approximate  accuracy  by  the  outcrops  of 
granite  on  the  south  and  conglomerate  on  the  north.  But 
east  of  the  railroad  the  breadth  of  the  belt  and  the  character  of 
its  northern  boundary  are  wholly  conjectural.  North  of  the 
line  as  drawn,  the  nearest  outcrop  of  any  kind  is  the  slate  on 
the  north  shore  of  Weymouth,  between  Rowe's  Hill  and 
Great  Hill,  and  this  is  referred  on  lithological  grounds  to 
the  newer  or  post-Cambrian  slates.  The  great  fault  I'epre- 
sented  as  bounding  the  Complex  on  the  north  and  sharply 
limiting  the  breadth  of  this  Cambrian  belt  will  appear  later 
to  be  a  structural  necessity.  West  of  the  railroad  its  posi- 
tion is  determined  by  the  common  boundary  of  the  slate 
and  conglomerate,  while  east  of  the  railroad  it  has  simply  been 
drawn  so  as  to  give  the  slate  a  nearly  uniform  breadth.  This 
great  displacement  would,  obviously,  follow  the  slate  belt 
closely,  since  the  vertical  strata  of  the  latter  afford  a  much 
easier  path  for  it  than  the  relatively  unyielding  granite. 

The  southern  boundary  belt  is  bordered  by  the  granitic 
rocks  on  both  sides  and  the  outer  or  southern  border  is  a  very 
obvious  fault  line,  as  shown  by  the  cuts  on  the  South  Shore 
Railroad  at  Weymouth  Station  and  both  east  and  Avest  of 
Weymouth  Highlands.  This  belt  is  most  clearly  and  satis- 
factorily defined  midway  of  its  length,  in  the  valley  of  Monati- 
quot  River,  where  it  is,  perhaps,  narrowest  (see  special  map, 
/^  PI.  :i^).  Between  East  Braintree  Mill  Pond  and  the  main 
line  of  the  Old  Colony  Railroad  there  are  no  outcrops  of  any 
kind,  but  fortunately  several  wells  in  this  valley  have  pene- 
trated the  slates.  West  of  the  railroad,  on  Monatiquot  Heights, 
it  is  i)roved  by  wells  and  street  excavations  for  a  breadth  of 


301 

fully  one  fourth  of  a  mile.  Farther  west  the  slate  outcrops 
on  the  shores  of  Quincy  Reservoir  and  beyond  that  it  probably 
passes  beneath  the  newer  sediments  of  the  Norfolk  Basin.  In 
its  eastern  half  this  belt  is  quite  clearly  broken  by  several  com- 
plementary faults,  the  outcrops  occurring  along  the  railroad, 
between  Weymouth  and  Weymouth  Highland  Stations,  on 
the  north  side  of  Commercial  Street  west  of  King  Oak  Hill, 
and  on  the  railroad  east  of  the  hill.  Between  the  railroad  and 
Weymouth  Back  River  the  modified  drift  obscures  everything, 
and  the  areal  geology  is  almost  wholly  a  matter  of  conjecture. 

Between  these  two  main  belts  of  Cambrian  strata  are  three 
minor  but  very  interesting  east-west  belts  in  the  valleys  of 
Hay  wards  and  Ruggles  Creeks,  and  in  the  Pine  Hill  tract. 
The  only  other  known  area  large  enough  to  be  mapped  is  the 
east-west  belt  two  and  a  half  miles  long  and  nearly  half  a  mile 
in  greatest  width  in  the  upper  valley  of  Pine  Tree  Brook. 
Its  erosion  has  determined  this  depression  between  the  southern 
or  felsitic  rano'e  and  the  northern  or  granitic  rano^e  of  the  Blue 
Hills.  East  of  Randolph  Avenue  it  is  well  defined  by  frequent 
outcrops  both  of  gi'anite  and  slate  (special  map,  PI.  'tB-)  ;  but 
west  of  the  avenue  it  is  indicated  mainly  by  the  topography 
and  the  fragments  of  slate  in  the  drift. 

If  Cambrian  belts  of  sensible  area  remain  to  be  discovered, 
they  must  be  small,  and  are  most  likely  to  exist  under  the 
modified  drift  of  North  Weymouth,  between  the  northern  and 
southei-n  boundary  belts,  and  in  the  upper  valley  of  Purgatory 
Brook,  south  of  Pine  Hill,  in  Braintree.  A  belt  of  slate 
underlying  Purgatory  Brook  might  very  well  extend  westward 
beneath  the  broad  sheet  of  effusive  felsite  between  Rattlesnake 
Hill  and  Braintree  Great  Pond. 


302 


NORMAL    LITHOLOGIC    CHARACTER    AND    PROBABLE    CONDI- 
TION OF  DEPOSITION. 

Through  the  influence  of  the  granitic  eruptions  the  Cambrian 
strata  have  been,  locally,  profoundly  modified  in  various  ways 
which  will  be  fully  described  in  a  later  section  on  the  contact 
phenomena  of  the  granitic  rocks.  The  deformation  (folding 
and  especially  faulting)  of  the  Cambrian  sediments  has  also 
led  to  the  local  development  of  important  secondary  lithologic 
and  petrologic  characters,  which  will  be  more  particularly 
noticed  hereafter.  But  it  is  desired  to  direct  attention  here 
only  to  the  normal  original  features  of  the  Cambrian  strata, 
or  such  characters  as  exist  independently  of  the  localized  acci- 
dents of  their  history. 

The  prevailing  and  almost  the  only  rock  is  slate.  Near  the 
granite  the  slate  is  largely  of  a  decidedly  metamorphic  char- 
acter,— very  massive,  hard,  and  semi-crystalline  ;  but  elsewhere 
it  is  soft  and  extremely  fine  or  impalpable,  a  typical  slate, 
with  rarely  any  suggestion  of  a  visibly  granular  or  arenaceous 
structure;  and  coarser  fragmental  rocks,  — sandstone,  quartz- 
ite,  etc., — are  wholly  wanting.  The  slate  varies  in  color  from 
grayish  green  to  greenish  gray,  dark  gray,  and  black,  and  also 
from  a  dark  reddish  brown  or  chocolate  color  to  reddish  black 
and  black.  As  usual,  the  greens  and  reds  are  more  or  less 
intimately  associated,  the  green  slates  being  often  banded  or 
blotched  with  red  or  brown,  and  vice  versa.  Apart  from  the 
color  distinctions,  whicii  depend  upon  varying  proportions  of 
carbon  and  different  degrees  of  oxidation  of  the  iron,  the  slates 
are  normally  very  homogeneous  and  indistinctly  stratified. 
Near  the  original  igneous  contacts  with  the  granite  they  are 
usually  extremely  massive,  and  in  the  proximity  of  important 
faults  they  are  sometimes,  as  in  the  railroad  cuts  in  Wey- 
mouth, abnormally  fissile.  But  elsewhere,  or  normally,  the 
slate  exhibits  a  more  or  less  distinct  cleavage,  which  appears, 


303 

in  general,  to  coincide  with  the  bedding,  though  it  is  often  dif- 
ficult or  well  nigh  impossible  to  make  out  any  indubitable  indi- 
cations of  the  stratification  ;  and  not  infrequently  the  cleavage 
cuts  the  bedding  at  very  oblique  angles.  The  black,  gray,  and 
greenish  slates  are  usually  slightly  pyritiferous  ;  and  the  red- 
dish brown  or  distinctly  ferruginous  slates,  especially,  are  com- 
monly somewhat  calcareous.  The  carbonate  of  lime  is  not 
now  evenly  disseminated,  but  the  distinctly  calcareous  portions 
of  the  slate  have  the  form  of  flattened  or  lenticular  concretions 
which  are  not  distributed  wholly  at  random,  but  tend  to  char- 
acterize occasional  definite  bedding  planes,  the  successive 
lenses  or  concretions,  which  must  have  been  originally  compar- 
able with  the  clay  stones  of  modern  clays,  are  often  more  or 
less  confluent,  tending  to  form  continuous  layers  of  calca- 
reous slate.  From  these  we  pass  almost  insensibly  to  thin 
beds  of  white  crystalline  limestone,  from  two  inches  to  two  feet 
or  more  in  thickness,  as-at  Mill  Cove.  In  the  neighborhood 
of  the  granite,  the  calcareous  concretions  and  layers,  and  the 
limestones  in  direct  proportion  to  their  earthly  impurities, 
have  been  profoundly  metamorphosed.  The  chief  secondary 
mineral  is  epidote  ;  and  this  epidotic  alteration  is,  perhaps, 
the  most  interesting  phase  of  the  contact  phenomena  of  the 
granite. 

The  impalpable  fineness  of  the  Cambrian  slates  of  the  Blue 
Hills  Complex,  their  calcareous  character,  and  the  entire 
absence  of  coarser  sediments,  are  plain  indications  that  they 
were  deposited  in  deep  quiet  water  remote  from  the  shore. 
From  this  inference  the  conclusion  naturally  follows  that  they 
are  a  mere  remnant,  isolated  by  plutonic  intrusions  and  erosion, 
of  a  widespread  series  of  Cambrian  strata.  It  may  be  noted, 
however,  that  in  the  numerous  areas  of  sedimentary  rocks  sup- 
posed to  be  of  Cambrian  age  north  and  W^est  of  the  Boston 
Basin,  quartzite  is  a  leading  lithologic  type,  indicating,  pos- 
sibly, that  the  shore  of  the  Cambrian  sea  lay  in  that  direction  ; 
while  to  the  southward,  in  Attleboro,  the  well-identified  Cam- 


304 

brian  strata  outcropping  from  beneath  the  Carboniferous  series 
are  distinctively  argiHaceous  and  calcareous  like  those  of  the 
Blue  Hills  Complex. 


COERELATION  AND  GENERAL  STRATIGRAPHY. 

The  Blue  Hills  Complex  embraces,  fortunately,  the  original 
locality  of  Cambrian  fossils  in  eastern  Massachusetts,  viz.,  the 
well-known  Paradoxides  Quarry  on  the  south  bank  of  Hay- 
ward's  Creek,  in  Braintree  :  and  the  Middle  Cambrian  age  of 
the  older  slates  may  be  regarded  as  well  determined  at  this 
point.  Through  the  courtesy  of  Mr.  T.  A.  Watson,  I  am 
able  to  announce  that  he  has  found  the  thorax  of  a  large 
specimen  o^ Paradoxides  liarlani  a  mile  southeast  of  the  Para- 
doxides Quarry,  in  the  prominent  ledge  of  massive  gray  slate 
banded  with  red  on  his  land  on  the  northeast  bank  of  Wey- 
mouth Fore  River,  in  Braintree.  We  are  also  indebted  to  the 
disinterested  zeal  of  Mr.  Watson  for  bringing  to  light  a  very 
similar  example  of  Paradoxides  harlani  found  about  twenty 
years  ago  by  Mr.  x4.1bert  Hobart,  in  the  thinly  laminated  green 
slate  with  occasional  streaks  of  red,  while  excavating  a  cistern 
at  his  house  on  Commercial  Street,  East  Braintree,  near  the 
corner  of  Liberty  Street,  and  a  little  more  than  a  mile  south  of 
the  quarry  on  Hayward's  Creek.  This  occurrence  is  entirely 
authentic  and  trustworthy,  and,  moreover,  it  is  fully  substanti- 
ated by  the  character  of  the  rock,  which  is  identical  with  that 
in  the  neighboring  ledges,  and  entirely  unlike  the  slate  of 
Mr.  Watson's  ledge  or  Hayward's  Creek.  The  specimens 
from  both  of  these  newly  discovered  localities  have  been  pre- 
sented by  Mr.  Watson  to  the  nuiseum  of  the  Boston  Society  of 
Natural  History.  In  view  of  this  additional  paleontologic  evi- 
dence, and  the  essentially  homogeneous  character  of  the  slates 
themselves,  where  unaltered  by  the  granitic  rocks,  we  may, 
perhaps,  fairly  assume  that  the  gray  and  green  slates  of  the 


305 

Blue  Hills  Complex  belong  mainly,  if  not  wholly,  to  the  Mid- 
dle Cambrian  or  Paradoxides  zone. 

The  Lower  Cambrian  or  Olenellus  zone  has  been  well  estab- 
lished by  Shaler  at  North  Attleboro,  and  by  Foerste  at  East 
Point,  Nahant.  The  Cambrian  beds  of  North  Attleboro  con- 
sist of  red  and  greenish  shales  with  occasional  bands  of  sand- 
stone and  very  limited  layers  and  patches  of  impure  red 
limestone,  in  which  the  fossils  occur.  Mr.  T.  A.  Watson  has 
found  on  Pleasant  Beach,  Cohasset,  several  small  water-worn 
bowlders  and  pebbles  of  precisely  similar  limestone  ;  and  more 
recently  Dr.  O.  H.  Howe  of  Cohasset  has  found  another  near 
the  Black  Rock  House  on  Jo'usalem  Road.  These  contain 
numerous  fossils,  which  Walcott^  has  identified  as  the  two 
Lower  Cambrian  types  Strajxirollina  remota  Billings  and 
HyoUthes  conimtinis  Billings.  These  specimens  are  found  at 
points  favorable  to  their  having  been  carried  by  the  ice  sheet 
from  the  Lower  Cambrian  outcrops  at  Nahant ;  and  although 
no  rock  of  similar  character  has  been  observed  in  place  on 
Nahant,  Mr.  Watson  has  made  its  occurrence  there  extremely 
probable  by  subsequently  finding  on  the  beach  at  Bass  Point, 
and  arovmd  the  western  end  of  Nahant,  a  number  of  larger  and 
more  angular  bowlders  and  several  water-worn  fragments,  all 
of  the  same  red  limestone  and  holding  the  same  species.  The 
well-known  beds  of  white  limestone,  on  East  Point,  may  have 
been  red  before  they  were  metamorphosed  by  the  great  dikes 
of  diabase.  At  any  rate,  red  beds  may  be  regarded  as  some- 
what characteristic  of  the  Lower  Cambrian  in  this  region. 
Foerste^  was  led  by  this  and  other  circumstances  to  suggest 
that  the  extensive  outcrop  or  series  of  outcrops  of  red  slate  on 
the  north  side  of  Mill  Cove,  in  Weymouth,  is  pi'obably  Lower 
Cambrian.  About  700  feet  of  strata  are  exposed,  dij^ping  at 
a  high  angle  to  the  south  or  toward  the  granite.     They  enclose 

iProc.  Biological  society  Washington,  vol.  7,  p.  155. 
2Proc.  Boston  society  natural  history,  vol.  24,  p.  262. 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  20. 


306 

numerous  small  concretionaTy  masses  of  limestone,  and  two 
beds  of  white  limestone  from  1  to  2  feet  thick.  The  calcareous 
matter,  as  already  noted,  has  been  largely  altered  to  epidote 
and  other  secondary  silicates  ;  and  the  absence  of  fossils  is, 
therefore,  not  surprising  ;  but  there  seems  no  reason  to  doubt 
that  the  Mill  Cove  beds  are  synchronous  with  the  Lower  Cam- 
brian beds  of  North  Attleboro,  and  underlie  the  Paradoxides 
or  Middle  Cambrian  beds  of  the  Blue  H'ffls  area.  In  the  vicin- 
ity of  Union  Street,  south  of  Washington  Street,  in  Quincy, 
about  half  a  mile  east  of  the  Quincy  railroad  station,  is  a  series 
of  outcrops  of  slate,  in  part  of  a  reddish  color,  inclosing  several 
thin  layers  of  limestone,  and  cut  and  metamorphosed  by  the 
fine  granite.  These  outcrops  .resemble  the  Mill  Cove  rocks 
very  closely,  and  may  also  be  provisionally  referred  to  the 
Lower  Cambrian  series,  especially  as  no  calcareous  beds  have 
been  observed  in  the  undoubted  Middle  Cambrian  of  this 
region.  The  prevailing  dip  of  the  Cambrian  beds  of  the  Blue 
Hills  area  is  southerly  at  very  high  angles  (usually  70°— 90°), 
and  the  beds  here  referred  to  the  Lower  Cambrian  are  thus 
seen  to  be  in  the  right  position  to  underlie  the  Middle  Cam- 
brian ;  and  there  are  no  known  facts  inconsistent  with  the  view, 
that  the  tvv^o  series  are  essentially  conformable.  The  only 
important  rock  referred  to  the  Cambrian  series  of  eastern 
Massachusetts  which  is  not  represented  in  the  Blue  Hills  area 
is  the  quartzite,  which  is  so  extensively  developed  in  the  region 
north  and  west  of  the  Boston  Basin.  The  fact  that  hard  sand- 
stone layers  occur  in  the  Lower  Cambrian  series  of  North 
Attleboro  suggests  the  reference  of  the  quartzites  to  that  hori- 
zon, together  with  the  limestones  of  Stoneham,  Newbury,  etc. 
If  the  quartzites  be  referred,  as  would  seem  reasonable,  to  the 
lower  part  of  the  Olenellus  zone,  their  non-occurrence  in  the 
Blue  Hills  Complex  might  be  explained  partly  by  the  greater 
distance  from  the  Cambrian  shore-line,  and  partly  by  the 
incompleteness  of  the  section  as  developed  in  the  existing  out- 
crops.    North  and  west  of  the  Boston   Basin,   the  prevailing 


307 

dip  of  the  Cambrian  beds,  as  at  Nahant,  is  northwesterly,  thus 
indicating  that  tlie  position  of  the  Boston  Basin  is  anticlinal 
with  reference  to  the  Cambrian  strata. 

The  data,  and  especially  the  palaeontologic  data,  are  still 
inadequate  foi'  a  definite  determination  of  the  general  succes- 
sion of  the  Cambrian  strata  of  the  Blue  Hills  Complex  ;  but  the 
following  sequence  seems  to  accord  best  with  the  facts  now  in 
hand,  and  may  be  stated  provisionally  :  — 

Lower  Cambrian  (Olenellus  zone)  :  — 

1.  Siliceous  series  (quartzite,  siliceous  slates,  etc.), 
not  exjDOsed. 

2.  Argillaceous  and  calcareous  series.  Keddish  brown 
to  reddish  and  brownish  black  slates,  often  banded  or 
interstratified  with  green  slates  and  characterized  through- 
out by  calcareous  segregations  (lenticular  concretions  or 
clay  stones)  and  more  or  less  continuous  calcareous  layers, 
Avhich  pass  upward  into  the  thin  beds  of  white  crystalline 
limestone. 

Middle  Cambrian  (Paradoxides  zone)  :  — 

1.  Greenish  and  greenish  gray  slates,  holding  the 
Paradoxides  fauna  and  often  banded  or  interstratified  with 
reddish  brown  slate,  and  changing  gradually  upward  to 

2.  Dark  gray  to  black  (carbonaceous)  slates  which 
have  a  great  thickness,  but  have  as  yet  afibi'ded  no  fossils. 

The  Middle  Cambrian  beds,  both  the  light  and  dark  series, 
are  frequently  pyritiferous ;  and  they  are  also  occasionally 
slightly  calcareous.  The  succession  is  believed  to  be  conform- 
able throughout,  and  the  Lower  Cambrian  red  beds  appear  to 
pass  gradually  upwai'd  into  the  Middle  Cambrian  green  beds. 
The  Lower  Cambrian  beds  appear  to  be  restricted  to  the  east- 
ern half  of  the  northern  boundary  belt  (including  the  Mill 
Cove  area),  the  Ruggles  Creek  belt,  and  to  Eldredge's  Hill  on 
the  northern  half  of  the  Hay  ward's  Creek  belt ;  while  the  west- 
ern half  of  the  northern  boundary  belt,  the  Pine  Tree  Brook 
belt,  the  southern  half  of  the  Hay  ward's  Creek  belt  (including 


308 

the  Paradoxides  quarry)  and  the  entire  southern  boundary 
belt  from  Monatiquot  Heights  to  King  Oak  Hill  are  Middle 
Cambrian. 

No  recognizable  trace  of  the  floor  upon  which  the  Cambrian 
strata  were  deposited  has  been  discovered  in  the  Blue  Hills 
area.  Whether  Archaean  or  Algonkian  rocks  underlie  the 
Boston  Basin  and  the  Blue  Hills  and  are  still  intact  at  a  great 
depth,  or  have  been  softened  and  melted  to  form  the  granitic 
series  which  now  everywhere  intersects  and  incloses  the  Cam- 
brian strata,  we  can  only  conjecture.  The  latter  view  would 
parallel  the  well-determined  occurrences  of  similar  rocks  in 
other  regions. 

The  most  satisfactory  conclusion,  then,  appears  to  be  that 
where  the  Blue  Hills  now  stand  was  an  off-shore  ai'ca  in  the 
Cambrian  sea,  and  that  over  this  area  were  deposited  v/ithout 
marked  unconformity  the  quartzites,  slates,  and  limestones  of 
the  Lower  Cambrian  and  the  great  slate  series  of  the  Middle 
Cambrian,  the  upper  part  only  of  the  Lower  Cambrian  being 
now  exposed  in  this  area.  The  highly  disturbed  condition  of 
the  Cambrian  beds,  and  their  intricate  relations  to  the  great 
masses  of  plutonic  rocks,  make  a  safe  estimate  of  the  thick- 
ness of  these  terranes  practically  impossible.  But  it  is  prob- 
ably very  greats  thousands  of  feet,  if  not  miles  ;  and  this 
assumption  is  unavoidable  if  we  suppose  that  the  floor  upon 
which  they  were  deposited  has  been  softened  and  extravasated. 
The  formation  of  these  great  banks  of  strata  involved,  of 
course,  enormous  waste  of  Algonkian  and  Archaean  rocks, 
and. the  contact,  if  it  still  exists,  must  be  unconformable;  but 
it  appears  impossible,  with  the  data  now  at  our  command,  to 
throw  any  stronger  or  clearer  liglit  upon  this  primary  problctn 
in  the  stratigraphy  of  the  Blue  Hills  Complex. 

The  pebbles  of  quartzite  holding  Lingula  reported  by  Pro- 
fessor W.   B.   Rogers  ^  in  the  Carboniferous  congk)mcrate  at 

'  Proc.  Boston  society  natural  history,  vol.  7,  p.  389-301. 


309 

Fall  River  and  Newport,  and  the  quartzite  fragments  showing 
typical  Scolithus  observed  by  the  present  writer  (not  hitherto 
published)  in  the  Carboniferous  conglomerate  on  Easton's 
Neck  near  Newport,  Rhode  Island,  are  best  explained  by 
supposing  that  Upper  Cambrian  or  Potsdam  strata  exist  or 
have  foi'merly  existed  somewhere  in  this  region.  If  recogniza- 
ble traces  of  this  terrane  still  remain,  they  may  be  wholly  con- 
cealed by  drift  deposits  or  even  by  Carboniferous  beds  derived 
in  part  from  their  waste.  Of  course  we  may  allow  this  scanty 
though  positive  evidence  its  full  weight  and  still  doubt  whether 
the  Potsdam  beds  once  covered  the  Blue  Hills  area ;  although 
the  arenaceous  character  of  the  Potsdam  sediments  and  the 
fact  that  they  rest  at  many  points  directly  upon  Algonkian  and 
Archaean  formations  indicate,  not  a  recession  of  the  sea,  but 
rather  that  it  had  advanced  on  the  land  and  was  actively  wearing 
away  the  crystalline  pre-Cambrian  rocks  which  we  suppose,  in 
eastern  Massachusetts,  to  have  lain,  as  now,  toward  the 
northwest.  The  main  point,  however,  and  the  only  one  upon 
which  we  can  insist,  is  that  we  have  some  evidence  that  deposi- 
tion continued  over  a  part  at  least  of  the  Cambrian  area  of 
southeastern  New  England  until  the  close  of  the  Upper  Cam- 
brian or  Potsdam  period. 

The  Potsdam  is  overlain  conformably  in  most  regions  by  the 
Ordovician  strata,  the  arenaceous  beds  giving  way,  through 
the  progressive  subsidence  of  the  sea-floor,  to  the  great  devel- 
opment of  calcareous  strata  and  shales  formed  during  the  suc- 
cessive periods  of  Ordovician  time.  These  strata,  so  far  as 
known,  are  wholly  unrepresented  in  southeastern  New  Eng- 
land ;  and  hence  we  might  infer  that  this  region  was  raised 
above  sea-level  at  the  close  of  Cainbrian  time,  instead  of  shar- 
ing-  in  the  Ordovician  subsidence.  To  this  conclusion  it  might 
be  added  that  although  this  movement  may  have  been  of  a 
massive  character,  matching  the  subsidence  in  Ordovician 
areas,  the  extreme  plication  of  our  Cambrian  strata,  even 
where  now   most  rigid,  is   a  slight  indication  that   they  were 


310 

mashed  up  at  an  early  period  in  their  history,  before  they  were 
thoroughly  lithified. 

In  view,  however,  of  the  fact  that  Ordovician,  Silurian,  and 
Devonian  strata  occur  in  western  New  England,  in  the  Con- 
necticut Valley  (Massachusetts  and  New  Hampshire),  in 
northern  and  eastern  Maine,  and  in  New  Brunswick  and  Nova 
Scotia  where  they  have  a  thickness  of  from  5,000  to  10,000 
feet  or  more  and  appear  to  rest  unconformably  upon  the 
highly  inclined  or  vertical  Cambrian  series,  the  best  conclusion 
seems  to  be  that,  following  the  deposition  and  disturbance  of 
the  Cambrian  strata  of  eastern  Massachusetts,  this  region 
probably  also  shai'ed  in  the  Ordovician  subsidence  and  was 
overspread  by  Ordovician,  Silurian,  and  Devonian  sediments, 
of  which  erosion  has  left  not  a  recoo-nizable  trace. 


311 


OEIGIN  OF    THE    GRANITIC   SERIES   AND 
DEVELOPMENT   OF   THE    COMPLEX. 

Although  it  is  impossible,  with  the  facts  now  at  oiu-  com- 
mand, to  trace  the  history  of  this  region  in  detail  through 
Ordovician,  Silurian,  and  Devonian  times,  we  know  that  the 
Cambrian  strata  were  strongly  folded  and  that  quite  certainly 
not  later  than  the  earlier  Devonian  epochs  they  were  invaded 
by  great  bodies  of  igneous  rocks.  These  include :  abyssal 
(plutonic)  types  (varying  phases  of  one  great  parent  batho- 
lite)  ;  intrusive  types  (dikes  and  sheets)  ;  and  effusive  (vol- 
canic) types  (sui-face  flows  of  lava).  The  plutonic  rocks  are 
prevailingly  granitic,  including,  however,  besides  various  types 
of  granite,  and  chiefly  as  contact  zones,  considerable  develop- 
ments of  diorite  and  quartz  porphyry.  The  intrusive  rocks 
include  diabase,  quartz  porphyry,  and  other  forms  of  aporhyolite 
(felsite),  as  well  as  innumerable  apophyses  of  granite,  while 
the  efl'usive  rocks  of  this  period  are  wholly  of  acid  character, — • 
quartz  porphyry,  compact  and  fluidal  felsites,  and  other  forms 
of  aporhyolite.^ 

With  the  exception  of  the  diabase,  these  igneous  types 
clearly  belong  to  one  genetically  related  or  consanguineous 
group,  and  since  the  granite,  which  embraces  several  distinct 
varieties,  largely  pi-edominates,  as  the  map  shows,  they  may 
be  collectively  designated  the  granitic  series.  The  diabase 
dikes,  although  clearly  older  than  the  granitic  series,  may  be 
most  conveniently  described  last. 

1  The  rocks  commonly  designated  as  felsites  are  devitrified  and  otherwise  altered 
forms  of  the  acid  voloanics  — rhyolite,  trachyte,  etc.;  and  Dr.  Florence  Bascom  has 
happily  proposed  (Bull.  U.  S.  geol.  surv.,  136,  p.  38)  to  call  them  aporhyolites,  etc. 
These  names  are  not  only  more  precise  and  imequivocal  than  those  they  are  de- 
signed to  replace,  but  they  have  also  the  great  merit  of  genetic  signification.  Here- 
after, in  this  work,  so  far  as  practicable,  or  whenever  the  composition  is  known 
with  sufficient  definiteness,  the  new  names  will  be  substituted  for  felsite,  except 
that  for  the  sake  of  brevity  the  good  descriptive  name  quartz  porphyry  —  a  variety 
of  aporhyolite  —  will  be  retained. 


312 

The  Cambrian  sediments  were  certainly  not  deposited  upon 
the  granitie  series,  by  which  they  are  now  intersected,  and  the 
transo-ressive  relation  of  the  o-ranitic  series  to  the  entire  Cam- 
brian  formation  of  this  region  is  clearly  seen  in  the  fact  that 
the  granite  is  found  in  igneous  contact  with  both  the  Lower 
and  Middle  Cambrian  strata.  The  fact  that  no  recognizable 
trace  of  the  ancient  floor  upon  which  the  Cambrian  strata  were 
deposited  has  been  discovered  in  the  region  of  the  Blue  Hills 
Complex  makes  it  necessary  to  suppose  either  that  the  igneous 
rocks  have  forced  their  way  up  through  this  floor,  accumulating 
above  it  in  the  Cambrian  strata,  or  that  the  pre-Cambrian 
Tocks,  as  well  as  a  great  volume  of  the  overlying  Cambrian 
sediments,  were  melted  and  absorbed  by  the  plutonic  magmas. 

According  to  the  first  view,  pre-Cambrian  rocks,  highly 
:inetamorphic  but  still  preserving  their  integrity,  may  be  sup- 
posed to  exist  at  the  present  time,  at  some  depth,  beneath  the 
granite  ;  while  the  second  view  compels  us  to  suppose  that  the 
entire  thickness  of  the  pre-Cambrian  solid  crust  was  fused  and 
the  identity  of  its  component  formations  irretrievably  lost. 
The  mechanical  difficulty  of  the  first  view  is  very  great,  since 
it  requires  us  to  provide,  in  the  Cambrian  strata  and  above  the 
solid  pre-Cambrian  floor,  a  continuous  horizontal  space  or 
cavity  for  the  reception  of  the  igneous  rocks  hundreds  of  square 
miles  in  area  and  of  vast  but  unknown  depth  ;  and  this  in  spite 
of  the  fact  that  the  Cambrian  beds  are,  throughout  the  entire 
region,  highly  inclined  or  vertical  and  thus  wholly  at  variance 
with  the  major  dimensions  of  the  massif.  The  chief  difficulty 
of  the  second  hypothesis  is  thermal — to  provide  the  heat 
required  to  melt  this  vast  area  and  thickness  of  the  sub- 
Cambrian  crust. 

The  relative  merits  of  these  rival  hypotheses  cannot  be  dis- 
cussed exhaustively  here.  In  the  opinion  of  the  writer,  the 
second  pi-esents  the  stronger  claims  to  acceptance  ;  and  the  dis- 
cussion of  the  origin  and  relations  of  the  granitic  series  which 
follows  is  based  upon  the  assumption  that  the  granite  is,  for 


313 

this  region,  now,  a  truly  fundamental  rock.  That  is,  it  is  not 
underlain  by  any  sedimentary  rocks,  nor  by  any  rocks  which 
have  once  held  a  more  superficial  position  in  the  crust  and  have 
escaped  fusion  during  their  subsequent  history. 

The  remnants  of  the  Cambrian  series  now  remaining  in 
eastern  Massachusetts  indicate  a  very  great  original  thickness, 
—  not  improbably  from  2  to  4  miles  ;  and  if  we  add  to  this 
thicknesses  of  Ordovician,  Silurian,  and  Devonian  strata  at  all 
commensurate  with  those  noted  in  the  Maritime  Provinces, 
it  is  obvious  that  the  total  volume  or  ag-orreofate  thickness  of 
the  pre-Cai"boniferous  formations  must  have  been  sufficient 
to  cause  a  notable  rise  of  the  isogeotherms  in  the  subjacent 
portion  of  the  crust  and  a  consequent  softening  of  the  crust 
at  all  levels  where  the  combined  influences  of  the  temperature, 
pressure,  and  degree  of  hydration  were  sufficient  to  overcome 
in  some  degree  the  fusing  point.  This  weakening  of  the  crust 
beneath  its  great  load  of  sediments  finally  led  to  its  collapse 
and  the  intense  plication  of  the  sti^ata  which  we  observe  to-day. 

The  deep  isoclinal  folding,  and  the  flow  of  the  rocks 
involved  in  the  development  of  slaty  cleavage,  led  to  an 
enormous  vertical  thickening  of  the  formations,  whereby, 
through  continued  rising  of  the  isogeotherms,  the  temperature, 
not  merely  of  softening,  but  of  aqueo-igneous  fusion,  invaded 
the  lower  part  of  the  water-soaked  or  hydrated  zone  of  the 
crust.  In  this  way  was  slowly  developed  at  a  great  depth  in 
the  earth  an  immense  body  of  molten  rock.  This  primary 
reservoir  of  magma,  it  will  be  noted,  was  the  result  mainly, 
not  of  an  uprising  of  molten  material  from  still  greater  depths, 
but  of  a  simple  invasion  of  the  hydrated  zone  by  the  abyssal 
heat.  It  is  probable,  however,  that  the  abyssal  heat  was  rein- 
forced in  an  important  measure  by  that  resulting  from  the 
transformation  of  mechanical  energy  during  the  mashing  up 
(folding  and  flowing)  of  the  strata  ;  and  this  auxiliary  heat,  it 
will  be  noted,  is  developed  largely  at  relatively  high  levels,  or 
where  it  is  most  needed  to  supplement  the  abyssal  heat. 


314 

Daring  the  gradual  conversion  of  a  heterogeneous  segment 
of  the  earth's  undercrust,  which  must  have  consisted  largely  of 
igneous  and  metamorphic  rocks,  to  an  incipient  batholite,  as 
the  great  body  of  molten  magma  may  be  called,  mutual  reac- 
tions of  the  parts  most  contrasted  in  composition,  —  free  silica 
and  free  bases,  alkaline  and  non-alkaline,  etc.,  —  would  pro- 
mote the  general  liquefaction  ;  and  through  the  slow  and  long- 
continued  process  the  principles  of  diffusion  and  convection  in 
liquids  would  tend  to  make  and  keep  the  magma  homogeneous 
and  give  it  a  somewhat  neutral  composition. 

Many  substances,  it  is  well  known,  fuse  at  temperatures 
below  their  normal  fusing  points  when  brought  in  contact  with 
fused  portions  of  more  fusible  substances.  The  fusion  (or 
solution)  of  gold  in  a  bath  of  molten  lead  is  a  familiar  instance. 
The  more  fusible  substance  appears,  in  general,  to  act  as  a 
solvent  for  the  less  fusible  substance.  This  corrosive  or  sol- 
vent'action  of  the  magma,  which  probably  obtains  in  some 
degree  in  all  cases  where  it  is  chemically  contrasted  with  the 
enclosing  rocks,  tends  to  give  the  batholite  sharp  and  definite 
boundaries.  The  only  rocks  in  this  region  known  to  be  older 
than  the  granitic  series,  with  the  exception  of  the  pre-Carbonif- 
erous  dikes  of  diabase,  are  the  Cambrian  slates  and  limestones. 
The  chemical  contrast  is  therefore  very  marked,  and  affords, 
perhaps,  a  sufficient  explanation  of  the  sharply  defined  contacts 
which  we  observe  at  all  points  between  the  igneous  and  sedi- 
mentary rocks  of  the  Complex.  At  no  point  has  any  appre- 
ciable blending  of  the  two  formations  been  observed,  such  as 
we  might  expect  between  rocks  of  like  nature  as  the  result  ot 
simple  fusion.  Any  movement  or  flowing  of  the  magma  rela- 
tively to  the  enclosing  rocks  would  operate  to  the  same  general 
end  of  giving  the  batholite  sharply  defined  boundaries.  The 
corrosive  action  of  fused  slags  on  a  clay  crucible,  the  material 
of  the  crucible  being  sensibly  unaltered  up  to  the  definite  surfiice 
beyond  wliich  it  has  been  completely  melted  or  dissolved  away, 
may  aid  us  in   explaining  or  understanding  the  almost  abso- 


315 

lutely  definite  boundaries  of  a  batliolite  developed  in  situ  by  a 
heat  invasion.  It  could  be  otherwise  only  when  the  magma 
and  the  enclosing  formation  are  throughout  of  closely  similar 
and  uniform  composition. 

Taking  a  nearer  view  of  the  problem  presented  by  the 
sharply  defined  boundaries  of  the  batholite,  it  may  be  noted 
that  the  important  and  unequal  changes  of  volume  in  the  vari- 
ous formations  affected  by  this  great  heat  invasion  must  have 
caused  extensive  rifting  and  Assuring  and  gravitational  read- 
justments in  the  still  unfused  cover  of  the  batholite  ;  and  the 
molten  magma  must  have  been  forced  into  innumerable  cracks 
in  the  highly  heated  Cambrian  strata,  isolating  many  masses 
which  were  subsequently  more  or  less  completely  absorbed  by 
the  magma.  It  is  thus  possible,  while  holding  that  the  batho- 
lite was,  in  the  main,  developed  in  situ  by  a  gradual  elevation 
of  the  isogeotherms  due  to  sedimentation  and  subsequent 
plication,  to  regard  the  actual  contacts  as  largely  or  chiefly 
mechanical,  the  product  of  rifting  and  flowing  :  and  I  am  in- 
clined to  regard  this  as,  on  the  whole,  the  more  probable  view. 
It  is  strongly  suggested,  also,  by  the  more  or  less  marked 
lamination  or  flow  structure  to  be  noted  at  many  points  in  the 
peripheral  portions  of  the  batholite. 

Owing  to  variations  in  the  structure,  composition,  and  degree 
of  hydration  of  the  segments  of  the  earth's  crust  affected  by  the 
thermal  invasion  and  the  consequent  variations  in  conductivity 
and  fusibility,  the  upper  surface  of  the  batholite  was,  probably, 
very  uneven,  rising  to  much  higher  levels  in  some  parts  than 
in  others.  Hydration  was  an  especially  potent  factor ;  and 
undoubtedly  its  influence  was  felt  to  the  greatest  depth  beneath 
and  in  the  deep  isoclinal  foldings  of  the  Cambrian  strata. 
Obviously,  also,  these  zones  of  deep  folding  must  have  experi- 
enced, even  above  the  upper  limit  of  fusion,  the  maximum 
increase  of  temperature  and  consequent  expansion.  T.  Mel- 
lard  Reade  has  demonstrated  that  under  the  conditions  postu- 
lated   here    cubic    expansion    is    resolved   into    linear    vertical 


316 

expansion,  giving  I'ise  thus  to  a  notable  elevation  of  the  sur- 
face ;  and  thence  follows  increased  activity  of  erosion,  which, 
in  turn,  involves  or  permits  a  more  rapid  escape  of  the  sub- 
terranean heat.  We  are  thus  led  to  the  conclusion  that  where 
the  incursion  of  the  batholite  into  the  Cambrian  strata  was 
most  extensive  its  subsequent  refrigeration  was  most  rapid. 

At  length  the  great  heat  invasion  gradually  declined  in  vigor 
and  finally  ceased  altogether.  The  batholite  had  attained  its 
maximum  limits  ;  and,  as  with  the  tide,  after  a  pause  the  ebb 
began.  Through  loss  of  heat  by  conduction  to  the  surface  the 
isogeotherms  receded  as  gradually  as  they  had  advanced. 
The  recession  was  slowest,  no  doubt,  over  those  areas  where 
the  batholite  had  made  the  slightest  advance  into  the  Cambrian 
strata,  because  the  depth  was  greatest  here,  and  also  because, 
as  we  have  seen,  these  were  probably  areas  of  miniriium  ero- 
sion. This  differential  surface  cooling  of  the  batholite  was 
undoubtedly,  as  we  shall  see,  an  important  factor  in  the  subse- 
quent development  and  differentiation. 

The  existing  heterogeneous  character  of  the  batholite,  espe- 
cially as  regards  composition,  next  calls  for  consideration.  The 
chief  chemical  contrast  is  that  between  the  diorite  and  granite. 
It  has  been  su2:o;ested  that  this  contrast  is  orio-inal,  the  diorite 
resulting  from  the  fusing  of  relatively  basic  and  the  granite  of 
relatively  acid  rocks.  The  main  difficulty  with  this  theory  is 
that  it  does  not  readily  account  for  the  remarkably  constant 
structural  relations  of  the  granite  and  diorite,  the  diorite  appear- 
ing always  to  be  the  older  rock  and  to  be  especially  character- 
istic of  tlie  more  superficial  parts  of  the  batholite,  forming 
normally  a  basic  crust  or  zone  enveloping  the  granite,  with  a 
more  or  less  perfect  gradation  between  the  two  rocks.  These 
relations  are  not  peculiar  to  the  batholite  of  eastern  Massachu- 
setts, but  characterize  the  plutonic  rocks  of  many  and  widely 
separated  regions  ;  and  if  we  accept,  as  is  certainly  necessary 
for  this  region,  the  view  that  the  batholite  is  due  primarily  to  a 
heat  invasion,  that  is,  to  the  remelting  in  situ  of  a  previously 


317 

solidified  portion  of  the  earth's  crust,  the  impossibility  of 
regarding  this  constant  relation  of  the  granite  and  diorite  as  a 
pre-existing  feature  of  the  crust  becomes  apparent.  This 
argument  is  strongly  reinforced  by  the  consideration  that, 
since  the  batholite,  then  deeply  buried  in  the  earth,  must  have 
remained  molten  for  an  immensely  long  period,  cooling  and 
solidifying  with  extreme  slowness,  molecular  diffusion  and 
convection,  and  especially  the  latter,  must  have  tended  to  make 
and  keep  the  magma  homogeneous.  The  magma  must  of 
necessity  be  coolest  at  the  top  ;  and  this  thermal  condition 
makes  convection  currents  inevitable,  the  hotter  material  rising 
and  the  cooler  sinking.  In  consequence  of  the  homogeneity 
due  to  this  constant  circulation,  the  magma  may  match  in 
composition  none  of  the  rocks  from  which  it  has  been  derived, 
presenting  in  all  directions  a  marked  lithological  contrast  with 
the  enclosing  formations. 

Accepting,  then,  the  view  that  the  batholite,  while  still 
molten,  was  substantially  homogeneous,  and  recognizing  that 
it  is  now  decidedly  heterogeneous,  we  are  forced  to  the 
conclusion  that  the  refrigeration  of  the  batholite  was  attended 
by  a  process  of  differentiation  by  which,  from  the  originally 
homogeneous  magma,  was  developed  the  entire  granitic  series 
noted  at  the  beginning  of  this  section.  The  differentiation  was 
partly  textural  and  partly  chemical.  The  differentiation  of 
texture  was  due  chiefly  to  variations  in  the  rate  of  cooling. 
The  chemical  differentiation  has  been  explained  as  resting 
primarily  upon  the  hypothesis  that  a  molten  magma,  even  if 
entirely  anhydrous,  is  essentially  a  solution.  It  is  not  sup- 
posed that  the  various  minerals, —  feldspars,  micas,  hornblende, 
etc., —  of  which  the  rocks  are  now  composed  existed  as  such  in 
the  molten  magma  ;  but  the  latter  contained  only  the  primary 
oxides,  —  silica,  alumina,  magnesia,  lime,  potassa,  soda,  iron 
oxide,  etc., —  which  at  the  moment  of  solidification  combined 
in  the  right  proportions  to  form  the  minerals.  Now,  regarding  ■ 
the    molten    magma    as    a    solution,  certain    of  these    oxides, 


318 

including  the  alkalies,  silica,  and  water,  may  be  regarded  as 
playing  the  role  of  solvents  for  the  others,  including  the 
alumina,  magnesia,  iron  oxide,  etc.  ;  and  Iddings  ^  supposes 
mao'matic  differentiation  to  result  chiefly,  at  least,  from  the 
application  to  such  a  solution  of  Soret's  principle  that  if  the 
temperature  of  a  solution  is  not  uniform,  the  substances  in  solu- 
tion tend,  in  obedience  to  osmotic  pressure,  to  become  concen- 
trated in  the  cooler  parts  of  the  solution  ;  and,  obviously,  during 
its  slow  refrigeration  the  cooler  part  of  the  batholite  must  have 
been  constantly  the  upper  part,  where  it  was  in  contact  with 
the  overlying  formations.  Therefore,  over  the  surface  of  the 
batholite  was  slowly  developed,  according  to  this  theory,  a 
zone  or  layer  of  more  basic  magma.  This  basic  outer  zone 
had  no  lower  boundary,  becoming  gradually  more  siliceous 
and  alkaline  downward  and  blending  with  the  vast  undiffer- 
entiated body  of  magma.  Eventually,  through  continued 
refrigeration,  it  solidified  and  formed  the  diorite,  which  thus 
becomes,  as  the  field  evidence  requires,  the  earliest  product  of 
magmatic  differentiation. 

Although  this  beautiful  theory  seemed  likely,  for  a  time,  to 
o-ain  general  acceptance,  Becker^  has  recently  disclosed  a  fatal 
weakness  in  it.  He  has  shown  that  molecular  segregation  in  a 
molten  magma  is  an  almost  infinitely  slow  process,  that  marked 
or  significant  results  would,  therefore,  require  an  immensely 
long  time,  even  in  a  geological  sense  ;  and  that  in  any  probable 
case  convection  currents  would  completely  and  continuously 
neutralize  the  effects  of  segregation  in  accordance  with  Soret's 
principle. 

Another  explanation  of  magmatic  differentiation  is  based 
upon  the  princi])le  that  a  complex  homogeneous  liquid  tends  to 
divide  at  a  lower  temperature  into  two  or  more  immiscible  liq- 
uids, one  of  which  solidifies  at  a  higher  temperature  than  the 
other.      Our  magma  of  neutral  composition  would  thus  naturally 

1  The  origin  of  igneous  roclvs.    Bull,  pliil.  soc.  Washington,  vol.  1'2,  p.  89-214. 

2  Amer.  journ.    sci.,  vol.  153,  ]).  21-40. 


319 

be  resolved,  as  it  slowly  cooled,  into  a  siliceous,  alkaline,  highly 
hydrated  and  fusible  portion  (granite)  and  a  basic,  ferruginous, 
relatively  anhydrous  and  infusible  portion  (diorite).  Segrega- 
tion by  molecular  flow  would  tend  to  cause  a  separation  of  the 
two  liquids  or  magmas,  especially  during  the  solidification  of  the 
more  basic  ;  and  thus,  no  doubt,  may  best  be  explained  many 
of  the  ill-defined  and  more  or  less  rounded  inclusions  of  diorite 
in  the  granite.  But  the  continuous  zone  of  diorite  covering 
the  granite  finds  its  best  explanation  in  the  assumption  that 
during  the  circulation  of  the  magma  due  to  convection  currents 
the  less  fusible  basic  portion  is  solidified  by  contact  with  or 
crystallizes  upon  the  relatively  cool  wall  of  the  batholite,  much 
as  moisture  in  the  air  of  a  room  separates  as  frost  on  the  cold 
window  pane.  This  theory  thus  virtually  resolves  itself  into 
an  explanation  of  the  dififerentiation  of  the  magma  and  formation 
of  the  diorite  by  fractional  crystallization  ;  and  we  are  not  enti- 
tled to  suppose  that  the  diorite  ever  existed  as  a  continuous 
body  of  molten  basic  magma. 

Since  the  preceding  paragraph  was  written,  Becker,^  in  a 
second  valuable  contribution  to  the  literature  of  rock-differen- 
tiation, has  brought  forward  the  principle  of  fractional  crystal- 
lization and  ably  applied  it  to  the  solution  of  this  problem  : 
and  the  writer's  confidence  in  the  soundness  of  this  explanation 
of  the  contrasted  and  yet  blending  and  obviously  related 
granite  and  diorite  is  thus  greatly  strengthened. 

The  batholite  had  now,  in  part  at  least,  a  basic  crust  or  outer 
layer.  The  shrinkage  due  to  continued  refrigeration  must  have 
involved  extensive  crustal  adjustments.  The  hardened  diorite 
was  cracked  and,  in  part,  completely  shattered,  and  slov^dy 
injected,  under  enormous  pressure,  by  the  more  acid  and  still 
molten  mao;ma  beneath  which  on  cooling  formed  the  basic  or 
dioi'itic  granite.  Under  the  conditions  postulated,  this  basic 
granite  must,  and  we  observe  in  the  field  that  it  actually  does, 

1  Amer.  Journ.  sci.,  vol.  154,  p.  257-261. 


320 

grade  into  the  diorite.  That  is,  where  their  original  and 
normal  relations  are  still  undisturbed  the  one  type  blends 
perfectly  with  the  other;  and  in  any  case,  between  the  most 
typical  diorite  on  one  side  and  granite  on  the  other  inter- 
mediate foi'nis  can  be  found.  Tlie  diorite  not  only  grades  into 
the  granite,  but,  as  previously  noted,  more  or  less  distinct 
segregations  of  diorite  are  of  frequent  occurrence  in  the  granite, 
especially  near  the  common  boundary  of  the  two  rocks.  These 
are  usually  rounded  and  minutely  irregular  in  outline,  and 
vary  in  size  from  mere  specks  to  masses  yards  in  diameter. 
Aside  from  these  basic  segregations  (the  familiar  dark  spots  in 
granite),  all  abrupt  passages  from  diorite  to  granite  and  all 
angular  inclusions  in  the  granite  are  due  to  dynamic  causes,  - — 
the  cracking  of  the  diorite  and  flowing  of  the  granite. 

In  the  region  of  the  Blue  Hills  Complex  the  diorite  now  forms 
only  a  few  widely-isolated  patches  ;  but,  although  these  are  to 
be  regarded  merely  as  ei'osion-remnants  of  a  once  continu- 
ous sheet,  it  appears  'probable  that  this  sheet  was  never 
coextensive  in  area  with  the  batholite.  In  the  Blue  Hills 
proper,  where  the  contact  of  the  granitic  series  with  the 
overlying  Cambrian  beds  is  exposed  at  many  points,  the  diorite 
is  wholly  wanting,  its  place  as  a  contact  zone  being  taken  by 
fine  granite  and  quartz  porphyry  or  other  forms  of  apo- 
rhyolite.  Moreover,  in  the  portions  of  the  complex  where  the 
granite  is  characterized  or  was  probably  once  characterized  by 
a  diorite  cover,  the  normal  granite,  —  meaning  by  that  term 
the  granite  forming  the  main  body  of  the  batholite  below  the 
contact  zone  and  beyond  the  chilling  influence  of  the  overlying 
formations,  —  is  distinctly  biotitic,  containing,  as  a  rule,  little 
or  no  hornblende  ;  while  in  the  Blue  Hills  area,  wliere  the 
contact  zone  is  fine  granite  and  aporhyolite,  the  underlying 
normal  granite  is  distinctly  hornblendic,  containing,  as  a  rule, 
little  or  no  biotite.  This  difference  in  the  normal  granites 
suggests  a  difference  in  the  depth  and  the  rate  of  cooling  at  the 
time  they  were  formed  ;  the  presumption  being,  from  the  anal- 


321 

ogy  of  modern  lavas,  that  the  biotitic  normal  granite  was  formed 
at  a  greater  dej)th  and  more  slowly  than  the  hornblendic  normal 
o^ranite.  The  contact  zones  have  a  like  sis^nificance.  The 
diorite  is  the  product  of  chemical  differentiation,  which,  as  we 
have  seen,  demands  long  time  and  consequently  slow  cooling 
and  great  depth  ;  while  the  fine  granite  and  aporhyolite  are 
the  products,  almost  exclusively,  of  textural  differentiation  due 
to  rapid  cooling  and  must  represent  a  portion  of  the  batholite 
which  originally  attained  an  exceptional  elevation  in  the  Cam- 
brian strata.  We  thus  find  that  the  theoretical  differentiation 
of  the  batholite  suggested  on  page  317  harmonizes  perfectly  with 
the  character  and  distribution  of  the  rocks. 

The  more  elevated  portions  of  the  batholite  necessarily  passed 
more  rapidly  than  the  deeper  portions  through  their  successive 
phases  of  development.  The  molten  state  did  not  persist  long 
enough  to  permit  any  marked  chemical  differentiation.  In 
fact,  where  the  magma  approached  nearest  to  the  surface,  or  at 
least  where  it  passed  tlirough  the  Lower  Cambrian  strata  and 
accumulated  in  the  Middle  Cambrian,  the  chilling  effect  of  the 
enclosing  rocks  was  so  marked  as  to  cause  a  relatively  sudden 
solidification  of  the  superficial  portion  of  the  magma,  thus  giving 
rise  to  the  zone  or  layer  of  fine  granite  and  porphyritic  aporhy- 
olite or  quartz  porphyry  which  now  envelops  the  normal 
granite.  This  is  in  either  case  a  true  contact  zone,  passing 
gradually  downward  into  the  normal  granite. 

In  the  eastern  half  of  the  Complex,  or  east  of  the  West 
Quincy  and  Quincy- Adams  Valley,  and  to  a  limited  extent 
farther  west,  the  contact  zone  is  composed  of  fine  granite  ;  and 
the  normal  type  of  fine  granite,  as  developed  in  eastern  Quincy, 
East  Braintree,  and  North  Weymouth,  is  believed  to  occur 
almost  entirely  as  a  contact  zone  between  the  normal  granite 
and  the  Cambrian  sti'ata,  the  latter  having  been  to  a  large 
extent  removed  by  subsequent  erosion.  In  like  manner,  along 
the  main  range  of  the  Blue  Hills,  from  Pine  Hill  westward, 
where,  we  may  suppose,  the  batholite  reached  to  still  greater 


322 

heights  in  the  Cambrian  series,  the  contact  zone  consists 
chiefly  of  quartz  porphyry,  the  product  of  a  still  more  rapid 
cooling. 

It  thus  appears  that  the  contact  zone  of  the  batholite  pre- 
sents, in  different  ai'eas  of  the  Blue  Hills  Complex,  three  more 
or  less  distinct  phases.  These  are  (1)  the  diorite,  covering 
the  biotitic  normal  granite  ;  and  (2  and  3)  the  fine  granite 
and  aporhyolite,  covering  the  hornblendic  normal  granite. 
These  may  be  regarded  as  essentially  contempoi-aneous  in 
development,  and  as  corresponding  to  varying  depths  in  the 
Cambrian  strata  of  one  and  the  same  oreat  batholite  or  reservoir 
of  magma.  The  essential  unity  of  this  trinity  is  seen  in  the  fact, 
to  be  more  fully  developed  in  the  detailed  descriptions  of  the 
rocks,  that  the  quartz  porphyry  phase  passes  gradually  into 
the  fine  granite,  and  the  latter,  in  turn,  blends  with  the  diorite. 

The  formation  of  the  contact  zones  was  accompanied  and 
followed,  as  already  noted  for  the  diorite,  by  cracking  and 
readjustment  of  the  overlying  rocks  which  permitted  extensive 
intrusions  and  outflows  of  the  enclosed  magma,  forming  dikes 
of  fine  granite  and  microgranite  and  both  dikes  and  surface 
flows  of  more  or  less  glassy  rhyolite  (obsidian,  etc.),  the  sub- 
sequent devitrification  of  which  has  given  rise  to  the  compact 
and  fluidal  forms  of  felsite  or  aporhyolite. 

But  it  is  an  important  fact  that  throughout  the  Boston  Basin 
and  eastern  Massachusetts  the  felsitic  rocks,  whether  intrusive 
or  efl^Lisive,  rest,  in  the  main  at  least,  directly  upon  the  various 
members  of  the  granitic  series,  and  not  upon  the  Cambrian 
strata  or  any  sedimentary  formation  ;  and  from  this  fact  the 
conclusion  necessarily  follows  that,  in  so  far  as  these  lavas  are 
truly  effusive,  their  extrusion  or  effusion  must  have  been  pre- 
ceded by  extensive  and  long-continued  erosion.  In  fact,  the 
erosion  must  have  been  sufficiently  extensive  to  remove  the 
sedimentary  cover  of  the  batholite  and  in  some  cases  the  entire 
thickness  of  its  contact  zone  down  to  the  normal  granite.  This 
raises  the  question   as  to  how,   if   tiiese  effusives  represent    a 


323 

previously  unsolidified  portion  of  the  batliolite,  it  had  escaped 
both  solidification  and  important  magmatic  differentiation  dur- 
ing the  long  time  required  for  the  denudation  of  the  batholite. 
As  regards  solidification,  the  best  answer  seems  to  be  that  the 
batholite  must  be  of  vast  depth  ;  and  that  while  denudation 
would  favor  solidification  by  cooling,  it  would  also  favor  lique- 
faction by  relief  of  pressure  and  by  permitting  the  access  of 
water  to  highly  heated  and  imperfectly  hydrated  zones  of  the 
batholite  ;  and  the  cooperation  of  these  causes  might,  conceiv- 
ably, induce  reliquefaction  in  material  which  had  solidified 
during  the  general  refrigeration  of  the  batholite.  According 
to  this  theory,  the  effusive  and  a  part  at  least  of  the  intrusive 
rocks  of  the  granitic  series  are  separated  by  an  important  time- 
interval  from  the  plutonic  types  ;  and  the  fact  that  they  are 
essentially  identical  in  composition  with  the  normal  granites  is 
an  additional  proof,  supplementing  that  afforded  by  the  homo- 
geneity of  the  great  body  of  normal  granite,  that  chemical 
differentiation  on  the  plan  of  fractional  crystallization  is 
dependent  upon  a  convective  circulation  of  the  magma  ;  and 
this,  originating  in  a  marked  thermal  contrast,  is  limited  to 
the  more   superficial  levels  of  tlie  batholite. 

The  sketch  or  outline  of  the  development  of  the  Blue  Hills 
Complex  embodied  in  the  preceding  paragraphs  is  necessarily 
somewhat  speculative.  But  the  writer  feels  that  it  is  based,  in 
the  main  at  least,  upon  sound  principles  ;  and  the  detailed 
descriptions  of  the  various  rocks  of  the  granitic  series  and  their 
relations  to  each  other  and  the  Cambrian  strata  which  follow 
will  show  to  what  extent  it  affords  a  consistent  explanation  of 
the  facts,  so  far  as  they  have  been  made  out. 

In  conclusion,  I  may  state  that  this  general  theory  of  the 
Complex  closely  parallels,  at  most  points,  that  proposed  by 
Dr.  Andrew  C.  Lawson  for  the  great  complex  of  sedimentary 
and  granitic  rocks  in  the  Rainy  Lake  region,  as  the  folio  win  o- 
paraphrase  of  the  part  of  his  summary  most  germane  to  our 
present    purpose  will    show.      Whatever  may  have    been    the 


324 

more  remote  history  of  the  granitic  rocks  of  this  region,  it 
appears  clear  that  the  magma  from  which  they  crystallized  as 
diorite,  granite,  etc.,  was  immediately  or  proximately  pro- 
duced by  the  fusion  of  the  basement  or  floor  upon  which  the 
Cambrian  strata  were  originally  deposited.  With  the  fusion 
of  this  floor  it  seems  probable  that  portions  of  the  superincum- 
bent strata  have  also  been  absorbed  into  the  general  magma, 
and  reappeared  on  crystallization  as  granite,  etc.  This  fusion, 
however,  only  extended  up  to  a  certain  uneven  surface,  which 
surface  now  constitutes  the  demarcation  between  the  granitic 
series  and  the  Cambrian.  Above  this  surface  or  upper  limit 
of  fusion,  the  Cambrian  formations  retained  their  stratiform  or 
bedded  disposition,  and  rested-  as  a  crust  of  hard  and  brittle 
rocks  upon  the  magma,  subject  to  its  metamorphic  influences. 
There  is  abundant  evidence  that  while  resting  iipon  this  plastic 
magma  the  crust  was  violently  disturbed,  folded,  crumpled, 
and  in  places  shattered.  This  disturbance  probably  served  to 
accentuate  the  sharpness  of  the  line  of  demarcation  between 
the  fused  and  unfused  rocks. ^ 

1  Geol.  surv.  Canada,  ami.  report,  1887-88,  F.  p.  139-142. 


325 


DESCRIPTIONS   OF   THE   ROCKS   OF   THE 
GRANITIC   SERIES. 

The  general  descriptions  which  follow,  and  the  lithological 
portions  of  which  are  in  part  quoted  from  Dr.  White's  paper, 
will  serve  to  give  a  more  complete  pictm^e  of  the  granitic  series 
and  the  mutual  relations  of  its  members  before  we  proceed  to  a 
detailed  study  of  its  distribution,  superinduced  structures,  and 
relations  to  the  Cambrian  strata.  The  follovving  classification 
is  that  indicated  in  the  preceding  outline  of  the  history  of  the 
Complex. 


CLASSIFICATION   OF    THE    ROCKS    OF    THE    GRANITIC    SERIES 

(PRE-CARBONIFEROUS). 

I.  Plutonic  rocks. 

1.  Normal  granites,  forming  the  main  body  of  the  batholite. 

(a)  Biotitic  normal  granite  (granitite). 

(b)  Hornblendic  normal  granite. 

2.  Rocks  forming  the  contact  zone  of  the  batholite. 

(a)  Diorite  and  basic  granite  (granodiorite). 

(b)  Fine  granite. 

(c)  Porphyritic  aporhyolite    (quartz  porphyry) ,  passing 

into  a  basic  quartzless  porphyry. 

II.  Intrusive  or    dike    rocks,  not    occurring  as  apophyses  of 

the  contact  zone. 

(a)  Microgranite. 

(b)  Aporhyolite. 

III.  Effusive  or  volcanic  rocks. 

(a)    Aporhyolite  (compact  and  fluidal  forms). 

IV.  Dikes  of  diabase  older  than  the  granitic  series. 


326 


PLUTONIC   EOCKS. 

JSforinal  Granites,  forming  the  Main   Body  of  the 
Batholite. 

Biotitic  Normal  Granite  (Granitite).  —  This  is  uudoubt- 
edly  the  most  abundant  and  widespread  and  probably  the  most 
truly  fundamental  of  all  the  rock  formations  in  or  bordering  the 
Boston  Basin.  In  its  areal  distribution  it  is  roughly  coex- 
tensive with  the  diorite,  which  we  suppose  to  have  once  formed 
a  continuous  contact  zone  over  this  type.  Together  with 
patches  of  the  diorite  it  forms  the  granitic  belt  from  eight  to  ten 
miles  wide,  extending  westward  from  the  shore  in  Cohasset  and 
separating  the  Boston  and  Narragansett  Basins.  East  of  Brain- 
tree  Great  Pond  it  borders  the  Blue  Hills  Complex  ;  but  west- 
ward from  this  point  it  is  separated  from  the  Complex  by  the 
Carboniferous  strata  of  the  Norfolk  Basin.  Nowhere  does  it 
form  an  essential  part  of  the  Complex  ;  that  is,  at  no  point  in 
the  South  Shore  district,  so  far  as  knov\^n,  is  the  biotitic  normal 
granite  closely  involved  with  the  Cambrian  strata,  erosion,  we 
must  suppose,  having  cut  deeply  enough  over  this  broad  area 
to  remove  the  last  vestige  of  the  Cambrian  series. 

This  granite  is  throughout  of  remarkably  uniform  character, 
as  befits  its  plutonic  origin.  It  is  of  medium  coarse  and  typi- 
cally granitic  texture  and  light  colored,  consisting  chiefly  of 
clear  and  colorless  quartz  and  white  to  light  gray  feldspars, 
with  a  liberal  sprinkling  of  biotite  in  rather  large  flakes,  and 
rarely  a  little  hornblende.  The  quartz  is  always  abundant, 
and  the  granite  must  be  regarded  as  of  decidedly  acid  character, 
although  Dr.  White  states  that  under  the  microscope  the  feld- 
spar appears  to  be  chiefly  or  entirely  plagioclase,  showing  the 
parallel  twinning  very  handsomely.  Macroscopic  observation 
shows,  however,  that  Cai'lsbad  twins  which  might  be  readily 
mistaken    for    orthoclase    are    fairly    common.       Dr.    White 


327 

also  says  that  the  biotite  is  idiomoi'phic  in  nearly  all  cases, 
although  sometimes  appearing  to  have  been  strained  or  more 
or  less  frayed  on  the  edges  ;  that  in  the  quartz  this  strain- 
ing is  often  quite  marked  ;  and  that  the  rock  seems  to  be 
devoid  of  accessory  minerals,  with  the  exception  of  a  few 
grains  of  magnetite  and  pyrite  and  an  occasional  wedge-shaped 
crystal  of  titanite.  In  most  cases  the  biotite  shows  more  or 
less  marked  chloritization,  and  the  alteration  product  has 
stained  the  other  minerals,  and  notably  the  feldspar,  bright 
green  ;  so  that  the  granite  usually  contains  feldspar  of  two 
colors  —  one  original  and  the  other  secondary.  Grains  of  epi- 
dote,  probably  also  of  secondary  origin,  have  been  observed  in 
a  few  instances. 

Notwithstanding  the  slowness  with  which  it  has  probably 
cooled  and  crystallized,  this  granite  is  by  no  means  exception- 
ally coarse,  scarcely  equaling  in  this  respect  the  more  super- 
ficial and  more  rapidly  formed  hornblendic  granites.  Below 
the  contact  zone  the  coarseness  of  crystallization  of  plutonic 
rocks  is,  we  may  suppose,  controlled  chiefly  by  the  viscosit}'^  of 
the  magma,  independently  of  the  rate  of  cooling  ;  the  viscosity 
must  be  in  general  inversely  proportional  to  the  degree  of 
hydration  of  the  magma  ;  and  hydration,  again,  must  usually 
diminish  downward.  Hence  it  follows  that  texture  is  not  an 
accurate  measure  of  original  depth  in  plutonic  rocks  ;  for  a 
truly  abyssal  magma  may,  in  spite  of  slow  cooling,  give  rise 
to  a  rock  of  merely  medium  texture. 

Very  rarely,  ill-defined,  vein-like  masses  manifest  a  ten- 
dency to  a  pegmatitic  development,  with  crystals  of  feldspar 
an  inch  or  more  in  diameter,  which  show  under  the  lens  a 
marked  graphic  granite  structure. 

9 

Although  not  rivaling  in  economic  interest  the  hornblendic 
normal  granite  of  Quincy  and  the  Blue  Hills,  and  therefore 
not  exposed  for  study  in  large  and  deep  quarries,  many  minor 
quarries  and  other  excavations  have  been  made  in  the  biotitic 
granite,  mainly  to  test  its  quality  or  to  obtain  stone  for  rough 


328 

local  uses.  One  important  exception  may  be  noted  now,  and 
tliat  is  the  "  seam-face"  granite  quarried  in  the  southwest  part 
of  Hingham  and  the  adjacent  part  of  Weymouth.  This  was 
briefly  described  in  Part  II  (pages  192-193),  at  a  time 
when  only  one  small  quarry  was  worked  in  it.  Since  then 
the  demand  for  the  stone  has  greatly  increased,  and  other 
quarries  have  been  opened.  The  quarry  first  referred  to  is 
about  one-half  mile  east  of  the  junction  of  Pine  and  Pleasant 
Sti'eets,  and  near  the  boundary  between  Hingham  and  Wey- 
mouth ;  and  one  of  the  new  quarries  is  about  one-sixth  mile 
east  of  Pleasant  Street,  near  Pine  Street,  in  Weymouth  ;  but 
the  new  quarries  are  chiefly  along  the  line  of  Whiting  Street  in 
Hingham.,  which  is  the  direct  continuation  of  Washington 
Street  in  Weymouth,  extending  from  the  town  boundary  south- 
eastward more  than  half  a  mile.  The  seam-face  granite 
extends  about  one-fourth  mile  north  of  Whiting  Street,  and  its 
southern  limit  appears  to  be  near  Derby  Street.  Its  area  is 
thus  roughly  at  least  one  mile  north-south  by  nearly  a  mile 
east-west,  or  approximately  a  square  mile,  partly  in  Hingham 
and  partly  in  Weymouth.  The  broad  development  in  this 
region  of  the  Liberty  and  Glad  Tidings  Plains  of  Lake  Bouve 
makes  it  impossible  to  determine  with  accuracy  the  limits  of 
this  interesting  rock,  which  is  virtually  a  more  finely  crystal- 
line facies  of  the  biotitic  normal  granite,  although  it  is  some- 
what variable  in  texture,  at  least  locally  and  near  the  borders 
of  the  area.  The  feature  to  which  this  granite  owes  its  chief 
interest  and  its  value  as  a  building  stone,  as  noted  in  Part  II, 
is  the  remai'kably  perfect  parallel  joint-structure.  (PI.  15.) 
"  The  joints  of  one  system  are  phenomenally  perfect,  close  and 
parallel,  dividing  the  granite  into  almost  absolutely  plane 
sheets,  varying  in  thickness  fi'om  half  an  inch  to  two  feet  or 
more,  but  mostly  from  six  to  twelve  inches.  The  granite  thus, 
in  a  general  view,  simulates  a  bedded  rock,  like  sandstone, 
very  closely,  and  the  surfaces  of  the  sheets  are  so  plane  and 
smooth,  and  the  grain  so  perfect,  that  blocks  suitable  for  build- 


329 

ing  purposes  are  obtained  with  remarkable  ease,  the  joint  struc- 
ture serving  the  same  useful  purpose  in  this  granite  as  in  the 
Roxbury  pudding  stone.  It  may  be  added  that,  as  in  the 
pudding  stone,  the  joint  faces  are  usually  variegated  and  beau- 
tified by  deep  and  rich  stains  of  iron  oxide. 

The  trend  of  this  system  of  joints  varies  from  N.  5°  E.  to 
N.  20°  E.  ;  and  the  hade  is  throughout  nearly  vertical 
(W.  0°  —  5°).  Very  perfect  cross  joints  are  seen  in  some 
of  the  quarries,  suggesting  a  distinction  between  block  jointing, 
vertical  sheet  jointing,  and  the  horizontal  sheet  jointing  so  fre- 
quently observed  in  ordinary  granite  quarries.  The  most 
northerly  quarry  is  about  one-fourth  mile  noi'th  of  Whiting 
Street.  Beyond  this  the  vertical  sheet  jointing  becomes  rap- 
idly less  distinct,  dying  out  as  the  granite  becomes  coarser  and 
merges  with  the  normal  granite  of  the  region. 

The  earliest  instance  of  the  use  of  the  seam-face  granite  was 
in  the  construction  some  seventy-five  years  ago  of  the  stone 
house  on  the  north  side  of  Washington  Street,  in  Weymouth, 
less  than  one  fourth  of  a  mile  from  the  Hingham  line  :  and  the 
little  quany  from  which  the  material  for  this  house  was 
obtained  may  be  seen  on  the  south  side  of  the  street  between 
the  house  and  the  town  boundary.  This  house  shows  to  good 
advantage  the  ready  adaptation  of  the  granite  to  architectural 
uses,  the  horizontal  surfaces  of  the  blocks  being  in  every 
instance  the  natural  joint  faces.  In  the  more  modern  use  of 
the  seam-face  granite,  however,  both  beauty  and  economy 
have  been  consulted  by  placing  the  stones,   as  a  rule,  on  edge. 

The  basic  inclusions  of  the  biotitic  normal  granite,  as  well 
as  its  relations  to  the  diorite,  basic  granite,  and  other  types, 
will  be  considered  as  these  are  described. 

Hornblendic  Normal  G-ranite.  —  This  type,  which  is 
often  but  improperly  called  syenite,  is  the  great  quarry  rock  of 
Quincy  and  the  Blue  Hills  ;  and,  economically,  it  ranks  as  the 
most  important  rock  formation  of  eastern  Massachusetts.  In 
the  district  under   consideration   its   distribution  is  practically 


330 

limited  to  the  Blue  Hills.  It  forms  the  irregular  belt  of  coarse 
granite  which  begins  on  Eldridge  Hill  in  eastern  Quincy  and 
includes  Payne  Hill,  Pine  Hill,  North  Common  Hill,  the 
quarry  district  of  West  Quincy,  and  the  entire  northern  or 
granitic  ridge  of  the  Blue  Hills  ;  and  it  appears  at  intervals 
along  the  southern  or  main  range  of  the  Blue  Hills,  where 
erosion  has  cut  through  the  covering  or  contact  zone  of  quartz 
porphyry.  Like  the  biotitic  granite,  it  is  of  surprisingly  uni- 
form character  ;  so  that,  except  for  slight  variations  in  color, 
which  are  chiefly  secondary,  and  the  concomitant  variations  in 
composition,  a  faii'ly  satisfactory  description  of  the  hornblendic 
granite  might  be  based  upon  a  single  specimen  taken  at  random 
in  almost  any  of  the  Quincy  quarries. 

Dr.  White  has  given  us  a  minutely  detailed  description  of 
this  important  type,  the  main  features  of  which  are  summar- 
ized here.  Macroscopically,  it  is  a  coarse  holocrystalline  aggre- 
gate of  light  gray,  dai'k  gray,  or  reddish  orthoclase,  and  clear 
or  smoky  quartz,  these  forming  the  bulk  of  the  rock,  with  dark 
green  or  black  hornblende  having  mai'ked  prismatic  cleavage, 
which  in  places  is  altered  to  a  lighter  green  chlorite.  The 
orthoclase,  which  occurs  commonly  in  Carlsbad  twins  from  5 
to  10  mm.  in  length,  is  the  predominant  constituent,  and  gives 
character  to  the  rock.  The  quartz  ranks  next  in  importance, 
the  hornblende  being  the  least  abundant  of  the  principal  con- 
stituents. According  to  Hawes  and  Merrill  ^  the  Quincy 
granite  also  contains  a  very  brittle  variety  of  pyroxene,  in  addi- 
tion to  the  hornblende,  which  tends  to  break  out  as  the  stone 
weathers,  leaving  a  polished  surface  of  the  stone  in  a  pitted 
condition.  This  is  not  uniformly  the  case  with  the  stone  now 
quarried,  however,  many  of  the  thin  sections  examined  show- 
ing no  pyroxene. 

Microscopically,  the  rock  presents  a  typically  granitic  tex- 


1  U.  r.  Merrill:  Building  and  ornamental  stones. 
Kept.  U.  S.  nat.  mus,,  188G,  p.  409. 
Tenth  census  of  U.  S.,  vol.  10,  p.  18. 


331 

ture,  without  differentiation  of  ground  mass  and  plicnocrysts. 
The  feldspar  is  practically  all  orthoclase,  with  possibly  a  little 
plagioclase.  It  is  for  the  most  part  little  altered,  and  occurs 
chiefly,  as  noted,  in  Carlsbad  twins.  Aniphibole  is  much  the 
more  abundant  dark  silicate.  It  is  usually  a  hornblende, 
apparently  of  a  variety  rich  in  soda,  in  densely  opaque  masses. 
On  the  edges,  however,  these  masses  are,  in  the  thinnest  sec- 
tions, seen  to  be  composed  of  many  fine,  rod-like  ciystals 
felted  together  and  fringed  with  projecting  needles.  This 
acicular  mineral  has  been  proved  by  Wadsworth'^  and  White  to 
be  glaucophane,  although  Dr.  White  supposed  at  one  time 
that  it  might  prove  to  be  riebeckite.  More  recently  Dr.  Henry 
S.  Washington  has  determined  by  chemical  analysis  that  both 
riebeckite  and  glaucophane  are  present  in  the  Quincy  granite, 
but  the  former  largely  predominating.  (See  page  332.)  Bio- 
tite  occurs  sparingly  as  brown  flakes  scattered  through  both 
quartz  and  feldspar.  In  portions  of  the  granite  local  segrega- 
tions of  various  minei^alizers  occur  in  small  quantity.  These 
are  especially  noticed  in  the  neighborhood  of  contacts.  Fluo- 
rite  is  thus  found  in  beautiful  bluish  purple,  partly  crystallized 
masses  of  both  microscopic  and  macroscopic  dimensions,  inter- 
mingled with  altered  pyroxene.  Aegerine  occurs  spanngly  in 
long,  bright  green  shreds  ;  and  doubly  terminated  crystals  of 
zircon  are  also  observed.  Pyrite  appears  to  be  of  rare  occur- 
rence, and  yet  quite  prominent  macroscopic  developments  of  it 
have  been  noted.  The  rusting  of  the  granite  superficially  and 
along  joint  cracks  is  undoubtedly  due,  however,  chiefly  to  iron 
oxide  derived  from  hornblende  and  biotite  ;  and  this  is,  doubt- 
less, true  of  all  the  granitic  rocks.  The  tendency  to  assume  the 
form  of  pegmatite,  with  more  or  less  interesting  developments 
of  amazon  stone,  allanite,  danalite,  annite,  etc.,  sometimes 
observed  in  the  similar  granite  of  Cape  Ann,  has  been  dis- 
tinctly noted  at  only  one  point  in  the  Blue  Hills  area.      This  is 

iM.  E.  Wadsworth  :  Descriptive  catalogue  of  American  and  foreign  rocks,  Boston, 
1883,  no.  71. 


332 

in  the  little  quarry  on  the  eastern  end  of  Rattlesnake  Hill.  In 
the  gray  granite  of  medium  texture  is  a  band  nearly  two  inches 
wide,  dike-like  in  form,  of  coarser  grain,  and  containing  slender 
crystals  of  hornblende  from  10  to  20  mm.  in  length.  A  vet- 
eran quarryman  has  informed  me  that  he  has  seen  in  one  of  the 
West  Quincy  quarries  a  vein  five  or  six  inches  wide,  with 
crystals  of  hornblende  as  large  as  his  finger.  Mr.  White's 
description  of  the  normal  granite  substantially  confirms  the 
descriptions  given  by  Wadsvvorth,  Hawes,  and  Merrill. 

A  typical  sample  of  this  important  type  of  granite  from  the 
Hardwick  Quarry,  Quincy,  was  analyzed  by  Henry  S.  Wash- 
ington,i  with  the  following  results  :  SiOo,  73.93  ;  TiOa,  0.18  ; 
AIO3,  12.29  ;  FeA,  2.91  ;  FeO,  1.55  ;  MnO,  trace  ;  MgO, 
0.04;  CaO,0.31;  BaO,  none  ;  Na.,0,4.66;  Kp,  4.63  : 
H,0,  0.41 1=  100.91.  From  this  analysis  Washington  has 
calculated  the  mineral  composition  of  the  granite,  as  follows  : 
Quartz,  30.2  ;  orthoclase,  27.2  ;  albite,  27.7  ;  riebeckite, 
12.3;  glaucophane,  2.0;   accessories,  0.6^=100.00. 

The  Dark  Gray  to  Blach  Phase  of  the  Hornblendic 
Normal  Granite. —  In  several  of  the  West  Quincy  quarries, 
and  notably  in  the  quarry  of  McGilvray  and  Jones,  west  of 
the  Granite  Railway  Quarry,  and  in  several  of  the  smaller 
quarries  south  of  the  quarry  railway,  including  the  Baker 
Quarry  and  the  quarry  next  west  of  the  Bunker  Hill  Quarry, 
and  in  a  ledge  on  the  south  side  of  Pleasant  Street,  near  the 
corner  of  Gun  Hill  Road,  in  Milton,  the  granite  is  in  part  of  a 
very  dark  gray  color,  suggesting  the  name  "  black  granite." 
This  dark  granite  blends  insensibly  in  all  directions  with  the 
normal  gray  granite,  in  which  it  forms  irregular  and  ill-defined 
or  cloud-like  masses.  It  is  very  clearly  a  darker  facies  of  the 
normal  gray  granite,  and  not  in  any  sense  a  distinct  rock. 
The  most  casual  examination  sliows  that  the  dax'k  color  is  not 
due  to  an  increased  proportion  of  liornblende,  but  the  explana- 
tion is  to  be  found  in  the  dark  inclusions  of  tlic  feldspar  and 

1  Amer.  jour,  sci.,  vol.  156,  p.  181. 


333 

quartz,  and  especially  the  former.  Dr.  White  says  the  dark 
color  of  the  orthoclase  proves,  under  the  microscope,  to  be  due 
to  innumerable  minute  needle-like  crystals  of  hornblende  which 
traverse  it ;  and  also  that  the  rock  contains  black  tourmaline  in 
bunches  of  blunt  needles.  In  general  tlie  black  granite  shows 
more  alteration,  especially  of  the  hornblende,  tlian  the  normal 
gray  granite,  agreeing  in  this  respect  with  the  red  granite  to 
be  described  later. 

In  Wilson's  Quarry,  near  the  junction  of  Center  and  Granite 
Streets,  the  normal  gray  granite  with  unaltered  hornblende  is 
separated  by  a  definite  and  approximately  plane  surface  slop- 
ing south  at  a  high  angle  from  a  distinctly  darker  gray  and 
more  altered  granite,  the  latter  overlying  the  former.  In  this 
instance  there  is  no  gradation,  but  the  contact  is  like  that  of 
two  distinct  rocks. 

Additional  light  is  thrown  upon  this  problem  of  the  relations 
of  the  light  and  dark  granites  by  the  facts  presented  in  the 
small  quarry  on  the  east  end  of  Rattlesnake  Hill.  The  granite 
is  here  of  medium  texture,  intermediate,  as  we  shall  see  later, 
between  the  normal  coarse  granite  and  the  quartz  porphyry  at 
the  western  end  of  the  hill.  Although  mainly  of  the  normal 
gray  color,  it  is  partly,  especially  in  the  upper  part  of  the 
quarry,  very  dark  gray  to  black.  This  black  granite  clearly 
forms  irregular  and  indefinite  masses  in  the  gray  ;  and,  as  in 
the  West  Quincy  quarries,  the  contacts  are  blending,  the  one 
type  merging  insensibly  with  the  other  through  a  zone  from  six 
inches  to  a  foot  or  two  in  breadth.  Other  contacts  in  the 
immediate  vicinity  of  these  blending  ones  are  as  sharp  and  defi- 
nite as  that  in  Wilson's  Quarry  ;  and  the  black  granite  then 
occurs  as  dark  stripes  and  streaks  in  the  gray.  Apparently, 
we  have  revealed  here  more  perfectly  than  elsewhere  in  this 
field,  the  actual  difiPerentiation  of  the  granitic  magma  by  segre- 
gation (blending  contacts)  accompanied  by  a  sluggish  flowing 
movement  (abrupt  contacts).  Flowing  is,  naturally,  antago- 
nistic to  magmatic  segregation  ;    but  before    the    former    had 


334 

completely  neutralized  the  effects  of  the  latter,  congelation 
supervened ;  and  thus  were  preserved  for  us,  and  finally 
exposed  to  observation  through  the  fortunate  location  of  this 
quarry,  several  important  phases  of  the  magmatic  process.  It 
is  surprising,  however,  to  find  that  the  differentiation  has  not 
appi'eciably  affected  the  silica  ratio,  the  gray  and  black 
granites  from  this  quarry  yielding  on  analysis,  respectively, 
70.98  and  70.95  per  cent  of  silica  :  and  from  this  we  may 
conclude  that  there  is,  probably,  no  essential  difference  in  com- 
position ;  the  contrast  in  color  being  due,  not  to  true  chemical 
differentiation  of  the  magma,  but  simpl}'  to  the  fact  already 
stated  that  in  the  black  granite  the  dark  constituent  is  in  part 
diffused  through  the  feldspar  and  its  color  value  thus  greatly 
increased.  The  cause  of  this  contrast  in  the  relations  of  the 
hornblende  and  feldspar  is  not  apparent ;  but  we  cannot  doubt 
that  it  is  an  original  feature  of  the  rock,  and  determined  in  some 
way  by  the  conditions  under  which  the  magma  crystallized. 

The  Red  Phase  of  the  Hornblendic  JSFormal  Granite. — 
The  color  contrast  most  frequently  exhibited  in  the  horn- 
blendic normal  granite  is  that  between  the  gray  and  red  varie- 
ties. In  quarries  and  ledges  all  over  the  area  occupied  by  this 
granite  it  is  often  reddish  superficially,  and  the  red  granite  fre- 
quently alternates  with  the  gray  at  the  surface  ;  while  in  cer- 
tain limited  parts  of  the  field,  as  on  the  northeast  and  south- 
east sides  of  the  Pine  Hill  area  and  in  all  the  eastern  half  of 
Quincy,  including  especially  the  Payne  Hill  district,  the  red 
color  extends  to  greater  depths,  and  some  quarries  are  worked 
exclusively  for  the  red  granite. 

The  gray  and  red  granites  are  equally  rich  in  quartz  ;  but  in 
addition  the  gray  granite  consists  of  gray  orthoclase,  with  an 
abundance  of  sliarpl}'  defined  and  fresh  hornblende  ;  while  the 
red  granite  is  made  up,  besides  the  quartz,  of  reddish  ortho- 
clase with  little  or  no  hornblende,  greenish  (chloritic)  altera- 
tion products  seeming  to  take  the  place  of  the  hornblende. 
These  facts  strongly  suggest  that  the  red  granite  is  merely  a 


335 

more  altered  and  oxidized  form  of  the  gray  granite.  The 
hornblende,  being  the  weakest  constituent,  has  been  most 
extensively  altered  ;  but  Dr.  White  also  notes  that  the  feldspars 
are  fractured  and  opaque,  and  seldom  show  twinning,  that 
small  octahedral  magnetites  occur  in  streaks  between  the  feld- 
spars and  a  reddish  limonite  alteration  product.  The  distri- 
bution of  the  red  granite  is  favorable  to  the  view  that  it  is  a 
superficial  and  secondary  phase  of  the  gray  granite,  since  it  is 
most  conspicuous  and  extensive  at  those  points  where  glacial 
erosion  was  least  severe  ^  ;  and  it  is  probable  that  everywhere 
in  the  Blue  Hills  area  the  red  granite  passes  gradually  down- 
ward into  the  truly  normal  and  original  gray  granite.  In  fact, 
this  vertical  gradation  in  color  may  be  observed  in  quite  a 
number  of  the  quarries,  and  several  of  the  clearer  and  more 
instructive  examples  will  be  noted  here. 

Commencing  with  the  West  Quincy  quarry  district,  that  is, 
the  complex  ridge  of  granite  extending  west  from  the  vicinity 
of  the  West  Quincy  Station,  we  find  that  along  the  north  side 
of  the  belt  of  quarries,  where  glacial  erosion  must  have  been 
most  effective,  the  granite  is  practically  all  gray.  Opposite 
the  end  of  Grove  Street  and  north  of  Blueberry  Swamp  is  a 
low  ridge,  composed  chiefly  of  till,  which  may  be  designated, 
from  a  prominent  tree  at  its  western  end.  Hangman's  Oak 
Eidge.  Toward  the  eastern  end  of  the  ridge  the  granite  out- 
crops and  is  well  exposed  in  several  small  quarries.  The  rock 
appears  to  be  quite  fresh  and  unaltered,  except  that  many  of 
the  minor  feldspars  are  oxidized,  varying  in  tint  from  pale 
yellow  to  deep  orange  and  red.  It  might  be  called  a  pink  and 
gray  granite,  and  undoubtedly  marks  an  incipient  stage  in  the 
development  of  the  red  granite.  In  a  small  quarry  east  of  the 
Jones  and  Desmond  Quarry  and  a  larger  one  southeast  of  the 
Fuller  and    Clarke   Quarries,   this    pink    and   gray   granite  is 

In  the  granite  of  the  Pike's  Peak  district  in  Colorado,  I  have  observed  that  where 
mechanical  erosion  is  most  rapid,  as  on  precipitovis  slopes  and  crags,  the  prevailing 
red  color  gives  way  to  gray,  showing  that  the  red  is  superficial  and  secondary. 


336 

clearly  underlain  by  the  normal  gray  variety.  Along  the  south 
side  of  this  quarry  belt  there  are  several  quarries  in  which  the 
secondary  color  phenomena  are  interesting.  In  the  small 
quarry  nearest  the  main  junction  of  the  quarry  railroad,  at  the 
east  end  of  Blueberry  Swamp,  the  granite  is  brown  above  and 
brownish  gray  below  ;  and  the  Dean  and  Horrigan  Quarry, 
one-fourth  mile  farther  east,  is  a  fine  example  of  the  relations 
of  the  red  and  gray  granites.  The  pale  brownish  red  color  is 
not  only  superficial,  but  obviously  confined  to  the  vicinity  of 
certain  prominent  and  very  rusty  joints.  A  few  feet  away 
from  these  joints  the  normal  gray  granite  reaches  the  surface. 
On  North  Common  Hill  the  conditions  seem  to  have  been 
exceptionally  favorable  to  glacial  erosion,  and  no  impoi'tant 
developments  of  red  granite  have  been  noted.  In  Wilson's 
Quarry,  south  of  North  Common  Hill,  the  dark  gray  granite 
previously  noted  is  overlain  by  ten  to  fifteen  feet  of  red  granite, 
the  occurrence  being  very  normal.  Undoubtedly  the  most 
important  development  of  the  red  granite  is  found  on  the  south- 
east side  of  Pine  Hill,  in  Braintree  ;  and  in  practically  every 
one  of  the  half  dozen  quarries  the  color  gradation  is  more  or 
less  distinctly  shown,  the  thickness  of  red  granite  over  the  gray 
varying  from  nothing  to  perhaps  twenty  feet.  Echo  Quarry, 
at  the  eastern  base  of  Payne  Hill,  shows  above  the  water  levels 
from  twenty  to  thirty  feet  of  reddish  granite,  while  the  granite 
from  the  deepest  part  of  the  quarry  is  a  very  pure  and  typical 
gray,  with  fresh  hornblende. 

In  all  these  examples  the  change  from  red  granite  above  to 
gray  below  is  gradual,  so  that  it  is  possible  to  select  a  blended 
series  of  specimens  ;  and  yet  the  change  is  rapid,  only  from  five 
to  ten  feet  of  intermediate  tints  separating  the  typical  red  granite 
from  -the  unaltered  gray.  But  even  in  the  gray  granite  the 
hornblende  is  usually  more  or  less  completely  decomposed,  and 
the  develo])ment  of  limonite  in  the  chloritic  and  micaceous  sec- 
ondary products  is  often  very  evident.  It  does  not  appear, 
however,  that  this  limonite  is  the  principal  cause  of  the  stain- 


337 

ing  of  the  feldspar,  but  that  is  probably  chiefly  due  to  the 
peroxidation  of  the  iron  originally  contained  in  the  gray  feld- 
spar or  its  dark  inclusions.  The  change  does  not  affect  all  the 
feldspar  crystals  simultaneously,  and  hence  we  find  in  the 
intermediate  stages  both  pink  and  gray  feldspars  ;  in  fact,  the 
two  colors  are  often  blended  in  the  same  crystal ;  and  the  indi- 
vidual crystals  of  oxidized  feldspar  vary  in  tint  from  a  faint 
yellow  to  a  decided  orange  or  brownish  red.  The  biotitic 
normal  granite  is  never  red,  except  very  superficially,  where 
stained  through  the  peroxidation  of  the  iron  in  the  biotite  ;  and 
we  may  safely  assume  that  the  feldspars  are  less  susceptible  of 
reddening  by  oxidation  than  those  of  the  hornblendic  normal 
granite,  on  account  of  their  smaller  proportion  of  iron,  as  indi- 
cated by  the  paler  colors.  The  greater  purity  of  the  feldspar 
in  this  respect  may,  perhaps,  be  referred  to  the  slower  cooling 
and  crystallization  of  the  magma,  permitting  the  absorption  of 
all  the  iron  by  the  dark  or  ferro-magnesian  constituents  of  the 
first  generation,  and  leaving  none  for  the  feldspars  of  the 
second  generation.  Or,  accepting  Dr.  White's  observation, 
that  the  feldspar  of  the  hornblendic  normal  granite  owes  its 
dark  color  wholly  to  included  hornblende,  we  may  say  that  the 
slower  cooling  of  the  magma  prevented  overlapping  of  the 
generations,  and  thus  permitted  a  more  perfect  separation  of 
the  minerals  by  successive  crystallization. 

Since  the  red  granite  usually  passes  gradually  downward 
into  gray  granite  in  which  the  hornblende  is  already  chloritized, 
it  follows,  that,  as  a  rule,  at  least,  the  chloritization  of  the 
hornblende  precedes  the  peroxidation  of  the  iron  in  the  feld- 
spar. Both  of  these  preparatory  steps  in  the  decomposition  of 
the  granite  are,  doubtless,  taking  place  with  extreme  slowness, 
comparatively  insignificant  results  having  been  accomplished 
since  the  close  of  the  glacial  period.  The  contact  of  the  red 
and  gray  granites,  as  exposed  in  the  quarries,  is  everywhere 
extremely  irregular,  the  reddening  having  penetrated  deeper  at 
certain  points,  as  in  the  vicinity  of  prominent  joint  cracks,  than 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  22. 


338 

at  others  ;  and  the  frequent  alternations  of  red  and  gray  gran- 
ites observed  at  the  surface  are  probably  best  explained  by 
supposing  that  in  these  localities  glacial  erosion  removed  the 
more  elevated,  but  not  the  deeper  portions  of  the  red  granite. 
A  comparison  of  our  red  granite  with  the  red  residuary  soil  of 
the  Southern  States  develops  the  interesting  fact  that  the  per- 
oxidation of  the  iron  and  development  of  the  red  color  precedes 
kaolinization,  or  the  complete  decomposition  of  the  feldspar,  in 
the  one  case  and  follows  it  in  the  other.  The  possible  climatic 
significance  of  this  striking  contrast  is  an  attractive  problem, 
the  investigation  of  which  would  lead  us  too  far  from  the  study 
of  the  Blue  Hills  Complex. 

The  Joint  Structitre  of  the  Hornhlendic  jSFormal  Granite. 
—  The  numerous  lai-ge  and  deep  quarries  in  the  hornblendic 
normal    granite  afford    ample  opportunity  to  study  the   joint 
structure  of  this  rock  ;  and  in  many  cases  it  is  the  most  impres- 
sive feature  of   the  quarries.       The  two    principal    classes  of 
joints  commonly  occurring  in  granite  are  typically  developed 
in  the  Quincy  quarries.      First,  the  parallel  and  intersecting 
joints,  which    are    usually  approximately  vertical,   and,  trav- 
ersing the  rock  in  two  or  more  directions  or  systems,  divide  it 
into    blocks   (block  jointing)  ;    although  one  system  may  be 
locally  more  strongly  developed  than  the  others,   giving  rise 
to  a  vertical  sheet  jointing,  of  which  we  have  noted  an  excep- 
tionally perfect  example  in  the  seam-face  granite  of  Hingham 
and  Wevmouth.     The  joints  of  this  class  are  undoubtedly  due 
chiefly  to    torsional    and    vibi-atory  movements  in   the  earth's 
crust,  and  may  be  regarded  in  general  as  earthquake  fractures. 
In  many  cases  subsequent  slipping    or  faulting   has  occurred 
along  these  planes,  developing  striated  and  polished  or  slicken- 
sided  surfaces  and  changing  a  simple  joint  to  a  master  joint. 
No  finer  examples  of  master  joints  or  shear  planes,  as  they  may 
be  called,  can  be  found  than  arc  presented  in  the  glistening  and 
almost  absolutely  plane  walls  of  some  of  the  quan-ies.      Second- 
ly, the  horizontal  joints,  or  more  accurately  the  joints  a])proxi- 


339 

mately  parallel  to  the  surface  of  the  ground.  'I'hcse  are  com- 
monly most  marked  in  the  superficial  portions  of  the  quarries, 
gradually  becoming  fewer  and  less  distinct  downward,  and 
giving  our  granite  hills  and  ledges  a  structure  comparable 
with  that  of  a  cabbage  or  an  onion.  The  joints  of  this  class 
are  especially  helpful  in  the  quarrying  of  granite,  since  they 
usually  obviate  the  necessity  of  undercutting.  In  the  opinion 
of  the  writer,  they  are  best  explained  as  due  to  superficial, 
diurnal  and  seasonal,  alternations  of  temperature,  being  analo- 
gous to  the  rifting  or  exfoliation  of  granite  ledges  and  blocks 
when  exposed  to  fire. 

Following  are  some  notes  on  a  few  of  the  more  perfect  or 
striking  examples  of  jointing  in  the  Quincy  quarries,  beginning 
with  the  West  Quincy  section.  A  marked  shear  and  probable 
fault  plane  of  the  Granite  Railway  Quarry  trends  N.  80°  W. 
and  hades  S.  25°,  directly  toward  the  large  dike  forming  the 
south  wall  of  the  quarry,  the  two  fissures  converging  down- 
ward. There  are  other  shear  planes  parallel  to  the  main  one. 
Massive  block  jointing  prevails  in  this  quarry,  as  indicated  by 
the  large  prismatic  block  or  monolith  lying  on  the  southeast 
side  of  the  quarry.  In  the  quarries  east  of  this  the  vertical 
master  joints  are  well  developed,  trending  in  both  north-south 
and  east-west  directions.  The  vertical  jointing  is  also  very 
fine  in  the  Townsend  and  Clements  Quarry  to  the  westward, 
with  only  traces  of  horizontal  sheet  jointing.  But  the  quarry 
immediately  south  of  this  is  a  superb  example  of  horizontal 
sheet  jointing,  the  sheets  varying  from  3  to  8  feet  in  thickness. 
In  the  Jones  and  Desmond  Quarry,  one  third  of  a  mile  further 
west,  the  horizontal  jointing  is  again  very  perfectly  developed, 
the  layers  of  granite  varying  downward  from  2  to  6  feet ;  and 
the  same  statement  may  be  repeated  for  several  other  quarries 
in  this  section.  In  the  quarry  on  the  west  side  of  Willard 
Street,  north  of  West  Quincy  Station,  the  vertical  joints  are 
strongly  marked,  the  principal  system  having  a  north-south 
trend. 


340 

Passing  to  North  Common  Hill,  we  find  that  the  east  and 
west  ends  of  the  Craig  and  Richards  Quarry  are  very  continuous 
and  perfect  and  very  rusty  vertical  joint  faces,  exactly  parallel 
with  the  small  dike  crossing  the  middle  of  the  quarry.  In  the 
quarry  next  west  of  this,  on  the  highest  part  of  the  hill,  the 
jointing  is  similar  —  north-south,  vertical,  and  very  perfect. 
The  lai-ge  and  deep  Churchill  Quarry  is  one  of  the  most 
impressive  examples  of  horizontal  sheet  jointing  in  the  district 
/,  (PI. -14)  ;  and  its  broad  northwest  wall  is  one  continuous 
vertical  master  joint.  In  the  quarries  south  of  this  are  many 
superb  examples  of  parallel  and  vertical  master  joints  and 
sheeted  zones,  which  are  often  remarkably  plane  and  con- 
tinuous ;  and  although  they  trend  in  all  directions,  the  prevail- 
ing trends,  as  throughout  the  quarry  district,  are  approximately 
north-south  or  east-west,  agreeing  in  this  respect  with  the 
dikes.  The  horizontal  sheet  jointing  is  well  developed  in 
nearly  all  of  the  quarries  on  this  hill. 

The  only  large  quarry  east  of  North  Common  Hill  is  Echo 
Quarry  at  the  eastern  base  of  Payne  Hill.  Some  of  the  joints 
here  are  wonderfully  perfect,  especially  the  slickensided,  sloping, 
east-west  joints  ;  and  one  of  these,  forming  part  of  the  north 
side  of  the  quarry,  is  a  magnificent  example  of  a  polished  and 
almost  absolutely  true  gi-anite  wall. 

Relations  of  the  Two  Types  of  Norynal  Granite.  —  The 
facts  bearing  upon  the  relations  of  the  hornblendic  normal 
granite  to  the  biotitic  normal  o-ranite  are  rather  meaijer  in  the 
region  of  the  Blue  Hills  Complex.  From  the  western  end  of 
the  Complex  as  far  east  as  Great  Pond  in  Braintree  the  two 
granitic  areas  are  separated  by  the  broad  trough  of  Carbonifer- 
ous strata  ;  and  farther  east  by  the  Cambrian  strata  and  drift 
deposits  of  the  Monatiquot  Valley  ;  while  still  farther  east,  in 
Weymouth,  the  hornblendic  normal  granite  is  Avanting,  being- 
covered  by  the  contact  zone  of  fine  granite.  But  the  biotitic 
normal  granite  of  Weymouth  seems  gradually  to  api)roxiniate 
in  character  to  the  hornblendic  type  as  we  advance  northward 


341 

toward  tlie  boundaiy  fault,  which  is  probably  the  only  real 
line  of  demarcation  between  the  two  o^ranites.  This  irreat  dis- 
placement  passes  under  the  north  end  of  King  Oak  Hill  and  is 
very  clearly  exposed  on  the  railroad  at  the  northeast  base  of 
the  hill,  where  the  Cambrian  slates  lie  asrainst  the  o-ranite. 
The  granite  is  here  greatly  crushed  and  profoundly  altered,  the 
dark  constituents  being  completely  chloritized.  But  the  red- 
dish color  and  general  aspect  indicate  that  it  was  originally 
mainly  a  hornblendic  granite. 

Farther  south,  in  the  railroad  cut  at  East  Weymouth  Station, 
the  granite  is  fresh  and  gray,  and  undoubtedly  mainly  biotitic, 
and  the  original  hornblende  has  in  general  suffered  the  chloritic 
alteration  ;  while  the  rock  shows  no  signs  of  reddening,  not 
even  at  the  surface.  Still  farther  south  the  evidence  is  of  the 
same  general  character,  and  the  two  normal  granites  are  prob- 
ably blended  through  a  zone  a  mile  or  so  in  width. 

Rocks  Forming  the  Contact  Zone  of  the  Batholite. 
Diorite  and  Basic  Granite  (  Granodiorite) .  — Although  the 
diorite  is  a  very  important  factor  in  the  geology  of  the  country 
north,  west,  and  south  of  the  Boston  Basin,  it  is  only  very 
scantily  developed  in  the  area  under  consideration.  In  fact, 
neither  in  the  Blue  Hills  proper,  nor  anywhere  strictly  within 
the  limits  of  the  Blue  Hills  Complex,  has  any  diorite  been 
observed  ;  but  this  rock  is  closely  associated,  as  previously 
noted,  with  the  biotitic  normal  granite,  which  lies  wholly  to 
the  south  of  the  Blue  Hills  Complex,  in  a  broad  band  sepa- 
ratino;  the  Boston  and  JSTarrao-ansett  Basins. 

A  general  survey  of  this  area  discloses  the  fact  that  the 
diorite  occurs  in  irregular  patches,  remnants  of  a  once  con- 
tinuous contact  zone,  and  that  these  patches  can,  with  a  few 
exceptions,  be  referred  to  a  discontinuous  belt  from  one  to  two 
miles  wide  near  the  northei'n  edo;e  of  the  o-ranite,  the  oi-anite  to 
the  southward  representing  a  swell  or  uplift  over  which  erosion 
has  cut  deeply  enough  to  remove  the  diorite  cover.  Com- 
mencing   at    the  northeastern  end  of   the   belt,  the  diorite  is 


342 

prominently  developed  in  Cohasset  about  the  Harbor  and  Little 
Harbor,  and  farther  west  north  of  the  railroad.  It  crosses  the 
railroad  as  a  well-defined  belt  north  of  Turkey  Hills,  and 
recrosses  it  at  Fort  Hill  in  West  Hingham  (see  sheet  1  of  the 
general  map).  Along  the  line  of  Weymouth  Back  River  the 
diorite  belt  is  shifted  to  the  south  two  miles,  reappearing  in 
very  typical  form  on  the  east  side  of  Whitman  Pond,  in  Wey- 
mouth. A  very  minor  belt  extends  from  the  vicinity  of  King 
Oak  Hill  to  Mt.  Pleasant,  south  of  Commercial  Street.  No 
important  outcrops  have  been  noted  in  Braintree  or  Holbrook, 
but  after  a  gap  of  five  miles,  and  beyond  the  boundary  of  the 
map,  the  main  belt  is  resumed  in  Randolph,  north  and  west  of 
the  village,  whence  it  extends  continuously,  but  much  mixed 
with  the  granite,  into  Stoughton,  where  it  is  two  miles  wide. 
There  are  extensive  outcrops  of  diorite  south  of  Rattlesnake 
Hill  in  the  eastern  part  of  Sharon  ;  and  from  Canton  village 
southwest  over  the  Moose  Hill  Range  to  Neponset  Reservoir, 
and  thence  across  the  north  part  of  Foxborough,  diorite  is  the 
prevailing  rock.  Important  outcrops  have  also  been  observed 
south  of  the  main  belt,  in  Avon  and  the  western  part  of 
Brockton . 

While  the  larger  patches  of  diorite  are,  doubtless,  true 
erosion  remnants  of  a  once  continuous  sheet  covering  the  gran- 
ite, the  smaller  ones  should  be  regarded  mainly  as  inclusions 
in  the  granite  due  to  the  cracking  and  shattering  of  the  diorite 
before  the  solidification  of  the  granite,  and  the  still  smaller 
ones  of  rounded  outline  as  segregations  from  the  granite 
magma,  these  only  as  a  rule  occuri'ing  widely  separated  from 
the  main  bodies  of  diorite. 

The  diorite  outcrops  of  Weymouth  lie  wholly  south  of  the 
great  boundary  fault  of  the  Complex  ;  but,  coming  within  the 
area  of  the  general  map,  they  may  be  briefly  described  here. 
The  principal  and  most  interesting  outcrop  is  that  in  the  woods 
on  the  east  side  of  Whitman  Pond.  This  is  in  every  respect  a 
typical  occurrence.     The  diorite,  in  bold  and  massive  ledges, 


343 

forms  a  compact  mass  extending  east  from  the  shore  of  the 
pond  about  one-fourth  mile,  with  a  maximum  width  one  half 
as  great.  Throughout  this  area  there  are  occasional  nari'ow 
and  highly  irregular  branching  dikes  and  dikelets  of  granite. 
These  increase  in  number  toward  the  border  of  the  diorite, 
until  they  form  at  some  points  a  complete  net-work,  and  the 
dikes  are  lost  in  a  breccia,  the  o-ranite  beino;  for  a  breadth  of 
several  or  many  yards  crowded  with  angular  fragments  of 
diorite.  The  relation  of  the  two  rocks  is  identical  with  that 
so  clearly  exposed  at  many  points  on  the  shores  of  Cohasset, 
Swampscott,  Marblehead,  etc.  A  small  exposure  of  this 
character  may  also  be  seen  on  Washington  Street,  Weymouth, 
a  short  distance  west  of  the  Town  Hall.  In  the  basic  granite 
on  the  northern  slope  of  Mt.  Pleasant  are  several  large  and 
many  small  bodies  of  diorite  ;  and  similar  but  more  obscure 
occurrences  extend  eastward  south  of  Commercial  Street,  as 
already  noted,  toward  King  Oak  Hill.  As  in  Cohasset  and 
Hingham,  and  throughout  the  entire  region,  the  structural 
relations  of  the  diorite  and  granite  are  always  essentially  the 
same  ;  that  is,  it  is  always  the  granite  that  cuts  and  incloses 
the  diorite  and  never  the  reverse  ;  and  hence  it  follows  that  the 
diorite  is  always  clearly  the  older  rock. 

The  diorite  is  a  typical  plutonic  rock  of  mildly  basic  com- 
position and  hypidiomorphic  holocrystalline  texture.  Mr. 
White  notes  that  it  contains  well-twinned  plagioclase  feldspars  ; 
and,  unlike  the  enclosing  granite,  is,  in  typical  examples, 
entirely  quartzless.  A  secondary  greatly  strained  and  bent 
hornblende  occurs  all  through  the  rock  and  is  its  most  prom- 
inent feature.  There  is  also  an  original  brown  hornblende ; 
but  this,  in  most  cases,  is  so  bleached  as  to  be  easily  mistaken 
for  a  variety  of  pyroxene.  Crystallized  apatite  occurs,  and 
secondary  epidote,  often  in  the  form  of  veinlets,  is  a  striking 
and  characteristic  feature  of  the  rock. 

The  basic  granite  or  granodiorite  is  relatively  less  important 
than  in  the  Cohasset  area,  being  little  more  than  a  transition 


344 

phase  between  the  diorite  and  the  normal  granite  ;  and  I  have 
observed  it  only  in  proximity  to  more  or  less  typical  diorite,  as 
east  of  Whitman  Pond  and  on  Mt.  Pleasant.  It  is  usually 
readily  distinguished  from  the  diorite  by  its  quartzose  character 
and  from  the  normal  granite  by  the  predominance  of  the  dark 
constituents —  biotite  and  hornblende,  and  the  frequent  occur- 
rence, as  in  the  diorite,  of  secondary  epidote.  Mr.  White 
says,  "Neither  macroscopically  nor  microscopically  is  any 
definite  demarcation  between  this  granite  and  the  diorite  per- 
ceptible. The  diorite  seems  simply  to  develop  wherever  the 
orthoclase  of  the  granite  diminishes,  and  the  chloritic  products 
lend  a  green  color  to  such  masses."  Mr.  White  further  notes 
that  the  hornblende  is  very  pleochroic,  and  with  the  prismatic 
cleavage  strongly  developed.  The  biotite  is  idiomorphic  when 
alone  and  not  broken  by  strains.  When  with  hornblende  the 
borders  are  irregular,  probably  due  to  resorption  into  the 
magma  for  the  benefit  of  the  hornblende.  The  biotite  con- 
tains various  minerals  between  its  leaves.  The  most  prominent 
of  these  is  rutile  ;  but  small  crystals  of  apatite  also  occur. 
Magnetite  is  plentiful,  with  some  ilmenite  and  secondary 
limonite  in  the  cracks. 

Relations  of  the  Diorite  to  the  Hiotitic  Normal  Granite. 
—  Although  its  structural  relations  show  that  the  diorite  is 
always  older  than  the  inclosing  granite,  yet  the  gradual  pas- 
sage through  the  granodiorite  to  the  biotitic  normal  granite 
which  the  larger  masses  exhibit  proves  that  the  diorite  is,  after 
all,  but  a  more  basic  phase  of  the  granite  of  somewhat  earlier 
solidification,  as  both  of  the  theories  of  magmatic  differentia- 
tion stated  in  the  preceding  pages  would  require  ;  and  in  its 
earlier  crystallization  alone,  therefore,  we  have  ample  evidence 
that  its  normal  position  relatively  to  the  granite  is  peripheral. 

The  minor  inclusions  of  diorite  especially  are  commonly  very 
angular,  and  shar[)ly  contrasted  with  the  enclosing  rock  ;  and 
these  are  undoubtedly  due  chiefly,  as  previously  noted,  to  the 
cracking  and  shatterinjy  of  a  once  continuous  zone  of  diorite 


345 

and  large  segregations  of  cliorite,  prior  to  the  solidification  of 
the  underlying  granite.  That  the  diorite  was  very  slowly  and 
gradually  injected  and  inclosed  by  the  viscous  magma  is  proved 
by  the  fact  that  adjacent  fragments  often  retain  tiieir  original 
orientation,  and  would  undoubtedly  fit  accurately  against  each 
other  if  the  inclosing  granite  were  removed.  Concerning  the 
more  scatterino-  and  isolated  inclusions  of  diorite  occurring  at  a 
distance  from  a  main  body  of  that  rock,  two  views  are  possible  : 
first,  they  are  simply  mechanical  inclusions  more  widely  sep- 
arated from  the  parent  mass  ;  or,  secondly,  they  are  segrega- 
tions from  the  inclosing  granite. 

Both  theories  are  required  to  explain  all  the  facts,  and  the 
best  criteria  for  determining  their  applicability  in  particular 
cases  are  form  and  diversity.  When  the  inclusions  are  sharply 
angular  in  form  or  diversified  in  texture  and  composition,  the 
first  explanation  is  plainly  indicated ;  but  when  they  are 
rounded  in  form,  with  frayed  or  more  or  less  indefinite  borders 
or  halos,  and  of  rather  uniform  character,  the  second  explana- 
tion seems  to  be  demanded.  On  testing  these  rival  explanations 
in  the  field,  I  find  that  the  mechanical  hypothesis  is  most  satis- 
factory as  a  rule,  and  almost  invariably  where  the  inclusions 
are  somewhat  crowded  or  a  conspicuous  feature  of  the  granite. 
As  an  example,  I  may  quote  from  my  note-book  a  description 
of  the  small  ledge  of  granite  on  Washington  Street,  Weymouth, 
west  of  the  Town  House.  The  diorite  inclusions  are  sharply 
defined,  in  part  distinctly  and  entirely  angular,  and  without 
perceptible  halos.  They  are  also  varied  in  texture  and  com- 
position, being  normally  and  evenly  crystalline,  porphyritic 
with  large  feldspars,  or  with  prominent  slender  hornblendes. 
Adjacent  and  even  contiguous  inclusions  may  be  strongly  con- 
trasted ;  and  one  inclusion  is  normally  crystalline  on  one  side 
and  porphyritic  on  the  other.  Evidently  these  inclusions  are 
not  indigenous  in  the  immediately  inclosing  granite  ;  but  they 
have  been  assembled  from  various  sources  as  hard  and  angular 
masses  while  the  granite  was  still  plastic. 


346 

It  can  hardly  be  doubted  that  the  true  segregations  of  diorite 
in  the  granite  were  all  formed  before  the  final  or  complete 
crystallization  of  the  inclosing  rock  ;  but  we  may  properly 
distinguish  between  those  formed  before  the  granite  magma 
had  become  too  stiffly  viscous  to  permit  their  displacement  and 
association  with  masses  of  diverse  origin,  and  those  formed  at 
a  later  period  when  the  magma  had  developed  a  good  degree 
of  rigidity.  The  latter  only  can  be  expected  to  exhibit  definite 
halos,  that  is,  bordering  zones  of  granite  relatively  depleted  of 
the  dark  constituents,  and  hence  lighter  colored  than  either  the 
segregation  within  or  the  normal  granite  without.  The  finest 
examples  of  the  latter  type  which  I  have  observed  occur  in 
some  parts  of  the  seam-face  granite,  in  both  Hingham  and 
Weymouth.  They  were  first  observed  and  appear  to  be  most 
abundant  in  the  most  easterly  quarry  on  the  south  side  of 
Whiting  Street  in  Hingham,  and  about  one-half  mile  east  of 
the  Weymouth  line.  They  also  occur  of  essentially  the  same 
character  in  the  Gilbreth  Quarry,  east  of  Pleasant  Street,  in 
Weymouth.  The  segregations  proper  are  quite  dai-k,  spherical, 
sharply  defined,  from  half  an  inch  to  an  inch  in  diameter,  being 
in  the  main  of  remarkably  uniform  size,  and  distributed  some- 
what sparingly  but  rather  evenly  through  the  granite,  adjacent 
segregations  being  usually  from  a  foot  to  a  yard  apart.  The 
pale  halo  varies  with  the  size  of  the  segregation  from  half  an 
inch  to  an  inch  in  breadth,  and  blends  externally  with  the 
normal  gray  of  the  granite.  Both  the  segregation  and  its 
halo  are  identical  in  composition  and  texture,  apparently,  save 
that  the  dark  constituents  have  been  removed  from  the  halo 
and  concentrated  in  the  segregation  ;  and  it  is  obvious  that  if 
the  dark  minerals  of  the  segregation  were  evenly  disseminated 
through  both  the  segregation  and  aureole,  both  phases  of  the 
segregation  would  be  effaced,  the  whole  becoming  indis- 
tinguishable from  the  inclosing  gray  granite. 

Where  the  magma  was  more  plastic,  molecular  flow  from 
without  inwards  seems  to  have  prevented  the  development  of  a 


347 

halo  or  zone  of  depletion  ;  and  the  segregations,  varying  in 
size  from  mere  specks  or  shreds  to  masses  several  inches  or 
even  several  feet  in  diameter,  are  characteristically  irregular 
and  indefinite  in  outline,  presenting  frayed  margins  which 
blend  almost  imperceptibly  with  the  inclosing  granite. 

Fine  Granite.  —  Although  this  name  appears  more  trivial 
and  less  desci'iptive  than  the  hornblende  granitite  employed 
by  Mr.  White  for  the  same  rock,  the  fineness  of  this  granite  is 
at  once  its  most  distinctive,  constant,  and  significant  character ; 
and  the  designation  j^ne  granite  will,  therefore,  best  serve  our 
present  purpose.  It  is  important  to  note,  however,  that  while, 
as  regards  composition  and  general  macroscopic  and  micro- 
scopic characters,  the  fine  granite  is,  perhaps,  quite  as  homo- 
geneous as  any  of  the  granitic  types,  yet  it  is  genetically  and 
structurally  composite.  It  occurs  chiefly  as  a  contact  zone 
bordering;  or  coverino-  the  normal  hornblendic  o-ranite ;  but 
partly,  also,  as  dikes  cutting  through  the  normal  granite,  both 
hornblendic  and  biotitic.  In  the  first  relation  the  fine  granite 
is  necessarily  older  than  the  normal  granite,  and  in  the  second 
relation  it  is  obviously  newer.  Where  occurring  as  a  contact 
zone  the  fine  OTanite  holds  the  same  general  o-enetic  and  struc- 
tural  relations  to  the  hornblendic  normal  granite  that  the  basic 
granite  and  diorite  hold  to  the  biotitic  normal  granite.  It  is 
then,  also,  of  relatively  basic  composition,  being  poorer  in 
quartz,  and,  as  a  rule,  richer  in  the  dark  silicates  (chiefly 
hornblende)  than  the  normal  granite.  The  fine  granite  of 
intrusive  habit,  on  the  other  hand,  is  of  relatively  acid  char- 
acter, containing  but  little  hornblende  or  other  dark  silicate, 
and  sometimes  passing  thi'ough  micro-granite  to  quartz  poi"- 
phyry,  with  the  quartz  partly  in  compact  blebs  or  crystals  and 
clearly  belonging  to  the  first  crystallization.  It  is  proposed  to 
discuss  here  onlv  the  fine  OTanite  occurring  as  a  contact  zone, 
reserving  the  intrusive  phase  or  dikes  of  fine  granite  for  a  later 
section. 

The  theory  that  the  biotitic  normal  granite  has  been  formed 


348 

under  an  exceptionally  thick  cover  of  overlying  formations 
affords  a  ready  and  natural  explanation  of  the  relatively  basic 
and  crystalline  character  of  its  outer  zone  of  basic  granite  and 
diorite.  The  material  remained  molten  long  enough  or  solidi- 
fied slowly  enough  to  permit  a  marked  magmatic  differentia- 
tion and  prevent  the  development  of  a  marked  contrast  in 
texture  betw^een  the  contact  zone  of  diorite  and  basic  granite 
and  the  normal  granite  within.  But  over  the  hornblendic  nor- 
mal granite,  formed  at  relatively  higher  levels  in  the  crust, 
these  relations  were  reversed.  In  other  words,  we  find  that 
over  the  biotitic  granite  the  original  contact  zone,  so  far  as  it 
has  survived  erosion,  is  contrasted  with  the  normal  granite  in 
composition  much  more  than  in  texture  ;  while  over  the  horn- 
blendic granite  the  opposite  relation  holds  between  the  normal 
type  and  its  contact  zone,  the  chief  contrast  being  textural,  — 
a  more  rapid  refrigeration  having  prevented  alike  marked  mag- 
matic differentiation  and  coarse  crystallization.  The  diorite  is 
areally  much  more  fragmentary  and  isolaterl  than  the  fine 
granite,  simply  because  it  is  on  the  upthrow  side  of  the  great 
boundary  fault,  erosion  having  cut  deeply  enough  here  to 
remove  not  only  the  last  vestige  of  the  Cambrian  strata,  but 
also  the  main  part  of  the  contact  zone  beneath  them. 

As  shown  on  the  map,  the  chief  developments  of  the  fine 
granite  as  a  contact  zone  are  in  Braintree,  north  and  south  of 
the  Monatiquot  Valley  belt  of  the  Cambrian  slates,  in  eastern 
Quincy,  and  north  of  Commercial  Street  and  King  Oak  Hill 
in  Weymouth ;  that  is,  in  that  part  of  the  Blue  Hills  Complex 
east  of  the  Blue  Hills  proper,  with  minor  developments  extend- 
ing interruptedly  westward  across  the  Pine  Hill  area  and  along 
the  north  side  of  the  Complex  into  East  Milton  and  the  valley 
of  Pine  Tree  Brook  as  far  as  the  Cambrian  slates  can  be  traced. 
In  fact,  every  considerable  body  of  the  Cambrian  strata  is 
bordered  by  fine  granite,  which  thus  forms  a  nearly  universal 
contact  sheet  or  zone  separating  the  Cambrian  sediments  and 
the  normal  granite.     In  the  Pine  Hill  district  and  generally 


349 

throughout  the  Blue  Hills  west  of  Willard  Street,  the  iinme- 
diate  contact  zone  or  border  of  the  Cambrian  slate  is  the  quartz 
porphyry,  which  passes  gradually  outward  through  fine  granite 
to  the  normal  granite  ;  and  in  the  southern  half  of  the  Pine 
Hill  district,  and  over  the  main  range  of  the  Blue  Hills,  east 
of  Randolph  Avenue,  where  the  sedimentary  rocks  are  now 
wholly  wanting,  the  fine  granite  continues  as  a  persistent  zone 
of  passage,  often  of  very  limited  thickness,  between  the  normal 
granite  below  and  quartz  porphyry  above.  Wherever  the 
Cambrian  strata  are  immediately  bordered  by  the  coarse  or 
normal  granite,  a  reasonable  explanation  may  be  found  either 
in  the  small  volume  of  the  slate  inclusion,  which  failed  in  con- 
sequence to  exert  an  appreciable  chilling  influence  upon  the 
granite  magma,  as  on  the  north  side  of  North  Common  Hill, 
or  in  a  fault,  as  at  the  extreme  west  end  of  the  Ruggles  Creek 
belt  of  slate. 

The  normal  thickness  of  the  contact  zone  of  fine  granite  has 
not  been  satisfactorily  determined.  It  probably  varies  greatly, 
—  from  fifty  feet  or  less  to  hundreds  of  feet,  being,  we  may 
suppose,  roughly  proportional  to  the  volume  of  the  bordering 
slate  as  it  existed  during  the  development  of  the  Complex. 
The  broad  areas  of  fine  granite  shown  on  the  map,  such  as  the 
Wyman  Hill  area  between  Monatiquot  Eiver  and  Hayward's 
Creek,  do  not  necessarily  indicate  exceptional  thicknesses. 
They  are  rather  to  be  regarded  as  approximately  horizontal 
portions  of  the  contact  zone,  —  rounded  summits  or  domes  of 
the  granite  batholite  from  which  the  original  covering  of  sedi- 
mentary rocks  has  been  worn  away,  but  leaving  the  much 
harder  contact  zone  almost  intact. 

Under  the  name  of  hornblende  granitite,  Mr.  White 
describes  this  rock  as  composed  largely  of  white  or  pink  feld- 
spar with  scattered  grains  of  dark  silicates  and  little  quartz. 
There  is  no  plagioclase  ;  and  the  orthoclase,  which  shows  no 
twinnino;,  is  shot  through  with  innumerable  acicular  crystals 
of  the  blue   hornblende  which    characterizes    the    hornblendic 


350 

normal  granite.  The  needles  gathered  into  compact  clusters 
also  form  masses  by  themselves  among  the  other  minerals. 
The  biotite,  in  small  brown  flakes,  is  quite  abundantly  scat- 
tered through  the  rock,  often  lying  directly  adjacent  to  the 
hornblende  masses.     ISTo    accessory   minerals    were    observed. 

The  inspection  of  many  specimens  of  the  fine  granite  from 
all  parts  of  the  field  has  convinced  the  writer  that  biotite  is 
rather  exceptionally  an  important  constituent  and  that  this  rock 
is  in  general  quite  closely  related  in  composition  to  the  horn- 
blendic  normal  granite,  although  usually,  as  previously  noted, 
of  more  basic  character,  as  indicated  by  the  larger  proportion 
of  the  dark  constituents,  chiefly  hornblende.  In  fact,  the  fine 
granite  undoubtedly  exhibits,  as  the  theory  requires,  a  dual 
relation,  passing  gradually  downward  into  the  normal  horn- 
blendic  granite  and  horizontally  into  the  diorite.  Toward  the 
south  and  east  it  certainly  becomes,  on  the  r/hole,  gradually 
darker  and  less  quartzose,  seeming  to  shade  into  the  diorite  as 
the  hornblendic  normal  granite  shades  into  the  biotitic  normal 
granite  ;  and  thus  is  explained  the  fact  that  no  contact  or  line 
of  demarcation  has  been  observed  between  the  two  normal 
granites  or  their  respective  contact  zones. 

This  general  gradation  in  the  composition  of  the  fine  granite 
must  appeal  to  any  one  who  goes  systematically  over  the  field. 
In  the  southern  part  of  the  Pine  Hill  district  and  over  the  main 
range  of  the  Blue  Hills,  or  where  the  fine  granite  separates  the 
normal  granite  and  quartz  porphyry,  it  is,  apparently,  identi- 
cal with  them  in  composition  and  combines  their  textural 
features,  being  distinctly  holocrystalline  like  the  former  and 
porphyritic  like  the  latter,  the  quartz  occurring  chiefly  in 
rounded  grains,  which  clearly  belong  to  the  first  crystallization. 
North  and  w^est  of  Payni3  Hill  the  fine  granite  is  usually 
a  light-colored  rock,  containing  but  little  hornblende  or  other 
dark  constituent,  and  consisting,  like  the  hornblendic  normal 
granite,  chiefly  of  orthoclase  and  quartz  ;  and  the  feldspar, 
even  more  genei'ally  than  in  the  normal  granite,  is  pink  or  red 


351 


in  the  zone  of  oxidation,  the  smaller  size  of  the  crystals  natur- 
ally favoring  the  more  rapid  and   complete   oxidation   of  the 
ferrous  iron.     East  and  south  of  Payne  Hill,   and  especially 
in  the  district  embracing  Hay  ward's  Creek  and  Wyman  Hill, 
the  fine  granite  is  on  the  whole  of  a  decidedly  more  basic  aspect. 
The  hornblende  with  some  biotite  is  more  prominent,  and  the 
red  color  is  much  less  characteristic  and  where  developed  less 
marked  and  more  superficial.      Some  more  or   less  important 
exceptions  to  this  generalization  have  been  noted  :   but  these 
occur  chiefly  near  the  contact  with  the  Cambrian  slate  or  where 
the  fine  granite  forms  apophyses  (dikes)  in  the  slate,  the  more 
acid  and  ruddy  phase  being  due  then,  probably,  to  the  mao-ma 
having  solidified  too  rapidly  to  permit  appreciable  differentia- 
tion.     Still  farther  east,  in  Weymouth,  the  fine  granite  is,  in 
the  main,  of  decidedly  darker  and  7nore  basic  aspect.     Both 
north  and  south  of  the  railroad  much  of  the   fine  granite    is 
poor  in  quartz,  rich  in  hornblende  and  biotite,  with  very  dark 
to  black  feldspars,  weathering  to  a  dull,  dark  brown,  but  not  to 
the  bright  pink  of  Quincy  and  Milton.      Some  of  the  rock  in 
the  vicinity  of   and  south  of  Mill    Cove    especially  is  almost 
black,    recalling   the   black   normal   granite.      Mr.   White  has 
noted  the  basic  character  of  this  rock,  and  Mr.  T.  W.  Vaughan, 
from    whose    unpublished    notes    I    am    permitted    to    quote, 
describes  it  as  chiefly  composed  of  dark  orthoclase  and  plagio- 
clase,  with  little  quartz  and  much  hornblende  and  augite,  and 
accessory  magnetite  and  apatite  ;  and  thus  intermediate  in  char- 
acter between  diorite,  basic  granite,  and  augite  syenite. 

Althouofh  the  orradation  or  blendino-  of  the  contact  zones,  as 
of  the  normal  granites,  is  probably  interrupted  by  the  bound- 
ary fault  as  well  as  concealed  at  most  points  by  the  drift  or 
the  Cambrian  strata,  in  the  ledges  along  Commercial  Street, 
between  Weymouth  and  North  Weymouth,  the  dark  and  basic 
fine  granite  seems  to  shade  into  the  diorite  and  to  have  at  some 
points  a  finely  gneissoid  structure.  On  Mount  Pleasant  the 
fine  granite  is  not  so  dark,  but  blends  perfectly,  apparently, 


352 

with  the  basic  granite  and  cliorite.  And  on  Liberty  Hill,  in 
Braintree,  the  biotitic  normal  granite  forming  the  body  of  the 
hill  and  well  exposed  in  the  little  excavations  on  the  summit  is 
covered  by  a  thin  surface  layer  or  veneering  of  fine  granite 
of  biotitic  and  dioritic  character,  which  is  usually  almost  micro- 
granitic  in  texture  and  distinctly  gneissoid. 

Relations  of  the  Fine  Granite  to  the  Hornhlendic  Normal 
Granite.  —  Although  this  topic  has  received  some  general 
consideration  in  the  preceding  pages,  certain  phases  of  it 
remain  to  be  presented.  We  have  noted  the  normal  position 
of  the  fine  granite  as  a  contact  zone  between  the  hornblendic 
normal  granite  and  the  Cambrian  strata,  in  the  eastern  half  of 
the  Blue  Hills  Complex,  and  its  principal  variations  in  com- 
position. The  variations  in  texture  next  claim  our  attention. 
Along  its  contact  with  the  sedimentary  rocks  the  fine  granite 
is  usually  visibly  finer  grained  for  a  few  inches  or  feet,  and 
forms  in  the  sediments  many  irregular  dikelets  or  apophyses  of 
almost  or  quite  microcrystalline  texture  to  be  described  in 
detail  in  a  later  section,  while  near  the  coarse  or  normal 
granite  it  becomes  perceptibly  coarser,  ar-d  the  two  types  are 
at  some  points  perfectly  blended.  Over  the  main  range  of  the 
Blue  Hills  and  in  the  southern  part  of  the  Pine  Hill  district, 
or  wherever  the  fine  granite  forms  a  contact  zone  between  the 
normal  granite  and  quartz  porphyry,  it  is  hardly  to  be  reck- 
oned as  a  distinct  type,  appearing  as  little  more  than  a  zone  of 
passage.  But  in  the  eastern  part  of  the  Complex,  and  gener- 
ally where  directly  boi-dering  the  Cambrian  slates,  it  is  not  to 
be  regarded  simply  as  a  gradation,  a  mere  selvage  of  the 
normal  granite  ;  for  it  is  throughout  the  main  part  of  its  thick- 
ness of  remarkably  uniform  texture,  becoming  rather  rapidly 
finer  near  the  slate  and  rapidly  coarser  near  the  normal  granite. 

The  entire  granitic  series  presents,  perhaps,  no  more  remark- 
able fact  than  the  uniformity  of  texture  through  thousands  of 
feet  and  possibly  miles  of  thickness  of  the  normal  granite, 
with,  as  we  approach  the  surface  of  tlie  batholite,  a  sudden  but 


gnidiial  passage  to  the  fine  granite,  which,  in  turn,  often 
exhibits  a  like  uniformity  of  texture  tlu'ough  scores  or  hundreds 
of  feet,  or  ahuost  to  the  extreme  hmits  of  the  batholite. 

That  the  fine  granite  is  not  a  later  intrusion  between  the 
normal  granite  and  slate,  or  the  normal  granite  and  quartz 
porphyry,  is  proved  by  its  gradual  passage  into  both  the  last 
named  rocks,  and  by  the  fact  that,  near  the  junction  of  the 
normal  orranite  and  fine  oTanite,  fragments  of  the  latter  are 
sometimes  inclosed  in  the  former  and  dikelets  of  the  normal 
granite  also  intersect  the  fine  granite,  showing  tliat  the  nor- 
mal granite  was  still  plastic  after  the  solidification  of  the  fine 
granite.  The  clearest  instance  of  the  inclosure  of  typical  fine 
granite  in  the  normal  granite  is  on  the  north  side  of  Glencoe 
Street  in  Quincy,  just  west  of  the  westerly  extremity  of  the 
Ruggles  Creek  belt  of  slate.  The  contact  of  the  two  granites, 
although  not  clearly  exposed,  is  evidently  somewhat  gradual, 
and  ano'ular  masses  of  the  fine  granite  from  six  inches  to  sev- 
eral  feet  in  diameter  are  inclosed  in  the  normal  granite  near 
the  contact.  Another  instance  occurs  on  Liberty  Hill,  in 
Braintree,  where  the  fine  granite  of  apparently  intermediate 
composition  and  gneissoid  structure  is  inclosed  in  the  biotitic 
normal  granite.  The  inclosures  are  numerous,  angular,  and 
sometimes  of  laro;e  size.  There  is  not  much  indication  here  of 
a  gradual  passage,  but  the  normal  granite  appears  rather  to  be 
eruptive  through  the  fine  granite.  A  few  steps  west  of  Howard 
Street,  Quincy,  on  the  private  road  to  Echo  Quarry,  along  the 
north  side  of  Haywai-d's  Creek,  the  contact  of  the  reddish 
normal  granite  and  the  relatively  basic  (dioritic-looking)  fine 
granite  is  cleai'ly  exposed,  and  is  a  definite  line  without  a  trace 
of  gradation  or  inclosed  fragments.  But  in  the  fine  granite, 
40  to  50  feet  from  the  contact,  are  very  distinct  and  unmis- 
takable dikes  of  the  typical  normal  granite  from  1  to  4  inches 
wide.  On  the  west  side  of  Howard  Street,  south  of  Hay  ward's 
Creek,  is  a  small  quarry  in  the  basic  fine  granite,  and  at  the 
upper  western  edge  of  the  quarry  the  contact  with  the  normal 

OCCAS.  PAPERS  B,  S.  N.  H.  IV.  23. 


354 

granite  is  exposed.  The  exposiu'e  is  not  especially  satisfactory, 
but  sufficient  to  show  that  the  contact  is  sharp  and  without 
sensible  gradation .      The  lichens  coverins^  the  normal   o^ranite 

o  or? 

make  it  difficult,  however,  to  determine  whether  there  are 
inclusions  of  fine  granite  or  not.  Perhaps  the  most  perfect 
exposure  of  this  contact  which  the  district  affords  is  that  on 
and  near  Commercial  Street,  on  the  south  side  of  Payne  Hill. 
It  is  straight  and  definite,  with  no  appreciable  gradation,  and 
inclosed  fragments  appear  to  be  wholly  wanting.  At  all  of 
these  points  the  two  granites  are  firmly  united  in  one  con- 
tinuous and  original  mass  and  with  not  the  slightest  indication 
of  faulting ;  and  yet  the  plane  of  contact  is  in  general  highly 
inclined  or  vertical. 

The  essential  facts,  then,  concerning  the  original  structural 
relations  of  the  normal  and  fine  granites  in  the  eastern  part  of 
the  Complex  are  :  (1)  each  type  is  substantially  homogeneous 
in  texture,  except  very  near  the  contacts  ;  (2)  the  mutual  con- 
tact of  the  two  granites,  although  distinctly  igneous  and  not 
due  to  faulting,  is  usually  either  absolutely  or  approximately 
abrupt  and  often  highly  inclined;  and  (3)  the  normal  granite 
cuts  and  incloses  the  fine  granite  as  clearly  (but  probably  less 
abundantly)  as  the  biotitic  normal  granite  cuts  and  incloses  the 
diorite.  To  account  for  these  facts,  two  views  are  suggested, 
both  of  which  are,  possibly,  required  for  a  complete  explanation. 

First,  starting  with  the  time  when  the  entire  body  of  gran- 
ite was  molten,  the  microcrystalline  border  or  selvage  resulted 
from  contact  with  the  relatively  cool  slate.  Continued  super- 
ficial cooling  gave  rise  to  convection  currents,  which  tended  to 
maintain  vuiiforaiity  of  composition  and  temperature  in  tiie 
magma.  During  the  slow  circulation  of  the  magma  the  more 
basic  and,  under  aqueo-igneous  conditions,  tlie  more  infusible, 
portions,  especially,  crystallized  on  the  inner  surface  of  the 
shell.  Finally,  the  magma  became  so  stiffly  viscous  that  con- 
vection ceased  and  with  it  the  general  process  of  differentiation 
dependent   upon  the   circulation  of  the    magma.       The    great 


355 

residual  body  of  relatively  acid  and  fusible  inaaina  continued 
to  cool  by  .conduction  ;  but  its  centripetal  solidification  did  not 
involve  increasing  size  of  grain,  since  the  coarseness  of  crys- 
tallization was,  probably,  determined  in  greater  measure  by 
the  degree  of  viscosity  than  by  the  rate  of  cooling.  During 
the  development  of  the  relatively  basic  zone  of  fine  granite  the 
convective  circulation  of  the  magma  determined  uniformity  of 
temperature  and  consequently  of  texture.  When  the  circula- 
tion ceased,  the  relatively  acid  magma  residuum  was  probably 
of  approximately  uniform  temperature  ;  and  since  its  subse- 
quent cooling  and  crystallization  must  have  been  centripetal, 
the  heat  evolved  by  the  crystallizing  process  must  have  tended 
to  maintain  the  thermal  equilibrium.  The  real  or  controlling 
factor  in  determining  the  scale  of  the  crystallization  was,  thus, 
the  viscosity  of  the  mao;ma  :  whereas,  with  the  fine  o-ranite  the 
rate  of  cooling  was  probably  the  dominant  factor. 

Second,  the  development  of  the  dioritic  crust  over  the 
biotitic  normal  granite  was  followed  by  extensive  and  com- 
plicated rifting  and  the  injection  and  inclosure  of  the  diorite  by 
the  still  molten  granite  magma,  and  it  is,  a  jyriori,  altogether 
probable  that  the  crust  of  fine  granite  over  the  hornblendic 
normal  granite  was  similarly,  though  less  extensively,  rifted 
and  injected.  Some  direct  proofs  of  this  have  been  cited,  and 
it  may  be  noted  that  the  general  lack  of  contrast  between  the 
two  granites  is  rather  unfavorable  to  the  ready  discovery  of 
such  structural  details.  Anyhow,  we  have,  apparently,  no 
reason  to  doubt  that  the  normal  granite  was  still  plastic  after 
the  solidification  of  the  fine  granite,  and  the  abrupt,  vertical, 
igneous  contacts  of  the  two  rocks  may  be  explained  as  resulting 
from  the  sinking  into  the  unsolidified  normal  granite  of  blocks 
of  the  fine  granite.  A  species  of  faulting,  whereby  fracture 
surfaces  of  the  solidified  crust  or  contact  zone  are  submerged 
in  or  brought  in  contact  with  the  still  unsolidified  magma 
below,  appears  to  the  writer  a  natural  and  essentially  inevi- 
table episode  in  the  development  of  a  great   batholite.     The 


356 

fine  orranite,  althouo-h  solid,  was  still  hot  enouo;h  so  that  it 
could  not  cause  an  immediate  or  sudden  crystallization  of  the 
contiguous  normal  granite  magma  with  its  lower  temperature 
of  solidification,  the  efi^ect  being  like  that  of  a  relatively  small 
and  highly  heated  body  of  slate  in  the  granite,  and  thus  the 
normal  granite  developed  its  normal  texture  right  up  to  the 
contact. 

The  first  explanation  would  yield  contacts  of  the  normal 
and  fine  o-ranite  showino-  some  gradation  in  texture  and  nor- 
mally  horizontal  or  only  moderately  inclined,  conforming  in 
direction,  approximately,  with  the  under  surface,  but  not 
necessarily  with  the  bedding,  of  the  Cambrian  strata.  The 
second  explanation,  on  the  other  hand,  calls  for  abrupt  and,  in 
general,  highly  inclined  contacts.  Inclosed  fragments  of  the 
fine  oranite  in  the  normal  o;ranite  are  consistent  with  either 
type.  Since  the  contacts  of  the  first  type  result  directly  from 
the  refrigeration  of  the  batholites  under  normal  conditions, 
they  may  be  called  the  normal  contacts  ;  while  contacts  of  the 
second  type,  although  due  to  crustal  displacement,  may  be 
best  designated  the  modified  normal,  or  subnormal,  contacts, 
the  true  fault  contacts  being,  of  course,  wholly  secondary  or 
subsequent  to  the  development  of  the  two  types  of  original  con- 
tacts, both  of  which  have  been  recognized  in  the  field. 

The  Payne  Hill  area  of  normal  granite  is  bounded  on  the 
north,  east  and  south,  and  possibly  on  the  west,  by  fine  gran- 
ite. The  contact  is  clearly  exposed  only  at  the  three  points, 
previously  cited,  but  it  belongs  unequivocally  to  the  second  or 
subnormal  type.  We  are  justified,  therefore,  in  regarding  this 
area  of  approximately  a  square  mile,  provisionally,  as  having 
experienced,  after  the  development  of  the  fine  granite  and 
before  the  solidification  of  the  normal  granite,  a  sharply  defined 
elevation  relatively  to  the  contiguous  areas,  whereby  its  cover 
of  fine  granite  was  carried  above  the  present  plane  of  erosion, 
and  a  sharp  line  of  demarcation  developed  between  the  normal 
granite   and    the   borderino;    fine    sfranite.      But    north   of  the 


0&7 

Payne  Hill  area  of  normal  granite,  the  northern  limit  of  which 
is  marked  approximately  by  Quincy  Avenue,  the  contact 
between  the  normal  granite  on  the  west,  as  exposed  in  the 
vicinity  of  Glencoe,  South,  South  Walnut,  and  Union  Streets, 
and  the  fine  granite  on  the  east,  shows  an  appreciable  grada- 
tion, the  zone  of  passage  having  usually  a  breadth  of  10  to 
20  feet.  As  we  should  expect,  the  contact  rises  in  anticlinal 
fashion  between  the  Ruggles  Creek  and  Union  Street  belts  of 
slate  ;  and  the  tongue  of  normal  granite  which  the  map  shows 
extending  eastward  into  the  fine  granite  between  South  and 
South  Walnut  Streets  marks  the  denuded  axis  of  the  anticline, 
which  pitches  gently  eastward  beneath  the  fine  granite,  afford- 
ing a  striking  confirmation  of  our  general  theory  that  the 
cover  of  Cambrian  strata  was  the  controlling'  factor  in  deter- 
mining  the  superficial  structure  of  the  batholite.  The  map 
also  shows  that  the  Pavne  Hill  area  of  normal  o-raiiite  holds  a 
somewhat  similar  but  less  symmetrical  relation  to  the  broad 
band  of  fine  granite  separating  the  Monatiquot  Valley  belt  of 
slate  from  the  narrower  northern  belts.  The  best  conclusion 
seems  to  be  that,  while  the  position  of  the  granite  between  the 
deep  and  narrow  troughs  of  slate  is  essentially  anticlinal  in 
both  cases,  the  contact  of  the  normal  and  fine  granites  is 
strictly  normal  in  the  northei'n  area  and  subnormal  in  the 
southern . 

Porphyritic  Aporhyolite  or  Quartz  Porphyry.  —  The 
aporhyolite  or  felsite  of  the  contact  zone  is  exclusively  of  por- 
phyritic habit :  and  since  phenocrysts  of  quartz  and  feldspar 
are  almost  invariably  associated  in  the  rock,  quartz  porphyry 
is  a  more  definite,  as  well  as  a  more  concise  and  familiar, 
name  than  porphyritic  aporhyolite. 

As  the  diorite  passes  gradually  into  the  fine  granite  on  the 
southeast  margin  of  the  Blue  Hills  Complex,  so  farther  west, 
in  the  Blue  Hills  proper,  the  fine  granite  gradually  gives  way 
to  the  quartz  porphyry.  But  in  the  second  instance  the  facts 
are  less    obscured    by  superficial    deposits,  and    of  the    truly 


358 

gradual  character  of  the  change  in  the  composition  of  the  con- 
tact zone  there  can  be  no  doubt.  It  is  true,  however,  that 
east  of  a  rather  definite  line  the  quartz  porphyry  is  wholly 
wanting,  while  farther  west  it  is  the  prevailing  contact  type. 
This  line  is  most  sharply  defined  at  its  most  eastern  point, 
which  is  marked  by  the  Quincy-Adams  Valley,  between  Payne 
Hill  and  Pine  Hill.  To  the  southward,  the  fine  granite  bor- 
dering the  Monatiquot  Valley  belt  of  slate,  without  associated 
quartz  porphyry,  extends  at  least  a  mile  farther  west  toward 
Braintree  Great  Pond,  and  to  the  northward,  along  the  north- 
ern border  of  the  Complex,  the  contact  zone,  so  far  as  exposed, 
is  composed  exclusively  of  fine  granite  for  more  than  two  miles, 
or  well  into  Milton.  In  other  words,  the  quartz  porphyry  is 
practically  limited  to  the  main  mass  of  the  Blue  Hills  or  the, 
topographically,  more  elevated  part  of  the  Complex.  The 
explanation  of  this  topographic  relation  will  be  considered  in  a 
later  section.  Taking  a  nearer  view  of  the  distribution  of  the 
quartz  porphyry,  we  find  that  it  is  limited  to  the  main  range  of 
the  Blue  Hills,  ending  eastward  in  Pine  Hill,  and  the  immedi- 
ate vicinity  of  the  Pine  Tree  Brook  belt  of  slate.  In  the  last 
named  district  and  in  the  northern  part  of  the  Pine  Hill  area 
it  borders  existing  bodies  of  Cambrian  slate  ;  but  elsewhere,  or 
for  the  entire  length  of  the  main  range,  from  Great  Blue  to 
Pine  Hill,  erosion  has,  apparently,  removed  almost  the  last 
vestige  of  the  Cambrian  cover,  and  the  only  contacts  to  be 
described  are  those  with  the  underlying  granite  and  the  effu- 
sive felsite. 

The  existing  slate  belts  mark  the  lines  along  wliich  the 
Cambrian  strata  extended,  unmelted,  most  deeply  into  the 
batholite  ;  while  the  intervening  areas  and  notably  the  main 
range  of  the  Blue  Hills  must  be  regarded  as  the  portions  of 
the  batholite  which,  by  absorption  or  otherwise,  attained  the 
greatest  elevation  in  tlie  cover  of  sedimentary  rocks.  Natu- 
rally  the  conditions  of  the  refrigeration  of  the  magma  at  its 
lowest  or  deepest  and  highest  or  most  superficial  contacts  were 


359 

somewhat  contrasted.  In  the  more  abyssal  region,  as  we  liave 
ah-eatly  seen,  solidification  must  have  progressed  more  slowly, 
thus  favoring  magmatic  differentiation  :  and  therefore,  follow- 
ing the  analogy  of  the  fine  granite  and  diorite,  we  should 
expect  to  and  do  find  that  the  quartz  porphyry  phase  of  the 
contact  zone  is  less  extensive  and  the  rock  of  more  basic  char- 
acter in  the  vicinity  of  the  existing  slate  contacts  than  else- 
where. 

The  quartz  porphyry  is  clearly  of  most  normal  character 
where  it  has  its  most  extensive  and  continuous  development, 
that  is,  over  the  main  range  of  the  Blue  Hills.  Save  where 
o-radino'  into  the  o-ranite,  it  is  throuo-hout  this  area  of  remarka- 
bly  uniform  character,  matching  in  this  respect  the  granite 
itself,  of  which  it  is  but  the  contact  phase.  It  agrees  closely 
in  color  with  the  granite,  the  prevailing  or  normal  tint  being 
bluish  gray,  varying  locally  as  in  the  granite  to  dark  gray  and 
black,  and  to  brownish,  reddish  and  purplish  hues  where  more 
or  less  oxidized.  On  Green  Street  in  Canton,  southwest  of 
Little  Blue  Hill,  are  ledges  of  a  typical  quartz  porphyry  with 
a  felsitic  ground  mass,  and  of  a  greenish  gray  color  except 
where  stained  irregularly,  by  peroxidation  of  the  iron,  to  a 
deep  red,  giving  the  rock  a  strikingly  blotched  or  conglomer- 
ate aspect.  A  bowlder  of  this  blotched  quartz  porphyry  was 
observed  on  Ponkapog  Hill ;  and  there  is  a  large  bowlder  of 
it,  of  darker  tint  and  bearing  a  most  striking  resemblance  to  a 
coarse  conglomerate,  on  Randolph  Avenue  at  the  junction  of 
High  Street.  The  same  rock  also  forms  ledges  along  the 
southern  base  of  the  hills,  both  east  and  west  of  Randolph 
Avenue ;  and  its  relations  to  the  basal  Carboniferous  con- 
glomerate, to  be  more  fully  described  in  a  later  section,  are 
such  as  to  indicate  that  this  partial  or  mottled  oxidation  of 
the  quartz  porphyry  is  not  recent  but  antedates  the  formation 
of  the  conglomerate.  Mineralogically,  also,  it  is  essentially 
identical  with  the  granite  ;  and  the  chief  contrast  between  the 
two  rocks,  as  before  noted,  is  textural. 


360 

The  quartz  porphyry  is  the  only  member  of  the  granitic 
series  in  which  the  porphyritic  texture  is  well  developed.  The 
usual  explanation  of  this  texture  requires  for  the  sharp  distinc- 
tion of  phenocrysts  and  ground  mass  an  abrupt  change  in  the 
conditions  of  crystallization,  either  a  sudden  increase  in  the 
rate  of  cooling  or  a  sudden  increase  of  pressure,  usually  the 
former.  But  the  geological  relations  of  the  quartz  porphyry 
seem  to  forbid  the  application  of  this  theory  in  explanation  of 
its  texture  ;  for  it  is  neither  intrusive  nor  effusive,  and  as  the 
strictly  sedentary  contact  zone  of  the  deeply  buried  batholite 
can  have  experienced  during  its  solidification  no  sudden  chill- 
ing or  increase  of  pressure,  and  any  pressure  changes  must 
have  been  felt  equally  by  the  immediately  subjacent  granite 
magma.  The  consideration  of  this  difficulty  has  suggested 
to  me  that  the  sudd.f^n  change  in  the  external  conditions  may 
be  dispensed  with,  and  the  porphyritic  texture  explained  on 
the  basis  of  a  slow  and  gradual  change  or  possibly  of  no 
change  at  all  by  taking  account  of  the  increasing  viscosity  of 
the  magma  and  thus  virtually  introducing  the  principle  of 
overcooling  or  supersaturation  ;  and  postulating,  as  of  course 
we  must,  a  somewhat  more  rapid  rate  of  cooling  in  the  super- 
ficial portion  of  the  batholite,  where  the  quartz  porphyry  was 
formed j  than  at  greater  depths,  where  the  granite  Avas  formed. 

At  the  beffinnino;  of  solidification,  when  the  crvstallizino- 
magma  had,  we  will  say,  its  maximum  liquidity,  each  growing 
phenocryst  of  quartz  and  feldspar  was  able  to  derive  material 
from  a  considerable  volume  of  magma.  But  with  continued 
coolins:  the  mao;ma  became  more  and  more  viscous  and  molec- 
ular  flow  more  restricted,  until  finally  the  molecular  flow  was 
unable  to  keep  pace  with  the  rate  of  solidification  and  new 
centers  of  crystallization  were  established  between  the  pheno- 
crysts, and  the  growth  of  the  phenocrysts  was  virtually 
arrested. 

Accordhig  to  this  view,  the  formation  of  the  phenocrysts 
or   the  porphyritic   texture   may  be   quite   independent  of  any 


361 

change  in  the  rate  or  condition  of  cooling,  being  conditioned 
solely  by  the  relation  of  the  rate  of  solidification  (equal  rate 
of  cooling),  which  may  be  absolutely  uniform,  to  the  rate  of 
molecular  flow,  which  varies  inversely  as  the  viscosity,  the 
viscosity  being  determined  chiefly  by  the  temperature  and  the 
proportion  of  water  or  other  mineralizers.  The  main  point  is 
that  the  change  from  centralized  to  difl'used  crystallization, 
from  the  formation  of  the  phenocrysts  to  the  formation  of  the 
gi'ound  mass,  will  be  abrupt.  In  the  granite  this  change  did 
not  occur,  because  solidification  was  so  slow  as  not  to  overtax 
molecular  flow,  and  allotriomorphic  crystallization  was  the 
result.  When  the  change  did  occur  in  the  porphyry,  the 
magma  was  analogous  to  an  over-cooled  liquid  or  supersatu- 
rated solution,  actually  liquid  but  potentially  solid  ;  and  it  is 
easy  to  see  that  the  molecules  would  add  themselves  much 
more  rapidly  to  a  crystal  half  a  millimeter  than  to  one  five 
millimeters  distant ;  and  that  the  new  centers  of  crystallization 
would  be  very  near  together,  A  rapid  solidification  of  the 
ground  mass  or  magma  residuum  is  thus  seen  to  be  probable. 
We  may  safely  assume  that  the  superficial  cooling  of  the  bath- 
olite  went  on  slowly  at  first  and  then  gradually  more  rapidly, 
M^hich  would  be  a  condition  much  more  favorable,  though  not 
strictly  essential,  to  the  development  of  the  porphyritic  tex- 
ture in  situ. 

Since  the  preceding  paragraphs  were  written,  Pirsson^  has 
expressed,  quite  independently,  a  somewhat  analogous  view  of 
the  origin  of  the  phenocrysts  of  deep-seated  igneous  rocks, 
holding  especially  that  the  viscosity  of  the  magma  is  an  impor- 
tant factor  in  the  process  ;  and  this  able  indorsement  inspires 
the  writer  with  increased  confidence  in  the  sedentary  origin  of 
the  quartz  porphyry  of  the  Blue  Hills  Complex. 

Mr.  White  describes  the  quartz  porphyry  as  a  massive  rock 
without  trace  of  flow  structure  and  showing,  macroscopically, 

iL.  V.  Pirsson:  On  the  phenocrysts  of  intrusive  igneous  roclis.    Amer.  journ.  sci., 
TOl.  157,  pp- 271-280. 


362 

a  compact  grayish  blue  ground  mass  with  numerous  irregu- 
larly arranged,  well-banded  phenocrysts  of  flesh-colored  ortho- 
clase,  from  one  or  two  up  to  live  mm.  in  length,  with  more  or 
less  equally  abundant  glassy  smoky  quartz  grains  scattered 
among  them.  Microscopically,  this  ground  mass  is  grano- 
phyric  and  largely  isotropic  with  characteristic  interlacing  lath- 
shaped  feldspars,  arranged  in  fluidal  texture  following  the  out- 
lines of  the  phenocrysts  :  in  part,  also,  it  is  glassy.  Into  this 
ground  mass  are  packed  large  quartz  and  feldspar  phenocrysts. 
The  quartz  is  idiomorphic,  without  inclusions  and  not  fractured, 
and  shows  beautiful  embayments  of  the  ground  mass.  The 
boundaries  are  sharply  outlined,  and  the  hexagonal  sections 
are  frequently  prominent.  The  feldspar  is  practically  all 
monoclinic  (orthoclase),  usually  in  rectangular  crystals  having 
rounded  corners  with  outlines  more  or  less  exactly  defined, 
and  conspicuous  Carlsbad  twinning.  Hornblende,  which 
makes  up  in  extreme  cases  about  one  fourth  of  the  mass  of  the 
rock,  forms  irregular  rounded  and  fractured  dark  green  grains, 
as  well  as  abundant  slender  bluish  transparent  needles  scat- 
tered tlirough  the  ground  mass  and  the  feldspar  phenocrysts, 
agreeing  closely  with  the  hornblende  of  the  normal  granite. 
Magnetite  occurs  sparingly  ;  and  transparent  ciystals  of  titan- 
ite  are  sometimes  abundant. 

A  typical  specimen  of  the  granitoid  quartz  porphyry  forming 
the  summit  of  Great  Blue  Hill  yielded  71.84  per  cent,  of 
silica.^  The  average  of  several  analyses  by  students  in  the 
chemical  department  of  the  Massachusetts  Institute  of  Tech- 
nology of  the  quartz  porphyry  from  the  eastern  part  of  the 
range  shows  :  SiO,,  74.21  ;  ALO3,  12.77  ;  Ye.jO,,  2.51  ;  FeO, 
2.04;  MnO,  trace;  CaO,  0.98;  MgO,  1.04;  Kfi,  5.44; 
Na^O,  2.17. 

The  most  marked  development  of  the  dark  gray  and  black 
quartz  porphyry  is  on  the  southwest  slope  of  Rattlesnake  Hill, 
where   it  is  well   exposed   in   ledges,  and   especially  in  a  mag- 

1  Occas.  papers,  Boston  soc.  nat.  hist.,  vol.  ,3,  ji.  91. 


nificent  cubic  bowlder  some  twenty  feet  in  diameter.  It  ia 
clearly  only  a  local  phase,  holding  essentially  the  same  rela- 
tion to  the  normal  quartz  porphyry  that  tlic  dark  granite  in 
the  quarry  at  the  east  end  of  the  hill  holds  to  the  normal  gray 
granite.  Mr.  White  says  that  the  dark  color  is  due  chiefly, 
as  in  the  granite,  to  the  black  feldspar  ;  and  the  feldspar  owes 
its  color  to  the  fine  needles  of  hornblende  which  aljound 
throughout  the  rock.  The  ground  mass  is  finely  microgra- 
nitic  ;  and  the  phenocrysts  are  mainly  orthoclase  in  handsome 
Carlsbad  twins  or  showing  peripheral  zonal  phases.  Plagio- 
clase  is  less  abundant  and  allotrioraorphic.  Besides  occurring 
in  fine  needles,  hornblende  also  forms  irregular  translucent 
flakes.  Much-crushed,  allotriomorphic  masses  of  bright  green 
pyroxene  were  observed,  and  handsome,  long,  transparent 
crystals  of  sillimanite.  Magnetite  or  menaccanite,  in  irregular 
grains,  occurs  sparingly,  associated  with  the  dark  silicates. 

It  is  possible,  apparently,  to  trace  a  perfect  gradation  from 
the  most  normal  or  acid  quartz  porphyry  to  the  decidedly  basic 
phase  characteristic  of  the  contact  zones  of  the  Pine  Hill  and 
Pine  Tree  Brook  slate  areas,  in  which  quartz  phenocrysts  occur 
but  sparingly  and  are  usually  wholly  wanting.  Macroscop- 
ically,  the  dark  and  fine-grained  ground  mass  of  this  basic 
variety  contrasts  strongly  with  the  large  and  crowded  pheno- 
crysts of  feldspar.  Mr.  White  finds,  however,  by  microscopic 
observation,  that  the  component  minerals  are  the  same  as  for 
the  normal  quartz  porphyry  and  granite,  differing  only  in  the 
proportions,  the  differentiation  originating  in  a  concentration 
of  the  dark  silicates.  Perhaps  the  clearest  instance  of  por- 
phyry intermediate  in  composition  between  the  acid  and  basic 
phases,  is  where  it  borders  the  aporhyolite  on  the  east  side  of 
Pine  Hill.  Especially  we  note  here  fewer  quartz  phenocrysts 
than  in  the  normal  quartz  porphyry  and  the  large  feldspar 
phenocrysts  characteristic  of  the  basic  porphyry.  Following 
the  contact  zone  farther  to  the  westward  around  the  aporhyo- 
lite, on  the  north  and  west  sides  of  Pine   Hill,  it  assumes  the 


364 

normal  acid  phase,  with  small  phenocrysts,  until  we  pass  the 
sharp  re-entrant  angle  at  the  western  base  of  the  hill,  when  it 
rather  suddenly  becomes  ultra-basic  in  character,  with  large 
feldspar  and  no  quartz  phenocrysts.  These  facts  suggest 
again  the  correlation  of  the  composition  with  the  depth  of  the 
contact  zone,  the  porphyry  becoming  more  basic  at  lower  and 
more  acid  at  higher  levels. 

Relations  of  the  Quartz  Porphyry  to  the  HornblencUc 
Normal  Granite.  — We  have  noted  the  gradual  replacement 
of  th^  fine  granite  by  the  qviartz  porphyiy  westward  across  the 
Complex,  the  fine  granite  becoming,  west  of  Pine  Hill,  in  the 
main,  merely  a  transition  phase  between  the  quartz  porphyry 
and  normal  granite  ;  the  gradation  in  the  composition  of  the 
quartz  porphyry  from  the  deeper  to  the  more  superficial  por- 
tions of  the  contact  zone  ;  and  the  essential  identity  in  compo- 
sition and  contrast  in  texture  of  the  quartz  porphyry  and 
granites  ;  and  we  have  now  to  consider  more  in  detail  the 
structural  relations  of  the  quartz  porphyry. 

That  the  quartz  porphyry  normally  overlies  the  granite  is 
proved  by  :  ( 1 )  the  natural  probabilities  of  the  case  as  indi- 
cated by  the  textures  of  the  rocks;  (2)  the  fact  that  in  the 
vicinity  of  the  slate  the  porphyry,  where  developed  at  all, 
invariably  forms  the  immediate  contact  zone,  separating  the 
sedimentary  rocks  from  the  fine  granite;  and  (3)  the  direct 
superposition  of  the  porphyry  on  the  granite,  as  may  be  seen  in 
numerous  sections  throughout  the  hills. 

The  first  proposition  requires  no  further  conwsidei'ation.  The 
second  is  illustrated  by  the  special  map  of  the  Pine  Tree  Brook 
slate  area  (PI.  21).  In  the  Pine  Tree  Brook  area  the  por- 
phyry, as  mapped,  is  very  discontinuous,  owing  in  part,  per- 
haps, to  lack  of  outcrops  ;  but  it  is  always  intimately  related  to 
the  slate  on  one  side  and  the  fine  ^ranite  on  the  other.  The 
most  prominent  development  is  in  the  hill  called  Little  Dome, 
where  it  is  crowded  with  large  phenocrysts  of  feldspar  and  has 
a  superficial   breadth  of  about  three  hundred  feet,  althougli  the 


365 

normal  thickness  can  hardly  exceed  fifty  to  one  hundred  feet. 
That  the  zone  of  porphyry  is  not  entirely  continuous  is  espe- 
cially evident  at  the  extreme  east  end  of  the  slate,  where  it  is 
closely  bordered  and  intersected  by  the  fine  granite,  with  sev- 
eral good  exposures  of  the  contact,  particularly  on  the  north 
side  of  Great  Dome.  Although  the  Pine  Hill  slate  band  is 
much  smaller,  the  boi'dering  zone  of  porphyry  is  practicidly 
unbroken,  indicating  a  relatively  elevated  portion  of  the  bath- 
olite  surface  ;  and  the  fact  that  the  outcrops  of  porphyry  can  be 
traced  from  either  end  of  the  slate  in  a  general  N.  W.-S.  E. 
direction  to  the  limits  of  the  Pine  Hill  area  is  a  plain  indication 
of  the  former  extension  of  the  slate  along  this  line.  The 
porphyry  passes  gradually  outward  at  all  points  into  a  like 
continuous  zone  of  fine  granite,  which  in  turn  blends  imper- 
ceptibly outward  with  the  much  coarser  normal  granite. 

Following  are  some  of  the  points  where  the  quartz  porphyry 
can  be  most  clearly  seen  to  overlie  the  granite  :  In  tracing  the 
contact  zone  of  the  Pine  Tree  Brook  slate  it  becomes  very 
evident  that  in  its  normal  relation  the  porphyry  overlies  the 
fine  granite,  and  some  ledges  are  plainly  porphyry  above  and 
granite  below,  with  a  gradual  but  rapid  transition.  On  the 
western  slope  of  Wampatuck  Hill  the  typical  quartz  porphyry 
passes  gradually  downward  into  the  fine  granite,  and  the  fine 
granite  near  the  base  of  the  hill  blends  with  the  normal  granite. 
The  broad  summit  of  Fox  Hill  is  composed  of  quartz  porphyry, 
which  passes  downward  on  all  sides,  but  especially  to  the  east, 
north  and  west,  into  broad  and  typical  developments  of  fine 
granite.  On  the  Broken  Hills,  and  especially  on  the  most 
southerly  and  principal  elevation  of  this  group,  another  broad, 
flat  summit  of  quartz  porphyry  passes  downward  on  three 
sides  —  north,  west  and  south  —  into  fine  granite.  This  rela- 
tion is  most  clearly  exposed  in  Scamaug  Notch  between  this 
hill  and  Nahanton  Hill,  the  abrupt  slopes  of  the  notcli  consist- 
ing of  quartz  porphyry  above  and  fine  granite  below.  The 
same  story  is  repeated  in  Sassamon  Notch,  between  Nahanton 


366 

and  Kitchamakin  Hills.  But  nowhere  is  the  superposition  of 
the  quartz  porphyry  on  the  granite  more  clearly  exposed  than 
in  Slide  Notch,  between  Kitchamakin  Hill  and  Chickatawbut 
Hill,  and  in  the  parallel  but  nameless  notch  between  the  east- 
ern member  of  Chickatawbut  and  the  main  hill.  Here  we  get 
in  nearly  vertical  sections  the  complete  range  from  typical 
quartz  porphyry  above  through  fine  granite  to  normal  granite 
below.  On  the  east  side  of  Tucker  Hill,  again,  the  quartz 
porphvry  is  seen  to  pass  gradually  dow^nward  into  fine  granite, 
and  this  is  the  only  point  on  the  western  half  of  the  main  range 
whei'e  the  fine  (jrauite  has  been  observed.  These  facts  and 
others  of  like  import  may  be  summarized  in  the  statement  that 
throughout  this  section  of  the  main  range  the  quartz  porphyry 
occurs  chiefl}  on  the  summits  and  upper  slopes  of  the  hills  and 
the  granite  on  the  lower  slopes  and  in  the  notches  dividing  the 
hills.  Hence,  although  the  quartz  porphyry  is,  east  of  Ran- 
dolph Avenue,  the  prevailing,  and  west  of  the  avenue  almost 
the  only,  surface  rock  of  the  main  range,  it  is  clearly  a  fair 
induction  that  these  are  throughout  essentially  granite  hills 
with  merely  a  surface  layer  or  veneering  of  quartz  porphyry. 

That  the  quartz  porphyry  is  not  a  later  intrusion  than  the 
granite  or  a  surface  flow  over  the  granite  is  proved  by;  (1) 
the  entire  absence  of  flow  structure  in  the  porphyry;  (2)  its 
gradual  passage  into  the  fine  granite  and  through  that  into  the 
normal  gTanite  ;  and  (3)  the  inclusion  of  innumerable  masses 
and  fragments  of  porphyry  in  the  fine  granite  and  to  some 
extent  in  the  normal  granite  where  the  intermediate  zone  of 
fine  granite  is  but  slightly  developed  or  wanting. 

Mr.  White  has  noted  {antect,  p.  361)  the  existence  of  a 
microscopic  fluidal  texture  in  the  ground  mass  of  the  quartz 
porphyry  ;  but  he  also  states  that  macroscopically  the  rock  is 
without  trace  of  flow-structure  ;  and  certainly  it  is  in  its  gen- 
eral aspect  no  less  massive  and  structureless  than  the  granite 
which  it  covers.  Of  the  gradual  passage  of  the  quartz  por- 
phyry into  the  granite  we  have  abundant   evidence  on  every 


367 

hand,  for  a  large  part  of  the  contact  zone  above  tlu;  normal 
granite  is  of  transition  character.  Mr.  AVliite  notes  that  the 
ground  mass  of  the  quartz  porphyry  is  in  part  cryptocrystaliine 
(felsitic)  and  even  slightly  glassy  ;  but  the  rock  is  also  in 
large  part  holocrystalline,  and  passes  in  the  most  insensible 
manner,  and  often  without  loss  of  phenocrysts,  into  the  fine 
granite,  which  thus  unites,  as  previously  noted,  the  characters 
of  both  the  quartz  porphyry  and  the  normal  granite.  In  other 
words,  the  ground  mass  of  the  quartz  porphyry  may  be  either 
felsitic  (aporhyolite  porphyry)  or  granitic  (granite  porphyry) 
depending  on  the  rate  of  cooling  ;  and  the  phenocrysts  gradu- 
ally fade  out  as  the  fine  granite  merges  downward  with  the 
still  coarser  normal  granite. 

A  large  part  of  the  area  mapped  as  quartz  jiorphvry  on  the 
main  range  of  the  Blue  Hills  is  a  blending  patch  v/ork  of 
aporhyolite  porphyry  (quartz  porphyry)  and  granite  porphyry: 
and  hence,  although,  as  previously  stated,  the  former  prevails 
at  the  higher  levels  and  the  latter  at  the  lower  levels,  the  accu- 
rate delimitation  of  the  two  porphyritic  types  is  impossible. 
Similarly,  the  granite  porphyry  blends  perfectly  with  both  the 
non-porphyritic  fine  granite  and  the  normal  granite  ;  and  in 
general  the  separation  of  the  quartz  porphyry,  fine  granite  and 
normal  granite  is  to  be  regarded  as  only  roughly  approximate. 

Inchisions  of  Porjohyry  in  the  Granite. — Among  the 
most  constant  and  characteristic  structural  features  of  the 
batholite  ai-e  the  inclusions  in  the  superficial  portions  of  the 
granite  of  the  rock  —  diorite,  fine  granite  or  quartz  porphyry 
—  forming  the  immediate  contact  zone.  These  inclusions  are 
least  abundant  for  the  fine  granite,  though  this  may  be  due 
in  part,  as  previously  noted,  to  the  fact  that  owing  to  lack  of 
contrast  they  are  less  conspicuous  and  have  generally  escaped 
observation.  But  they  are  a  prominent  feature  in  the  rela- 
tions of  the  diorite,  and  the  quartz  porphyry  is  in  this  respect 
strikingly  similar  to  the  diorite.  The  question  recurs,  also, 
as  to  whether  the  inclusions  are  chiefly  mechanical,  that  is,  due 


368 

to  the  cracking  of  the  contact  zone  and  its  injection  by  the 
still  molten  granite  magma,  or  are  to  be  explained  as  segrega- 
tions from  the  granite  magma  ;  and,  as  before,  Mr.  White 
reo-ards  the  segregation  theory,  with  the  greater  favor,  and  I 
find  myself  in  substantial  agreement  with  him,  although  recog- 
nizing, as  in  the  case  of  the  diorite,  that  both  explanations  are 
required  to  account  for  all  the  phenomena. 

These  inclusions  are  evidently  a  contact  feature,  occurring  in 
the  fine-grained  and  porphyritic  phases  of  the  granite  only,  as 
a  rule,  near  its  junction  with  the  quartz  porphyry.  This  rela- 
tion is  so  general  that  where  continuous  bodies  of  the  quartz 
porphyry  are  now  wanting  in  the  vicinity  of  the  inclusions  we 
may  fairly  infer  its  former  existence,  subsequent  erosion  having 
cut  just  deeply  enough  to  remove  the  quartz  porphyry  covering 
of  the  granite  without  obliterating  these  imbedded  masses. 
Although  the  inclusions  are  a  fairly  constant  feature  of  the 
quartz  porphyry-granite  contact,  they  are  much  more  pro- 
fusely developed  in  some  parts  of  the  field  than  in  others. 
They  especially  characterize  the  contact  zones  of  the  slate 
areas,  occurring  by  hundreds  and  thousands  in  the  granite 
encircling  the  slate  of  Pine  Tree  Brook  and  Pine  Hill.  Tliey 
are  also  fairly  abundant  at  most  points  around  the  large  body 
of  compact  and  fluidal  aporhyolite  of  the  Pine  Hill  area,  and 
in  the  granite  and  granite  porphyry  of  Rattlesnake  Hill  and 
vicinity.  West  of  Randol})h  Avenue  they  are  practically 
wanting,  and  east  of  the  avenue  they  become,  in  general, 
larger  and  more  numerous  eastward,  increasing  from  two  or 
three  inches  to  twice  as  many  feet  in  maximum  diameter. 
On  Rattlesnake  Hill  they  rarely  exceed  six  inches  in  diameter, 
while  around  the  Pine  Hill  and  Pine  Hill  Brook  slate  areas 
many  inclusions  are  from  six  inches  to  six  feet  in  diameter. 

Perhaps  the  finest  exhibition  of  porphyry  inclusions  is  in  the 
high,  glaciated  ledge  of  granite  near  the  railroad  on  the  east 
side  of  Pine  Hill,  opposite  the  end  of  Liberty  Street.  They 
are  also  dis})layed  to  good  advantage  in  some  of  the  quan-ies 


369 

of  red  granite  on  the  south  side  of"  Pine  Hill  ;  and  the  granite 
quarry  on  the  north  side  of  Rattlesnake  Hill  may  be  mentioned 
as  an  exceptionally  favorable  point  for  the  study  of  the  smaller 
inclusions.  In  several  of  the  quarries  these  dark  inclusions 
have  proved  seriously  detrimental  to  the  usefulness  of  the 
stone. 

The  inclusions  are  not  evenly  distributed  in  the  granite,  but 
proximity  to  the  quartz  porphyry  and  other  things  being 
equal,  the  inclusions  may  be  crowded  and  widely  scattered  in 
contiguous  portions  of  granite. 

The  inclusions  are  usually  more  or  less  rounded  in  outline, 
but  often  also  sharply  angular  or  jagged,  and  whether  rounded 
or  angular  almost  invariably  decidedly  irregular,  rarely  approx- 
imating to  the  globular  or  ellipsoidal  forms  of  typical  segi'e- 
gations,  such  as  the  diorite  nodules  in  the  seam-face  granite  of 
Weymouth  and  Hingham.  The  boundaries  of  the  inclusions 
are,, as  a  rule,  rather  sharply  defined,  but  very  rarely  absolutely 
so  ;  while  in  many  cases  they  are  more  or  less  blending,  the 
inclusions  merging  gradually  with  the  granite.  Not  infre- 
quently the  boundaries  of  individual  inclusions  are  sharp  at  some 
points  and  vague  or  blending  at  others  ;  and  very  rarely,  in- 
deed, the  inclusion  is  bordered  by  an  indistinct  halo,  half  an 
inch  to  several  inches  wide,  of  lightei'-colored  granite. 

Mr.  White's  description  of  the  inclusions  may  be  summa- 
rized as  follows  :  They  are  of  a  dark  bluish  gray  color,  much 
finer-o;rained  than  the  enclosino-  granite,  and  due  to  concentra- 
tion  of  the  dark  silicates  of  the  granite,  in  much  the  same  way 
as  the  diorite  inclusions  of  the  biotite  granite.  Although  the 
outlines  of  the  inclusions  are  fairly  defined,  yet  there  is,  in 
many  cases,  a  perfect  gradation,  without  line  of  demarcation  ; 
and  hence  they  are  clearly  segregations  and  not  inclusions. 
They  are  devoid  of  spheroidal  or  concentric  structure,  and 
resemble  the  differentiations  observed  in  the  granites  of  Maine, 
New  Brunswick  and  other  regions.  Tunnels  in  the  Wasatch 
Eange,    in    Utah,    showed  that   such  masses   did   not    extend 

OCCAS.   PAPERS  B,  S.  N.  H.  IV.  24. 


370 

deeply  into  the  rock,  so  they  may  represent  the  first  step 
in  the  transition  from  the  normal  deep-seated  granite  to  the 
more  rapidly  cooled  surface  porphyries,  which  they  closely 
resemble.  Microscopically  they  present  the  same  minerals  as 
the  enclosing  granite,  but  in  different  proportions,  and  packed 
together  in  smaller  pieces,  producing  a  fine-grained  gi-anitic 
texture.  A  little  more  plagioclase  is  noticeable,  while  the 
dark  o-reen  silicates,  both  mica  and  hornblende  and  their  altera- 
tion  products,  are  much  more  abundant  than  in  the  mass  of 
the  granite. 

It  is  an  important  and  significant  fact  that  the  inclusions  are 
throughout  of  remarkably  uniform  and  distinctly  basic  character, 
matching  the  quartz  porphyry  of  the  contact  zone  only  where 
that  has  its  most  basic  development,  as  in  the  vicinity  of  the  Pine 
Tree  Brook  and  Pine  Hill  slate  areas,  where  we  have  continuous 
bodies  of  highly  basic  porphyry  which  is  lithologically  indis- 
tinguishable from  the  inclusions.  The  phenocrysts  of  these 
large  bodies  of  basic  porphyry  and  of  the  porphyry  inclusions 
are  much  larger  than  those  of  the  normal  quartz  porphyry, 
averao:ino;  from  two  to  three  times  as  laro-e,  or  from  five  to  ten 
millimeters  in  their  larger  diametei'S ;  and,  although  often 
crowded  in  the  rock,  and  again  rather  scattering,  they  are 
chiefly  feldspar,  for  the  quartz  phenocrysts,  although  large, 
occur  but  sparingly,  and  are  often  entirely  wanting.  In  many 
cases,  and  usually  in  the  continuous  bodies  of  basic  porphyry, 
the  large  feldspar  phenocrysts  are  as  closely  packed  as  they  can 
be,  and  the  dark  ground  mass  is  restricted  to  the  angular  inter- 
stices between  them. 

I  have  already  noted  that  the  basic  porphyry  bordering  the 
Pine  Tree  Brook  and  Pine  Hill  slate  areas  probably  represents 
the  deepest  part  of  the  contact  zone  ;  and  this  conclusion  is 
sustained  by  the  striking  likeness  of  this  porphyry  to  the  por- 
phyry inclusions,  for  the  latter  have  certainly  been  formed  at 
the  bottom  of  or  entirely  below  the  contact  zone.  At  the 
higher  levels,  where  the  normal  quartz  porphyry  was  formed. 


371 

no  iippreciable  cheniical  (lifferciitiatioii  ot"  the  iiuigina  occiifixid  ; 
and  it  is  contrasted  with  the  underlying  granite  only  in  texture. 
Similarly,  in  the  deeper  parts  of  the  batholite,  where  the  nor- 
mal oranite  was  formed,  we  find  no  notable  niasmatic difleren- 
tiation  ;  but  this  is  confined  to  the  deeper  part  of  the  contact 
zone  of  quartz  porphyry,  and  to  the  granite  immediately  under- 
lying the  more  elevated  and  acid  portions  of  the  quartz 
porphyry. 

The  facts  suggest  that  the  more  elevated  part  of  the  contact 
zone,  composed  of  acid  quartz  porphyry,  represents  a  relatively 
rapid  cooling  of  sedentary  magma,  the  cooling  having  been  too 
rapid  to  permit  appreciable  circulation,  and  consequent  chemical 
differentiation  ;  and  that  the  normal  granite,  forming  the  vast 
interior  body  of  the  batholite,  represents  the  slow  and  gradual 
cooling  of  a  magma  too  stiffly  viscous  and  too  little  stimulated 
by  surface  cooling,  for  effective  circulation  and  differentiation  ; 
while  we  may  reasonably  suppose  that  between  the  successive 
stages  in  the  development  of  the  batholite  represented  by  the 
acid  quartz  porphyry  and  the  normal  granite,  that  is,  between 
the  periods  of  rapid  cooling  and  brief  circulation,  and  of  slow 
cooling  and  no  circulation,  intervened  a  period  of  moderately 
slow  cooling  and  sufficiently  prolonged  and  efficient  circulation 
to  permit  the.  moderate  chemical  differentiation  and  the  de- 
velopment of  the  larger  phenoerysts  of  the  basic  porphyry  of 
the  deeper  parts  of  the  contact  zone,  and  of  the  porphyry  in- 
clusions beneath  the  more  elevated  and  acid  portions  of  the 
contact  zone. 

It  is  a  fact  of  considerable  significance  in  its  bearing  upon 
the  origin  of  the  basic  porphyry  inclusions,  that  they  are  found 
not  alone  in  the  granite  and  granite  porphyry,  but  also,  occa- 
sionally, in  the  acid  and  subacid  quartz  porphyry.  Inclusions 
of  basic  porphyry  in  the  more  acid  porphyry  of  the  contact 
zone  have  especially  attracted  my  attention  in  the  subacid  por- 
phyry with  large  phenoerysts  bordering  the  aporhyolite  on  the 
east  side  of  Pine  Hill ;  but  they  also  occur  in  other  parts  of 


372 

the  field.  The  porphyiy  inclusions  in  the  porphyry  are 
indistinguishable  from  those  in  the  granite,  being  dark,  fine- 
grained, coarsely  porphyritic,  free  from  quartz  phenocrysts, 
sharply  defined  as  a  rule  and  often  angular. 

The  porphyritic  structure  of  the  basic  porphyry,  both  of  the 
contact  zone  and  the  inclusions,  is  a  plain  indication  that 
crystallization,  alike  of  the  phenocrysts  and  the  ground  mass, 
did  not  accompany  but  followed  differentiation.  And  when 
we  consider  the  excess  of  feldspar  and  dearth  or  absence  of 
quartz  in  much  of  the  basic  porphyry,  it  appears  probable 
that  the  differentiation  does  not  consist  wholly  in  a  concentra- 
tion of  the  ferromagnesian  silicates,  but  partly  in  an  elimina- 
tion of  the  free  silica,  by  virtue,  we  may  suppose,  of  its 
greater  affinity  for  the  mineralizing  or  liquefying  agent  — 
water.  During  the  slow  movement  of  the  magma  past  the 
■cooling  surface  of  the  batholite  the  imperfectly  hydrated  basic 
constituents  are  separated  by  the  increased  sluggishness  of 
flow  from  the  more  hydrated  and  mobile  silica,  and  accumu- 
late in  a  stiflly  viscous  mass  in  which  porphyritic  crystalliza- 
tion subsequently  takes  place. 

An  alternative  view  is  that  the  basic  porphyry  owes  its 
basic  character  to  the  local  absorption  of  slate  by  the  magma  ; 
but  this  does  not  account  satisfactorily  for  the  uniformly  acid 
character  of  the  normal  quartz  porphyry,  or  for  the  inclusions 
of  basic  porphyry  beneath  the  normal  i)orphyry.  The  hypoth- 
esis of  a  spontaneous  differentiation  incidental  to  the  cooling 
and  solidification  of  the  magma  appears,  therefore,  most 
probable. 

The  essential  kinship  of  the  basic  porphyry  inclusions  and 
the  continuous  sheets  or  contact  zone  of  identical  lithological 
character,  is  obvious.  In  both  the  Pine  Tree  Brook  and  Pine 
Hill  slate  areas  the  continuous  bodies  of  basic  porphyry  become 
outward,  or  away  from  the  slate,  gradually  less  and  less  con- 
tinuous, until  we  have  completely  isolated  inclusions  of  por- 
phyry in  the  granite  ;  and  these  gradually  become  smaller  and 


373 

more  scattering,  until  they  disappear  altogether.  In  some 
cases,  however,  the  basic  porphyry  and  granite  are  blended  in 
composition,  and  the  outer  limit  of  the  inclusions  is  rather 
indeterminate.  In  either  case  we  have  indicated  a  gradual 
dying  out  of  the  ])rocess  of  segregation,  in  consequence,  we 
may  suppose,  of  the  diminishing  energy  of  the  magmatic  cir- 
culation. 

Coming  now  to  the  question  as  to  the  real  significance  of  the 
porphyry  inclusions  as  distinguished  from  the  continuous  bodies 
of  basic  porphyry,  the  evidence  for  the  two  rival  hypoth- 
eses —  segregation  and  mechanical  inchision  —  may  be  sum- 
marized as  follows  :  Segregation  is  sustained  by  the  contrast 
in  composition  between  (1)  the  inclusions  and  the  enclosing 
granite  and  (2),  in  many  cases,  the  inclusions  and  the  super- 
jacent porphyry  ;  by  the  occui'rence  of  the  inclusions  in  the 
quartz  porphyry  as  well  as  in  the  granite  ;  by  the  rounded 
forms  of  many  of  the  inclusions  ;  and  by  the  frequent  partial 
blending  of  the  inclusions  and  granite.  Mechanical  inclusion 
is  most  favored  by  the  usual  absence  of  halos  and  concentric 
structure  ;  by  the  irregularity,  angularity  and  sharp  definition 
of  the  forms  in  many  cases  ;  by  the  irregularity  in  size  and 
distribution  ;  and  by  the  porphyritic  texture.  But  segregation 
develops  sharp  boundaries  if  time  is  allowed  for  perfect  work  ; 
the  porphyritic  texture,  as  we  have  seen,  is  consistent  with  the 
segregation  theory,  if  we  conceive  crystallization  to  follow 
seo'reo-ation ;  and  the  irreo;ularities  in  form  and  distribution 
are  explained  if  in  addition  we  conceive  movement  of  the 
magma.  This  movement  of  a  stiffly  viscous  magma  may  also 
lead  to  cracking  and  breaking  of  the  segregations,  as  proved  by 
a  specimen  of  granite  containing  large  oval  concretions  from 
Slattemosse,  Sweden,  and  now  in  the  geological  collection  of 
the  Massachusetts  Institute  of  Technology.  The  best  general 
view,  therefore,  appears  to  be  that  segregation  on  a  large 
scale  and  subsequent  crystallization  developed  a  continuous 
zone  of  basic  porphyry  ;  and  that  as  the  energy  of  the.  process 


374 

diminished,  or  where  it  was  priraaril}^  weak,  it  became  more 
or  less  localized,  developing  isolated  segregations  which  were 
subject  to  various  accidents — ^distortion,  cracking  and  crowd- 
ing—  during  the  gradual  stiffening  of  the  enclosing  magma. 

With  a  view  to  obtaining  a  more  definite  idea  of  the  degree 
of  chemical  differentiation  indicated  by  the  segregations  of 
basic  porphyry  in  the  granite,  two  specimens  vvere  selected, 
one  from  the  quarry  on  the  north  side  of  Rattlesnake  Hill, 
and  the  other  from  one  of  the  quarries  on  the  south  side  of 
Pine  Hill,  each  of  which  showed  typical  basic  porphyry  en- 
closed in  typical  granite.  Determinations  of  the  silica  were 
made  for  both  the  granite  and  the  porphyry  in  each  specimen, 
with  the  following  results  : 

Rattlesnake  Hill,  porphyry,  58.96  per  cent,  of  Si02. 
"  "     granite,       71.00  "  " 

Pine  Hill  porphyry,  59.25  "  " 

"       "  granite,       72.30  "  " 

A  clear  differentiation  of  12  to  13  per  cent,  of  silica  is  thus 
definitely  established. 


INTRUSIVE    OR    DIKE     ROCKS,    NOT    OCCURRING    AS    APOPH- 
YSES  OF    THE   CONTACT   ZONE. 


Prior  to  and  during  the  development  of  the  contact  zones, 
the  Cambrian  strata  were  injected  by  innumerable  apophyses, 
varying  in  lithological  character  with  the  contact  zone.  The 
subsequent  complete  erosion  of  the  Cambrian  series  from  the 
diorite  areas,  in  the  region  of  the  Blue  Hills  Complex,  has 
obliterated  the  apophyses  of  diorite,  although  tliey  are  well 
preserved  in  some  parts  of  the  Boston  Basin  ;  but  the  apoph- 
yses of  fine  granite  and  quartz  porphyry  are  still  important 
structural  features  of  the  Blue  Hills  Complex,  and  will  be 
farther  considered  among  the  contact  phenomena  of  the 
batholitc. 


375 

It  is  proposed  in  this  section  to  take  account  only  of  tlie  in- 
trusions of  magma  postdating  the  formation  of  the  contact 
zone,  and  antedating  the  complete  solidification  of  the  batho- 
lite,  being  due  to  the  cracking  of  the  hardened  exterior  of  the 
batholite  by  shrinkage  or  displacement  and  its  injection  by  the 
still  molten  interior  poi'tion.  Of  course  they  can  be  recognized 
as  belonging  to  this  type  only  where  cutting  the  contact  zone 
or  the  normal  granite  beneath  it ;  and  it  goes  without  saying 
that,  being  derived  from  the  residual  magma  of  the  batholite, 
they  must  be  of  relatively  acid  composition  ;  while  the  condi- 
tions of  cooling  must  relate  them  texturally  to  the  rocks  of  the 
contact  zone.  Hence  the  intrusive  rocks  are  practically  limited 
to  two  types  —  microgranite  and  aporhyolite.  Much  of  the 
granite  is  actually  macrocrystalline,  but  it  averages  much  finer 
than  the  fine  granite  of  the  contact  zone,  and  microgranite  is  a 
convenient  and  distinctive  designation. 

Microgranite.  —  Dikes  and  dikelets  of  fine  granite  and 
microgranite,  often  in  complexly  branching  systems  or  groups, 
are  a  common  feature  of  the  dioi'ite  and  basic  granite,  and 
occiu-  more  rarely  in  the  underlying  biotitic  normal  granite.  In 
the  fine  granite  of  the  contact  zone  they  are  naturally  incon- 
spicuous, and  but  few  have  been  observed.  In  the  hornblendic 
normal  granite  they  are  also,  as  a  rule,  few  and  far  between  ; 
while  in  the  quartz  porphyry  they  are  limited  to  the  deep- 
seated  basic  phase,  bordering  the  slate  areas.  In  other  words, 
they  appear  to  be  limited  to  those  types  of  the  granitic  series  in 
which  their  occurrence  is,  on  theoretic  grounds,  most  probable. 
The  intrusive  fine  granite  or  microgranite  appears  to  be, 
throughout,  of  more  acid  character  than  the  fine  granite  of  the 
contact  zone,  probably  matching  the  normal  granite  closely  in 
composition. 

No  systematic  search  for  these  post-granitic  dikes  of  granite 
has  been  made  ;  but  the  following  may  be  mentioned  as  typical 
occurrences  :  In  the  diorite  and  basic  granite,  on  the  east  side 
of  Whitman  Pond,  and  on  Mt.   Pleasant,   Weymouth.;  in  the 


376 

biotitic  normal  granite,  in  the  railway  cutting  at  East  Wey- 
mouth Station,  and  in  many  other  ledges  in  Weymouth  and 
Braintree  ;  in  the  hornblendic  normal  granite  in  the  quarry  on 
Franklin  Street,  Quincy,  a  little  south  of  the  summit  of  Payne 
Hill,  and  on  the  east  side  of  Echo  Quarry  at  the  eastern  base 
of  Payne  Hill ;  in  the  basic  porphyry  of  the  contact  zone,  m 
the  first  ledges  east  of  Randolph  Avenue  at  the  junction  with 
Highland  Street,  and  in  the  first  ledge  west  of  Little  Dome. 
The  dike  on  Payne  Hill  is  two  feet  wide,  but  the  others  are  in 
general  less  than  six  inches. 

Aporhyolite.  —  The  intrusive  fine  granite  is  constantly 
prone  to  pass  through  mici'Ogranite  to  true  felsite  or  aporhyo- 
lite, as  may  be  so  clearly  seen  on  the  shore  in  Cohasset ;  and 
these  two  types  of  intrusives  are  not,  therefore,  to  be  sharply 
distinguished.  In  the  area  of  the  Blue  Hills  Complex,  how- 
ever, dikelets  of  aporhyolite  cutting  the  granitic  series  are 
rarely  observed;  and  I  have  made  no  notes  upon  them.  One 
dikelet  an  inch  wide  and  traceable  for  twenty  feet  or  so,  in 
the  normal  granite  of  the  small  quarry  on  the  east  side  of  the 
Pine  Hill  tract  opposite  the  end  of  Liberty  Street,  attracted 
my  attention  by  reason  of  its  resemblance  to  the  basic  quartz 
porphyry  of  the  neighboring  contact  zone.  But  it  proves  on 
close  examination  to  be  a  dikelet  of  fine  granite,  and  is  spe- 
cially distinguished  only  by  its  porphyritic  structure.  The 
o-eneral  absence  in  both  the  intrusive  p'ranites  and  aporhvolites 
of  the  porphyritic  character  is  an  important  and  significant 
fact. 


EFFUSIVE    OR   VOLCANIC   EOCKS. 

Aporhyolite  (fliiidal  forms) .'_ — Although  the  dikelets  of 
aporhyolite  in  the  Blue  Hills  Complex  are  so  insignificant,  the 
larger  intrusive  masses  oF  this  type  occur  on  a  scale  of  such 
magnitude  as  to  make  them  factors  of  the   first  importance  in 


377 

the  geological  structure  of  the  Blue  Hills.  These  masses,  so 
far  as  traced  out  (see  map),  take  the  form  of  two  enormous 
dikes  trending  with  the  axis  of  the  main  range,  ..one  on  the 
north  and  the  other  on  the  south,  two  to  three  miles  in  length 
and  five  hundred  to  two  thousand  feet  in  maximum  breadth. 
These  great  masses  of  aporhyolite  are  not  only  strongly  con- 
trasted in  ma2['nitude  with  the  dikelets  of  microo-ranite  and 
aporhyolite,  but,  although  in  part,  certainly,  true  dikes,  they 
are  throughout  essentially  effusive  in  character,  and  it  is 
impossible  to  draw  a  line  between  them  and  unquestionable 
flows.  Again,  thei-e  is  at  least  a  fair  probability,  as  the  sequel 
shows,  that  they  are  in  large  part  truly  effusive  in  origin. 
For  these  reasons  it  appears  best  to  treat  them  as  virtually  a 
part  of  the  effusive  rocks.  The  most  easterly  and  southerly 
of  these  great  dike-like  areas  of  aporhyolite  extends  from  the 
vicinity  of  the  quarries  on  the  south  side  of  the  Pine  Hill  tract 
westward,  with  increasing  breadth,  over  the  summit  and  south- 
ern slope  of  the  main  mass  of  Pine  Hill,  attaining  on  the 
western  slope  of  the  hill  a  breadth  of  nearly  two  thousand  feet, 
and  then,  narrowing  abruptly  to  half  this  breadth,  it  crosses 
Willard  Street,  expands  to  a  breadth  of  fully  two  thousand  feet 
south  of  Kattlesnake  Hill,  and  forms  the  entire  southeast  half 
and  summit  of  Wampatuck  Hill.  In  this  vicinity  it  is  some- 
what split  up  by  included  masses  or  islands  of  quartz  por- 
phyry ;  and  beyond  this  its  direction  is  abruptly  changed  to 
south-southwest,  its  breadth  is  greatly  diminished,  and  it 
skirts  the  abrupt  southern  slopes  of  the  Broken  Hills,  as  a 
dike  tw^o  hundred  to  five  hundred  feet  wide,  as  far  as  the 
lower  end  of  Sassamon  Notch.  Beyond  this  it  is  lost  beneath 
the  drift  in  the  valley  of  Bouncing  Brook  and  does  not  reap- 
pear among  the  ledges  of  Hawk  Hill  and  Southeast  Ridge, 
makina^  the  entire  leno-th  of  the  dike  about  two  and  one  half 
miles.  As  the  map  clearly  shows,  the  two  dikes  are  overlap- 
ping, the  overlapping  portions  being  parallel  and  about  three 
thousand  feet  apart.      The  northern  and  western  dike  is  less 


378 

satisfactorily  exposed  than  the  other ;  and  consequently  its 
size  and  continuity  are  not  so  well  known.  Its  general  trend 
is  southwesterly  ;  and  it  first  appears  on  the  extreme  north- 
west end  of  Fox  Hill  with  a  breadth  of  500  feet.  To  the 
northeast  it  is  lost  beneath  a  swamp,  and  beyond  the  swamp 
are  only  ledges  of  normal  granite.  To  the  southwest  it  can 
be  traced  with  a  fair  degree  of  continuity  and  an  approximately 
uniform  breadth  for  about  three  thousand  feet ;  and  then  it  is 
completely  lost  beneath  the  broad  area  of  till  which  blots  out 
several  square  miles  of  Blue  Hills  geology,  for  nearly  a  mile, 
to  the  vicinity  of  the  school-house  fit  the  junction  of  Forest 
and  Hillside  Streets.  Scattering  outcrops  indicate  that  it 
extends  thence  along  the  south  side  of  Hillside  Street  as  far 
as  Hillside  Pond.  Assuming  its  continuity  between  these 
extremes,  the  dike  is  certainly  two  and  possibly  three  miles 
long. 

These  large  bodies  of  intrusive  aporhyolite  are  of  fairly  uni- 
form lithological  character.  Taking  a  broad  view,  it  may  be 
stated  that  they  show,  macroscopically,  a  general  gTadation  in 
texture  from  the  borders  iuAvard,  the  felsitic  and  fluidal  textures 
being,  as  a  rule,  most  marked  near  the  margins.  In  the 
peripheral  areas,  also,  the  fiuidal  lines  usually  conform  closely 
in  direction  with  the  margin.  This  rock  is  contrasted  with  the 
quartz  porphyry  not  only  by  its  flow  structure,  but  by  its 
relatively  non-porphyritic  character  as  well.  The  porphyritic 
texture  is  rarely  a  striking  feature,  the  phenocrysts  being  in 
general  small  or  scattering  and  mconspicuous,  and  sometimes 
wholly  wanting. 

Mr.  White's  descriptions  are  based  wholly  upon  material 
from  the  eastern  dike  ;  and  the  flow  structure  is  such  a  charac- 
teristic feature  that  he  calls  the  rock,  by  way  of  distinction 
from  the  quartz  porphyry,  fluidal  aporhyolite.  Of  the  fluidal 
a])orhyolite  of  Wampatuck  Hill  he  says  :  In  the  field  it  is  a 
compact^  jaspery,  purplish  black  rock  with  brilliant  Hesh-colored 
feldspars,    in    crystals    up    to  three  to  five  millimeters    long, 


379 

scattered  through  it.  It  also  shows  in  places  a  very  little 
smoky  quartz  in  fine  grains,  and  occasionally  hundles  of  needles 
of  yellowish  tourmaline.  The  weathered  surfaces  show  the 
contorted  flow  structure,  giving  that  peculiar  wavy,  or  often 
knotted,  appearance  so  common  in  blast  furnace  slags  and 
modern  volcanics.  By  polarized  light  the  base  is  found  to  be 
completely  deviti'ified  and  microfelsitic ;  and  in  the  devitrifica- 
tion the  silicates  crystallized  out,  leaving  the  residual  silica  in 
the  form  of  quartz  occupying  the  interspaces.  Thin  sections 
reveal  a  well-marked  fluidal  texture  of  the  ground  mass,  even 
when  it  is  wanting  as  a  macroscopic  feature.  The  particles 
either  assume  lenticular  outlines  as  they  flow  around  the  pheno- 
crysts,  or  else  they  form  narrow  chains  of  spherulites.  Parti- 
cles of  the  dark  silicates,  or  of  iron  oxide,  too  minute  for 
identification,  are  also  scattered  through  the  rock.  The  pheno- 
crysts  are,  on  the  whole,  neither  large  nor  abundant ;  but  they 
include  both  quartz  and  orthoclase.  The  quartz  phenocrysts 
are  usually  with  well-marked  crystal  outlines,  but  are  also 
crowded,  and  sometimes  with  inclusions.  In  some  of  the  sec- 
tions, also,  the  quartz  shows  cracks  and  undulatory  extinction 
due  to  movement  of  the  magma  after  the  quartz  crystallized. 
The  orthoclase  phenocrysts  are  sharply  defined,  rectangular 
crystals,  and  but  little  altered.  Strains  and  microperthitic 
intergrowth  are  noticeable,  frequent  twinning  on  the  Carlsbad 
law,  and  minute,  perfectly  formed  feldspar  crystals  of  similar 
form  intergrown  in  the  larger  ones,  with  the  same  orientation. 
The  accessory  minerals  include,  very  sparingly,  rutile,  zircon, 
hematite  and  biotite. 

On  the  summit  of  Pine  Hill  the  flow-structure  takes  the 
form  of  a  distinct  and  beautifid  banding.  Mr.  White  describes 
this  rock  as  a  devitrified  glass  witli  more  or  less  contorted  bands 
of  darker  jaspery  aporhyolite  with  few  pink  feldspar  pheno- 
crysts interbanded  with  lithoidal  felsite  of  a  reddish  color.  The 
latter  is  a  fine  breccia,  and  the  two  kinds  of  texture  are 
arranged  in  alternating  bands,  as  shown  on  weathered  surfaces. 


380 

This  banding  differs  from  the  ordinary  flow-structure  in  having- 
the  lines  almost  free  from  undulations.  The  general  color  of 
the  rock  is  reddish  black,  the  alternating  finer  bands  being 
nearly  black.  Microscopically  the  composition  and  texture  of 
the  banded  aporhyolite  does  not  differ  materially  from  the  more 
typically  fluidal  variety.  The  average  of  several  partial 
analyses  of  this  type  by  students  in  the  Massachusetts  Institute 
of  Technology  shows:  SiO„  74.52;  AW,,  13.95;  Fe.Og. 
2.72. 

Sti'ucture  and  Contact  Pheno'inena  of  the  Aj)orhyolite. — 
It  will  be  in  order  to  supplement  this  general  description  of  the 
intrusive  felsites  by  some  contact  and  structural  details  before 
proceeding  to  discuss  the  orio-in  and  o-eneral  o-eoloo-ic  relations 
of  the  dikes. 

The  eastern  dike  begins,  on  the  east,  immediately  above  the 
most  westerly  granite  quarry  on  the  south  side  of  the  Pine  Hill 
tract.  The  intrusive  aporhyolite,  or  felsite,  as  we  may  call  it 
for  the  sake  of  brevity,  a  finely  and  indistinctly  porphyritic 
gray  rock,  is  here  separated  from  the  coarse,  red,  normal 
granite  by  a  narrow  band  (20  to  30  feet)  of  coarsely  por- 
phyritic and  snbacid  quartz  porphyry.  The  contact  of  the 
porphyry  and  granite  is  not  exposed  at  this  point ;  but  the 
junction  of  the  felsite  and  as  quartz  porphyry  is  plainly  exposed. 
It  is  a  definite  line,  with  no  appreciable  gradation,  and  with  no 
indication  of  faulting,  the  two  rocks  being  firmly  welded 
together  in  an  unquestionable  igneous  contact.  This  contact 
trends  north-south  for  perhaps  fifty  feet,  following  the  brink  of 
a  low  cliff  overlooking  on  the  west  a  partially  swampy  depres- 
sion several  hundred  feet  wide.  At  tlie  north  end  of  this 
exposed  north-south  contact,  the  felsite  cuts  abruptly  across  the 
porphyry  eastward  for  ten  feet,  and  possibly  farther — 100  to 
200  feet.  This  may  be  interpreted  as  simply  the  thin  tapering 
end  of  the  felsite,  or  as  an  apophysis  of  the  main  dike  cutting 
through  tiie  por[)hyry  and  penetrating  the  granite  ;  and  it  is  a 
fact  strongly  supporting  the  dike  or  intrusive  theory    of   the 


381 

felsite,  and  negativing  the  idea  that  the  contact  may  be  a  fault. 
The  contact  cannot  be  traced  farther  to  the  nortli  or  soiitli  ;  but 
on  crossing  the  depression  to  the  westward  from  either  end 
admirable  exposures  of  the  contact  are  found,  marking  the 
diverging  north  and  south  walls  of  the  felsite,  which  thus 
appears,  but  for  the  apophysis  referred  to,  to  terminate  east- 
ward in  a  blunt  cul-de-sac  of  granite  lined  with  porphyry. 

Crossing  the  quarry  and  depression  from  the  south  end  of 
this  north-south  contact,  in  a  west-southwest  direction  for 
several  hundred  feet  the  contact  is  exposed  again,  and  at  inter- 
vals for  about  one  hundred  feet,  trending  west-southwest. 
Advancing  from  the  south  the  pink  normal  granite  passes 
gradually  but  rapidly  into  the  coarse  quartz  porphyry  ;  and  a 
hundred  feet  or  less  of  this  brings  us  to  its  sharply  defined 
contact  with  the  felsite.  The  contact  is-  quite  straight  and 
simple  with  only  slight  indications  of  apophyses  or  tongues 
of  felsite  in  the  porphyry,  or  of  inclosed  fragments  of  porphyry 
in  the  felsite.  The  two  rocks  are,  however,  firmly  welded 
together ;  and  the  felsite  is  visibly  more  compact  near  the 
contact  than  it  is  two  inches  and  farther  away  ;  while  its  indis- 
tinct fluidal  structure  conforms  with  the  contact,  which  is 
nearly  vertical,  hading  N.  5°  and  less.  Farther  west  on  this 
line  the  granite  and  porphyry  are  wholly  wanting  at  the  sur- 
face ;  but  the  felsite  ledges  continue  on  nearly  the  same  course 
at  frequent  intervals  or  almost  without  a  break  to  Willard 
Street,  and  stretch  northward  from  this  line  over  the  entire 
southern  slope  of  Pine  Hill.  The  felsite  of  this  entire  area  is 
of  fairly  homogeneous  character  —  mostly  gray  or  brownish 
gray,  incipiently  crystalline  or  porphyritic  and  frequently 
showing  indistinct  fluidal  lines,  which  are  usually  nearly  ver- 
tical and  trend  east-west  parallel  with  the  southern  border  of 
the  felsite.  At  some  points,  also,  the  felsite  is  more  or  less 
brecciated  in  a  coarse  way. 

From   the   abrupt   eastern   end    of  the   felsite,    its   northern 
boundary  can  be  traced  in  a  very  direct  line  about  ISF.  80°  W., 


382 

with  frequent  exposures,  to  a  point  a  short  distance  north  of 
the  summit  of  Pine  Hill.  As  before,  a  continuous  band  of 
coarse  subacid  porphyry  separates  the  felsite  and  granite ;  and 
the  contact  of  the  felsite  and  porphyry  is  very  clearly  exposed 
in  a  number  of  ledges.  It  hades  S.  5^  or  less  to  20°  or  more  ; 
the  two  rocks  are  always  firmly  welded  together ;  and  the 
porphyry  is  seen  at  every  point  to  change  gradually  northward 
throuo;h  granite  porphyry  or  fine  granite  to  the  normal  granite, 
the  breadth  of  the  porphyry  varying  usually  from  100  to  200 
feet.  At  several  points  the  felsite  seems  to  penetrate  and 
inclose  the  porphyry  ;  and  two  exposures  are  particularly 
satisfactory,  showing  angular  fragments  of  porphyry  from 
minute  grains  to  those  six  inches  or  more  in  diameter  inclosed 
in  the  felsite,  but  only  near  the  Contact  ;  while  narrow  dikelets 
of  felsite  penetrate  the  porphyry,  partially  detaching  fragments. 
Near  the  porphyry,  the  granite,  as  we  have  seen;  is,  in  its 
turn,  characterized  by  numerous  inclusions  (segregations)  of 
basic  porphyry  of  all  sizes  up  to  a  foot  or  several  feet  in 
diameter,  and  these  also  occur  to  some  extent  in  the  normal 
porphyry.  On  the  northeast  slope  of  Pine  Hill  the  coarsely 
porphyritic  subacid  porphyry  rather  abruptly  gives  way  to  the 
normal  acid  porphyry  ;  but  the  contact  phenomena  are  other- 
wise unchanged.  The  change  is  marked  by  a  deep  and  nar- 
row transverse  ravine,  suggesting  a  possible  fault  plane. 

The  summit  of  Pine  Hill  is  a  magnificent  exposure  of 
banded  and  coarsely  brecciated  felsite,  the  originally  contin- 
uous banding  described  by  Mr.  White  having  been  broken  up 
and  the  fragments  enveloped  in  a  felsitic  paste  in  which  fluidal 
lines  may  also  be  traced.  A  part  of  the  banded  felsite  is 
minutely  but  very  profusely  and  perfectly  spherulitic.  All 
over  the  western  and  southern  slopes  of  the  hill  are  broad 
ledges  of  gray  and  brown  more  or  less  banded  felsite.  At  a 
point  four  or  five  hundred  feet  north-northwest  of  tlie  summit, 
the  contact  seems  to  shift  abru[)tly  to  the  east  about  tJu-ce  hun- 
dred feet,   possibly  a  fault,   but  probably  the  original  form  of 


383 

the  contact:  and  then  it  takes  a  northwesterly  course,  and 
holds  it  without  important  change  for  about  a  thousand  feet,  or 
nearly  to  the  bottom  of  the  hill.  Here  the  direction  of  the  con- 
tact experiences  another  abrupt  change,  and  it  trends  south- 
southwest  across  tlie  western  slope  of  the  hill  for  fully  a  thou- 
sand feet  to  a  point  near  the  Quincy-Braintree  line  and  about 
seven  hundred  feet  east  of  Willard  Street,  where  it  turns 
sharply  upon  itself  and  runs  north  for  at  least  five  hundred 
feet.  Along  this  entire  zigzag  boundary,  from  the  extreme 
east  end  of  the  felsite  to  Willard  Street,  the  felsite  is  bordered 
continually  by  the  zones  of  porphyry,  and  fine  granite  or 
granite  porphyry.  As  previously  noted,  the  porphyry  varies 
somewhat  in  character.  On  the  east  side  of  Pine  Hill  it  is 
subacid,  with  large  phenocrysts  and  but  few  quartz  pheno- 
crysts  ;  across  the  northern  and  western  slopes  it  is  the  normal 
acid  quartz  porphyry  with  rather  small  and  numerous  pheno- 
crysts of  both  feldspar  and  quartz  ;  but  at  the  extreme  western 
base  of  the  hill,  where  the  contact  turns  abruptly  to  the  north, 
the  porphyry  suddenly  assumes  its  most  basic  phase,  with  a 
fine  dark  ground  mass,  large  and  crowded  feldspar  phenocrysts, 
and  no  macroscopic  quartz.  The  segregations  of  basic  por- 
phyry in  the  granite  are  practically  a  constant  feature  of  the 
contact. 

In  the  vicinity  of  Willard  street,  outcrops  on  the  line  of  the 
dike  are  wanting  for  about  half  a  mile  ;  but  south  of  Rattle- 
snake Hill  the  outcrops  are  very  prominent  and  sufficient  to 
prove  a  great  breadth  for  the  dike,  and,  as  previously  noted, 
several  included  masses  or  islands  of  quartz  porphyry.  West 
of  Willard  Street  the  porphyry  zone,  which  possesses  through- 
out this  part  of  the  field  its  normal  acid  character,  becomes 
much  broader  on  the  north  and  west  sides  of  the  dike,  and 
of  rather  indefinite  extent  on  the  south  and  east  sides.  The 
intrusive  aporhyolite  or  felsite  skirts  the  southern  base  of 
Rattlesnake  Hill ;  and  the  southern  slope  of  the  hill  is  quai-tz 
porphyry ;    while  the    broad    summit,   and    northern    slope  of 


384 

the  hill  and  all  the  ledges  between  the  hill  and  Willard  Street 
are  granite  porphyry,  passing  southward  into  the  quartz  por- 
phyry, and  northward  into  the  normal  granite,  and  charac- 
terized throughout  by  numerous  inclusions  of  basic  porphyry. 
At  the  southernmost  angle  of  Rattlesnake  Hill  the  contact  of 
the  felsite  and  quartz  porphyry  is  nearly  exposed,  and  the 
latter  rock  is  locally  dark  gray  to  black,  as  previously  noted, 
and  as  may  be  so  well  seen  in  the  large  detached  cubic  block 
or  bowlder,  which  nearly  covers  the  contact. 

Westward  the  granite  porphyry  gives  way  largely  to  the 
fine  granite  ;  and  both  of  these  transition  phases  of  the  granite 
are  somewhat  interrupted  by  the  coai'se  normal  granite  of 
Rattle  Rock,  and  the  western  slope  of  Wampatuck  Hill  ; 
while  the  precipitous  southwestern  slope  of  Rattle  Rock  is  sug- 
gestive at  least  of  local  faulting  as  a  possible  cause  of  the 
irregularities.  With  these  exceptions,  the  conditions  are  un- 
changed across  Wampatuck  Hill.  The  contact  of  the  felsite 
and  quartz  porphyry  is  readily  traced  by  numerous  outcrops 
in  a  full  and  graceful  curve  over  the  northern  and  western 
slopes,  and  is  admirably  exposed  for  study  at  a  point  50  to 
100  feet  northwest  of  the  summit.  It  is  an  absolutely  sharp 
and  definite  line  ;  the  two  rocks  are  firmly  welded  ;  the  quartz 
porphyry  shows  no  alteration  or  gradation  ;  the  felsite  is  sen- 
sibly more  dense  and  felsitic  ;  and  it  shows  indistinct  fluidal 
structure  parallel  with  the  nearly  vertical  contact.  Similar 
contact  exposures  have  also  been  observed  south  of  Wam- 
patuck Hill,  on  the  southern  margin,  and  around  the  quartz 
porphyry  inclusions  or  islands  of  the  dike  ;  while  along  the 
abrupt  and  craggy  southeastern  slo[)es  of  the  Broken  Hills  the 
contact,  telling  always  the  same  story,  is  ex[)osed  re[)eatedly 
and  more  or  less  continuously. 

East  of  Randolph  Avenue  the  western  dike  is  similar  to  the 
eastern.  The  p()r|)hyritic  texture  is  very  indistinct  or  wanting  ; 
the  rock  is  felsitic  and  fiiiidal  near  the  margins  and  more  mas- 
sive  and  crystalline  towai'd    the    middle  ;    the  flow   structure, 


385 

wlicre  developed,  is  usually  vertical  or  nearly  so  and  closely 
parallel  with  the  margins,  assuming  at  one  point  a  very  pro- 
nounced character  ;  the  contact  is  a  sharp  line,  without  gra- 
dation ;  and  the  bordering  rocks  pass  outwards,  as  usual, 
through  normal  quartz  porphyry  and  granite  porphyry  or  fine 
granite  to  the  normal  granite.  On  the  north  side  of  the  dike, 
however,  and  about  1500  feet  west  of  its  most  easterly  out- 
crops, the  felsite  is  bordered  for  several  hundi-ed  feet  by  slate, 
with  a  clear  exposure  of  the  contact  at  one  point.  It  is  a 
good  igneous  contact;  and  the  slate  is  highly  altered,  —  hard, 
massive  and  very  obscurely  bedded,  but  apparently  striking 
obliquely  against  the  felsite.  The  metamorphism  of  the  slate 
is  undoubtedly  to  be  attributed  mainly,  however,  to  the  inclos- 
ing granite  and  porphyry. 

West  of  Randolph  Avenue  the  few  scattering  outcrops  of 
felsite  are  of  very  uniform  character, — dai*k  purplish  gray, 
non-porphyritic,  and  obscurely  fluidal.  No  contact  phenomena 
are  exposed.  The  lithological  contrast  between  the  felsite 
outcrops  east  and  west  of  Randolph  Avenue,  might,  perhaps, 
suo;o;est  their  reference  to  different  dikes  ;  but  the  sino-le  con- 
tinuous  dike  appears  to  be,  on  the  whole,  the  best  interpretation 
of  the  meagre  data.  Equally  striking  lithological  contrasts 
occur  in  the  eastern  dike  ;  and  some  of  the  felsite  on  the  west 
side  of  Pine  Hill  is  very  similar  to  that  west  of  Randolph 
Avenue. 

Relations  of  the  A.j)orhyolite  to  the  Batholite. — The 
intrusive  relation  of  the  fluidal  aporhyolite  or  felsite  forming 
these  great  dikes  to  the  quartz  porphyry  and  granite  is 
unquestionable.  It  is  clearly  newer  than  and  cuts  the  contact 
zone  of  the  batholite,  and  may,  perhaps,  be  fairly  assumed  to 
have  represented  at  the  time  of  its  intrusion  a  deep-seated  and 
still  unsolidified  magma  residuum.  West  of  Willard  Street 
the  phenomena  are  simply  those  of  great  dikes  formed  by  a 
highly  viscous  magma  forced  up  through  the  successive  phases 
of  the  granite  and  the  overlying  contact  zone  of  quartz  por- 

OCCAS.  PAPEKS  l;.  8.  N.  H.  IV.  25. 


386 

pliyry,  and  developing  at  almost  all  points  flow  structure  par- 
allel with  the  walls.  The  fact  that  the  dikes  are  bordered 
almost  continuously,  at  the  present  surface,  by  the  quartz 
porphyry  and  but  seldom  by  the  granite,  suggests  a  bending 
down  or  local  subsidence  of  the  contact  zone  near  the  dikes 
consequent  upon  the  extrusion  of  such  great  volumes  of  plastic 
mao-ma. 

East  of  Willard  Street  the  zone  of  quartz  porphyry  separat- 
ing the  felsite  and  granite,  and  continuous  around  all  the  zig- 
zags of  the  boundary,  is  a  narrower  and  more  definite  and  at 
the  same  time  a  more  puzzling  feature.  On  one  side  it  meets 
the  felsite  in  a  perfectly  sharp  line  of  demarcation,  and  on  the 
other  side  blends  gradually  through  granite  porphyry  or  fine 
granite  with  the  normal  granite.  It  is  unquestionably  older 
than  the  felsite,  which  incloses  fragments  of  the  quartz  por- 
phyry ;  and  it  must  be  older  than  the  granite,  to  which  it  so 
clearly  holds,  as  elsewhere,  the  relation  of  a  contact  zone. 
Hence  we  seem  forced  to  the  conclusion  that  after  the  develop- 
ment of  the  contact  zone  of  quartz  porphyry  in  the  usual  man- 
ner the  extrusion  of  the  magma  now  represented  by  the  felsite 
took  place  in  such  a  way  as  to  form,  not  great  dikes  extending 
up  through  the  Cambrian  strata,  possibly  to  the  surface,  but 
rather  a  laccolitic  accumulation  between  the  contact  zone  and 
the  Cambrian  cover,  with  a  bending  down  or  falling  in  of 
the  edo-es  of  the  contact  or  porphyry  zone  sufficiently  marked 
to  account  for  the  great  width  of  the  dike,  for  the  narrowness 
of  the  porphyry  zone,  for  the  fact  that  in  spite  of  very  unequal 
erosion  the  felsite  is  nowhere  found  in  contact  with  the  granite, 
and  for  the  high  inclination  of  tlie  felsite-porphyry  contact. 
We  may  conceive  that  these  laccolitic  conditions  extended  at 
least  from  the  eastern  slope  of  Pine  Hill  to  Wampatuck  Hill, 
where  the  marked  narrowing  of  the  dike  occurs  and  the  por- 
phyry zone  becomes  broader  and  less  sharply  defined. 

South  of  Rattlesnake  and  Wampatuck  Hills  and    west    of 
Willard  Street  and  its  continuation  as  Granite  Street  in  Brain- 


387 

tree,  is  a  broad  area  of  drift,  cliieliy  modified,  crossed  by 
Purgatory  Road,  and  the  Quincy-Braintree  boundary.  The 
outcrops  scattered  over  this  area,  both  east  and  west  of  the 
town  boundary,  are  all  felsite,  the  most  southerly  exposures 
being  on  the  line  of  West  Street,  while  the  most  easterly  form 
a  long  rocky  north-south  ridge  about  one  third  of  a  mile  west 
of  Granite  Street.  This  expanse  of  felsite  is  mainly  a  grayish, 
obscurely  porphyritic,  massive  or  fluidal  variety,  not  unlike 
much  of  that  on  Pine  Hill.  Its  distribution  is  somewhat  sug- 
gestive of  a  southward  extension,  between  the  quartz  porphyry 
beneath  and  the  formerly  overlying  Cambrian  strata,  of  the  lac- 
colite  or  sill  already  postulated  in  explanation  of  the  broad  Pine 
Hill-Wampatuck  body  of  felsite.  A  sill  extending  south  from 
a  laccolitic  trough  between  the  contact  zone  of  the  batholite 
and  its  Cambrian  cover  would,  perhaps,  best  express  the  idea. 
An  alternative  and,  to  my  mind,  a  more  acceptable  view  is 
that  the  felsite  is  truly  effusive,  post-dating,  as  explained  on 
page  322,  the  erosion  requisite  to  lay  bare  the  batholite,  and 
occupying  in  its  broader  dike-like  development  between  the 
east  side  of  Pine  Hill  and  the  summit  of  A'\^ampatuck  Hill,  a 
steep-walled  valley  due  to  the  erosion  of  a  deeply  included  body 
of  slate.  Depressions  having  this  origin  exist  in  the  modern 
topography,  the  valley  of  Ruggles  Creek  being  a  good  example. 
If,  now,  we  conceive  such  an  erosion  trough,  with  its  wall  of 
quai'tz  porphyry  —  the  contact  zone  of  the  eroded  slate  — 
essentially  mtact,  as  the  Ruggles  Creek  valley  is  to-day  bor- 
dered by  the  contact  zone  of  fine  granite,  to  be  traversed  lon- 
gitudinally by  a  fault  fissure  with  the  down-throw  to  the  south, 
as  the  valley  of  Ruggles  Creek  unquestionably  is,  the  outflow 
of  acid  lava  filling  the  depression  and  connecting  fissure,  and 
flowing  away  to  the  south,  would  seem  to  account  for  all  the 
facts  as  they  are  now  developed  in  the  field  ;  and  the  extension 
of  the  large  body  of  Cambrian  slate  south  of  Pine  Hill  west- 
wai-d  beneath  the  fluidal  aporhyolite  would,  accoixling  to  this 
view,  appear  extremely  probable. 


388 

According  to  the  first  of  these  two  views  the  felsite  is  intru- 
sive and  closely  connected  in  time  with  the  development  of  the 
batholite,  and  effusive  rocks  are  now  wanting  in  the  Blue  Hills, 
and  possibly  always  have  been  ;  while  according  to  the  second 
view  the  felsite  is  effusive  and  widely  separated  in  time  from 
the  formation  of  the  underlying  and  deeply  eroded  batliolite, 
and  effusive  rocks  are  still  a  prominent  feature  of  the  Complex. 


DIKES  OF  DIABASE  OLDER  THAN  THE  GRANITIC  SERIES. 

Dikes  newer  than  the  Cambrian  strata  and  older  than  the 
granites,  have  been  certainly  identified  in  only  one  locality. 
This  is  the  valley  of  Pine  Tree  Brook,  east  of  Randolph 
Avenue  (see  special  map,  PI.  26).  We  find  here  an  extensive 
series  of  distinctly  basic  or  trappean  dikes,  varying  in  width 
from  a  few  inches  to  30  and  40  and  even  300  feet,  conforming 
closely  in  trend  with  the  strike  of  the  slate,  and  clearly  cut  by 
the  dikes  and  masses  of  granite  and  porphyry  Avhich  traverse 
the  slate  in  various  directions.  In  no  instance  do  they  extend 
into  the  granitic  rocks  ;  but  they  are  frequently  seen  to  be 
squarely  cut  off  by  the  latter  ;  and  several  inclusions  of  very 
similar  trap  have  been  observed  in  the  granite  ;  and  in  one 
instance,  to  be  more  fully  described  in  a  later  section,  two  small 
but  clear  dikes  of  granite  traverse  one  of  the  large  dikes  of 
trap. 

Except  for  textural  differences  due  to  variations  in  size  the 
dikes  are  of  very  uniform  character,  consisting  of  finely  crystal- 
line black  or  dark  green  diabase,  whicli  is  composed,  according 
to  Mr.  White,  of  twinned  plagioclase  witli  a  tendency  to 
ophitic  texture,  and  small  secondary  feldspar  rods  ;  of  augite, 
more  or  less  completely  uralitized  to  green  hornblende  which 
shows  chloritic  decomposition  ;  and  of  numerous  crystalline 
o-rains  of  mao-netite. 

No  characters  are  known  by  whicli  these  })rc-granitic  dikes 


389 

can  be  certainly  distinguislied  from  the  nmnerous  post-granitic 
dikes  of  this  region,  except  their  rehitions  to  tlie  enchjsing 
formations  —  tlie  later  series  cutting  the  granite  and  porphyry 
and  also  the  Carboniferous  strata  (conglomerate  and  newer 
slate),  while  the  earlier  series  cuts  only  the  Cambrian  strata 
and  is  cut  by  the  granitic  rocks. 

It  is,  therefore,  quite  possible,  and  even  probable,  that  some 
of  the  dikes  of  other  areas  of  the  Cambrian  slates,  which  are 
not  known  to  extend  into  the  granitic  rocks  or  Carbonifei'ous 
strata,  may  belong  to  the  pre-granitic  series  ;  and  a  complete 
chronological  classification  of  the  basic  dikes  of  the  region  is 
thus  seen  to  be  impossible  at  the  present  time. 

The  dip  of  the  slate  in  the  Pine  Tree  Brook  area  is  always 
high  or  approximately  vertical ;  and  the  fact  that  the  pre- 
granitic  dikes  conform  closely  vv^ith  the  slate  in  strike  and  dip, 
being  at  almost  all  points  distinctly  interbedded,  indicates  that 
they  were  formed  after  the  folding  of  the  Cambrian  series,  and 
before  the  great  heat  invasion  which  developed  the  granitic 
magma  in  relatively  superficial  portions  of  the  earth's  crust 
was  well  advanced.  Prior  to  the  softening  of  the  sub-Cam- 
brian crust,  due  to  the  close  folding  and  consequent  great 
vertical  thickening  of  the  Cambrian  strata,  and  the  deposition 
over  them  of  unknown  but  probably  great  volumes  of  later 
formations,  the  deep  fissuring  requisite  to  tap  the  abyssal  basic 
magmas  was  possible,  but  we  have  no  evidence  of  effusive 
igneous  activity  at  this  time.  The  probabilities  are  rather  that 
the  dikes  penetrated  only  deeper  portions  of  the  Cambrian 
strata,  where  they  were  most  thoroughly  indurated,  and  sus- 
ceptible of  effective  cracking.  With  the  subsequent  softening 
of  the  sub-Cambrian  crust  by  the  slow  rising  of  the  isogeo- 
therms,  continuous  fissures  or  effective  cracking  became  impos- 
sible at  the  lower  levels,  and  the  basic  intrusions  necessarily 
ceased. 

In  the  earlier  folding  of  the  Cambrian  strata  which  the  pre- 
granitic  dikes  suggest,  is   indicated  an  unconformity  between 


390 

the  Cambrian,  and  later  formations,  such  as  has  been  clearly 
proved  in  other  regions  ;  and  the  fact  that  the  Ordovician  and 
Silurian  strata  were  not  involved  in  the  primary  folding  of  the 
Cambrian,  will  help  to  account  for  their  subsequent  complete 
or  nearly  complete  removal  by  erosion. 


SUMMARY  OF  THE  HISTORY  OF  THE  COMPLEX. 

The  deposition  upon  a  floor,  of  which  no  recognizable  trace 
now  remains,  of  a  great  thickness  of  Cambrian  strata,  — 
slates,  limestone,  and  quartzite,  —  was  followed  by  a  period  of 
disturbance,  during  which  the  sub- Cambrian  floor  was  probably 
much  faulted  and  fissured,  and  the  Cambrian  strata  suflFered 
deep  isoclinal  folding,  and  were  invaded  by  an  abyssal  basic 
magma,  forming  the  pre-granitic  diabase  dikes. 

The  great  vertical  thickening  of  the  Cambrian  strata  due  to 
the  folding  and  the  subsequent  deposition  over  them  of  impor- 
tant thicknesses  of  Ordovician,  Silurian  and  Devonian  strata, 
caused  a  marked  and  lono'-continued  rise  of  the  isooeotherms  — 
a  great  heat  invasion  which  slowly  softened  and  melted  the 
sub-Cambrian  crust  and  considerable  volumes  of  the  Cambrian 
beds,  developing  a  vast  body  or  batholite  of  acid  magma  above 
the  abyssal  basic  magma,  and  forming  numerous  apophyses  in 
the  unfused  Cambrian  strata. 

The  gradual  refrigeration  of  the  batholite  was  attended  by 
various  processes  of  differentiation,  —  chemical  and  textural, 
—  giving  I'ise  to  the  two  types  of  normal  granite  —  biotitic  and 
hornblendic  —  forming  the  main  body  of  the  batholite,  and  to 
a  contact  zone  composed  of  three  distinct  types — diorite,  fine 
granite  and  quartz  porphyry. 

When  the  refrigeration  of  tlie  batholite  Avas  well  advanced, 
and  after  the  overlying  formations  had  sufl^ered  extensive 
erosion,  renewed  crustal  adjustments  led  to  the  displacement  of 


391 

considerable  volumes  of  still  unsolidified  or  possibly  remcltcd 
acid  magma,  forming  both  intrusive  and  effusive  masses  — 
dikes  of  fine  granite  and  dikes  and  flows  of  aporliyolite  (fel- 
site). 


STEUCTURAL  DETAILS  AND  CONTACT  PHENOM- 
ENA OF   THE    CAMBRIAN   STRATA. 

We  have  studied  the  general  distribution,  probable  succes- 
sion, conditions  of  deposition  and  normal  lithologic  and  pale- 
ontologic  characters  of  the  Cambrian  strata  of  the  Blue  Hills 
Complex.  We  have  noted,  also,  the  subsequent  plication  of 
the  Cambrian  strata  and  the  development  beneath  them  and 
in  the  lower  portion  of  the  deeply  folded  mass  of  sediments  of 
a  vast  body  of  magma  the  differentiation  of  which  has  given 
rise  to  the  entire  series  of  granitic  rocks.  And,  finally,  we  have 
described  each  of  these  igneous  types  as  fully  as  appeared 
desirable  for  a  work  of  this  character,  and  discussed  its  genetic 
and  structural  relations. 

Hence  we  are  now  prepared  to  return  to  the  Cambrian  out- 
crops for  a  more  detailed  structural  study,  and  to  note  the 
varying  modes  and  degrees  of  the  alteration  of  these  ancient 
sediments  by  the  igneous  rocks  in  which  they  occur  as  mere 
isolated  remnants  of  a  once  continuous  formation . 


THE   SOUTHEEN   BOUNDARY   BELT. 

This  band  of  Cambrian  strata,  as  previously  noted,  marks 
the  southern  border  of  the  Blue  Hills  Complex,  from  Wey- 
mouth Back  River  westward  for  nearly  six  miles  to  the  vicinity 
of  the  Quincy  Reservoir  in  Braintree,  beyond  which,  appar- 
ently,   the    Cambrian    sediments    pass    beneath    the    effusive 


392 

felsites  and  the  Carboniferous  strata  of  the  Norfolk  Basin. 
The  sharply  defined  southern  margin  of  the  belt  is  deter- 
mined by  the  great  boundary  fault,  which,  with  an  important 
up-throw  to  the  south,  appears  to  account,  at  once,  for  the 
existence  of  both  this  belt  and  the  broader  belt  of  Carbon- 
iferous strata  succeeding  it  to  the  westward  :  for  the  relatively 
elevated  position  of  the  biotitic  normal  granite  —  the  deepest 
or  most  abyssal  member  of  the  batholite ;  and  for  the 
rather  abrupt  passage  from  the  biotitic  to  the  hornblendic 
normal  granite.  The  northern  margin  of  the  belt,  on  the 
contrary,  is,  wherever  exposed  and  probably  at  all  points,  an 
oriofinal  isrneous  contact  with  the  fine  oranite  of  the  contact 
zone.  Corresponding  to  these  strongly  contrasted  and  highly 
characteristic  northern  and  southern  contacts  with  the  granitic 
rocks,  is  to  be  noted  a  significant  and  striking  difference  in 
the  character  of  the  Cambrian  slates.  They  are,  in  the  main, 
strongly  laminated  —  fissile  and  shaly,  parallel  Avith  the 
boundary  fault,  and  become  more  so  as  we  approach  this  great 
displacement ;  while  in  the  vicinity  of  the  igneous  contact  they 
become  rapidly  hard  and  massive,  and  very  near  the  granite 
are,  in  general,  entirely  devoid  of  lamination. 

This  belt  consists  chiefly  of  the  non-calcareous  green  to  dark 
gray  and  black  Middle  Cambrian  slates  ;  but  toward  the  east 
the  calcareous  red  and  green  Lower  Cambrian  slates  come  in 
on  the  northern  margin,  and  at  last  apparently  form  the  entire 
breadth  of  the  belt.  This  belt  is  broken  by  several  transverse 
faults,  and  the  sections  thus  resulting  wdl  be  described  sepa- 
rately, beginning  on  the  east. 

The  King  Oalc  Hill  A.rea. — This  area  may  be  regarded 
as  extending  from  Weymouth  Back  River  westward  under  the 
north  end  of  King  Oak  Hill  to  the  stream  tributary  to  the 
head  of  Mill  Cove.  The  only  outcrops  are  in  the  vicinity  of 
Commercial  and  Church  Streets  west  of  the  hill,  and  on  the 
railroad  northeast  of  the  hill.  At  the  last-mentioned  locality 
the  slate  is  found  at  the  nortii  end  of  a  lono;  cut  in  the  normal 


393 

granite  (puge  341).  The  slate  and  granite  are  ex[)()se(l  within 
a  foot  of  each  other  ;  and  the  bedding  of  the  shite  is  parallel 
with  the  contact:  strike,  N.  8()°-90°  W.  ;  dip,  N.  85^ 
The  slate  is  greenish  gray,  soft  and  shaly,  and  has  a  breadth 
in  the  cut  of  about  forty  feet.  On  the  southwest  side  of  the 
track,  at  about  this  distance  from  the  granite,  is  a  well,  which 
evidently  was  sunk  in  the  slate,  and  the  debris  from  the  well 
is  all  soft,  purplish  red  slate,  with  calcareous  nodules  and 
streaks  and  even  thin  lines  of  nearly  pure  limestone.  The 
slate  varies  in  structure  from  massive  to  shaly  and  semi- 
crushed  ;  and  slight  developments  of  white  mica  (sericite) 
occur  on  some  of  the  shear  planes.  Minute  pseudomorphic 
cavities,  and  specks  of  limonite,  in  part  of  the  slate  indicate 
that  it  was  once  finely  speckled  with  pyrite. 

The  contact  of  the  slate  and  granite  is  a  very  obvious  fault. 
The  essential  facts  are:  (1)  the  coarse  normal  granite  shows 
no  gradation  in  texture  toward  the  slate;  (2)  the  slate  shows 
no  signs  of  igneous  metamorphism  ;  and  (3)  both  the  slate 
and  granite  are,  especially  quite  near  the  contact,  much  sheared 
and  slickensided  and  even  comminuted.  The  dikes  in  the 
granite  are  also  much  shattered  and  slickensided. 

On  Church  Street  and  in  the  angle  between  Church  Street 
and  Commercial  Street  are  several  obscure  but  unquestionable 
ledges  of  a  bright  green,  soft  and  fissile  slate  ;  and,  apparently, 
a  more  massive  purple  slate,  but  that  was  not  seen  in  situ. 
Neither  variety  is  appreciably  calcareous.  The  strike  of  the 
green  slate  is  about  east-west,  and  the  dip  vertical ;  and  the 
total  breadth  of  the  outcrop  less  than  300  feet. 

In  and  south  of  Commercial  Street,  and  also  immediately 
north  of  Commercial  Street  a  few  rods  to  the  west,  between 
Church  and  Essex  Streets,  is  fine  granite,  passing  southward 
into  normal  granite  and  enclosing  large  dikes  of  trap.  The 
contact  with  the  slate  is  nowhere  exposed  ;  but  the  fissile  char- 
acter of  the  slate  is  a  plain  indication  that  the  contact  is  a 
fault.     The  fine  granite  is  exposed  for  about  one  hundred  feet 


394 

north  of  Commercial  Street ;  and  two  hundred  feet  farther 
north  brings  us  to  several  important  ledges  of  slate  with  a 
total  breadth  of  about  two  hundred  feet.  The  slate  is  mainly 
greenish  to  gray,  but  includes  some  dark  purple  layers  with 
calcareous  lenses  and  streaks.  The  strike  is  JS".  80°  W,,  and 
the  dip  S.  85°.  In  the  meadow  a  few  rods  to  the  northward 
is  an  obscure  ledge  of  fine  dark  granite.  If  this  is  a  genuine 
ledge,  the  breadth  of  this  section  of  the  slate  belt  is  limited  to 
about  five  hundred  feet.  But  probably  it  is  a  bowlder ;  and 
following  the  topographic  indications  we  may  extend  the  slate 
notth  of  the  railroad,  where  it  probably  meets  the  fine  granite 
north  of  the  cemetery  in  an  igneous  contact. 

The  large  dike  exposed  in  the  salt  marsh  at  the  head  of 
Mill  Cove  may  be  the  same  as  that  which  outcrops  obscurely 
in  the  large  gravel  excavation  north  of  King  Oak  Hill.  With 
this  exception,  no  reliable  outcrops  are  known  between  East 
Street  and  Weymouth  Back  River :  but  there  are  several 
places  where  slate  has  the  appearance  of  being  almost  in  situ  ; 
and  there  seems  to  be  no  alternative  but  to  extend  this  belt  of 
slate  through  to  the  great  transverse  fault  of  Weymouth  Back 
River. 

The  Whites  Nech  Area. — The  Mill  Cove  fault,  termi- 
nating the  King  Oak  Hill  area  on  the  west,  is  a  very  obvious 
necessity ;  for  on  the  west  side  of  the  north-south  brook  tribu- 
tary to  the  head  of  the  cove  not  a  trace  of  slate  can  be  found 
in  continuation  of  the  ledges  on  Church  Street  and  the  east 
side  of  the  brook  ;  but  the  fine,  dark  granite,  in  close  and 
numerous  ledges,  seems  to  extend  continuously  from  Commei- 
cial  Street  almost  to  the  railroad.  In  other  words,  the  slate 
is  cut  squarely  oflp;  and  the  brook  must  be  regarded  as  mark- 
ing approximately  a  transverse  fault.  This  conclusion  har- 
monizes well  with  the  straio^ht  and  ororoe-like  character  of  the 
valley  of  the  brook  south  of  Commercial  Street ;  and  the 
copious,  never-failing  Pavonia  Spring  on  Commercial  Street 
is  certainly  suggestive  of  an  important  displacement. 


395 

A  few  yards  south  of  the  ruih-oad,  on  the  west  side  of  the 
brook,  the  granite  ends  abruptly  in  a  vertical  face,  on  which 
are  traces  of  slate.  Tracing  this  contact  to  the  west  brings  us 
obliquely  to  the  railroad  and  the  long  cut,  10  to  25  feet  deep, 
which,  trending  in  a  west-southwest  direction,  gives  a  continu- 
ous oblique  section  of  the  Cambrian  slates,  near  the  northern 
edge  of  the  granite,  for  about  1500  feet  across  the  southern 
end  of  White's  Neck.  This  exceptionally  satisfactory  and 
instructive  section  is  wholly  in  the  dark,  carbonaceous  Middle 
Cambrian  slate.  The  slate  is  very  jfine,  soft  and  homogeneous  ; 
and  the  fresh  surface  is  usually  dark  gray  to  black,  but  it 
becomes  brownish  or  bronzed  by  exposure.  The  strike  varies 
from  E.-W.  to  N.  70°  W.  ;  and  the  dip  is  S.  75°-90°.  The 
slate  shows  immense  disturbance,  but  this  is  not  indicated  by 
folds  or  contortions  so  much  as  by  faults  and  fractures.  It  is, 
in  general,  highly  fissile,  lustrous,  broken,  warped,  and  cross- 
jointed,  many  of  the  joints  being  accompanied  by  slipping  and 
drag  flexures.  In  fact,  all  the  phenomena  ai'e  indicative  of 
proximity  to  an  important  fault.  The  surface  ledges  show 
that  the  fine,  dark  granite  is  near  by  on  the  south  ;  and  toward 
the  east  end  of  the  cut  the  granite  veers  to  the  northward  and 
enters  the  cut,  the  actual  contact  being  clearly  exposed.  It 
shows  a  vertical  and  even  overhanging  wall  of  granite  trending 
N.  70°  E.  for  one  hundred  feet,  making  an  angle  of  twenty 
degrees  with  the  slate,  which  ends  abruptly  against  it ;  and  then, 
bearing  more  to  the  east,  the  granite  recedes  from  the  cut  at 
a  point  500  feet  from  its  eastern  end.  The  granite  wall  is 
irregularly  undulating,  but  smooth,  and  shows  very  plainly 
the  effects  of  differential  movement  of  the  granite  and  slate  in 
a  definite  and  approximately  vertical  direction,  accompanied 
by  enormous  friction.  Internally,  the  granite  near  the  contact 
has  been  minutely  crushed  and  sheared,  exhibiting  in  vertical 
section  a  distinct  gneissic  structure  ;  while  the  whitened  and 
structureless  slate  near  the  contact  may  be  regarded  as  a  mere 
re-lithified  and  bleached   pulp.      Both  slate  and  granite    bear 


396 

witness,  in  bleaching  and  decomposition,  to  more  or  less 
marked  hydrothermal  action  accompanying  the  faulting,  the 
granite  being  in  part  completely  kaolinized  ;  while  the  slate 
is  specially  characterized  tliroughout  the  middle  part  of  the 
section  and  especially  near  the  granite,  by  numerous  irregular 
transverse  veins  and  masses  of  quartz,  from  half  an  inch  to 
six  or  eight  feet  wide,  and  increasing  rapidly  in  size  and  num- 
ber toward  the  granite. 

At  the  east  end  of  this  long  cut  the  slate  can  be  traced 
northward  across  the  strike  almost  continuously  for  about  500 
feet.  The  color  of  the  slate  changes  northwai-d  from  black  to 
the  green  and  red  normally  underlying  the  black  and  suggest- 
ing a  passage  to  the  Lower  Cambrian  ;  while  the  high  south- 
erly dip  continues  unchanged.  This  northward  extension  of 
the  section  makes  it  very  clear  that  the  quartz  veins  are  limited 
to  the  neighborhood  of  the  granite-slate  contact. 

There  are  no  other  outcrops  on  White's  Neck  north  of  the 
railroad ;  but  both  the  geologic  and  topographic  indications 
favor  the  view  that  the  entire  promontory  is  underlain  by  slate, 
and  it  has  been  so  represented  on  the  map. 

The  Fore  Rive}'  Area.  —  This  area  includes  both  sides  of 
Weymouth  Fore  River,  between  White's  Neck  and  the  mouth 
of  Smelt  Brook  ;  and  the  principal  exposure  of  the  strata  is  the 
long  railroad  cut  along  the  north  side  of  Mt.  Pleasant,  imme- 
diately east  of  Weymouth  station.  (See  special  map,  PI.  19.) 
In  fact  this  is,  thanks  to  the  widening  of  the  cut  a  dozen  years 
ago,  an  exceptionally  interesting  and  instructive  exhibition 
of  the  Cambrian  slates  in  secondary  or  dynamic  contact  with 
the  granite  ;  and  it  is  doubtful  if  elsewhere  in  the  Boston  Basin 
an  important  displacement  is  so  admirably  exposed  for  study 
as  this. 

The  slate  is  very  similar  to  that  in  the  White's  Neck  cut,  — 
dark  gray  to  black,  lustrous,  soft,  and  eminently  fissile  or 
shaly.  Occasional  thin  streaks  are  calcareous  or  pyritiferous. 
The  marked  but  irregular  cleavage  appears  to  conform  with 


397 

the  bedding,  wliich  is  otherwise  very  obscure.  The  strila;  is 
N.  70°-80°  E.,  and  the  dip  N.  60°-90°.  Besides  the  cleavage, 
the  slate  is  traversed  by  curving  longitudinal  joints  or  shear 
surfaces,  so  that  the  slate  Avail  facing  north  or  away  from  the 
granite  presents  a  succession  of  rounded  protuberances  of  lentic- 
ular outline  several  feet  or  yards  in  diameter.  There  are  also 
straight  transverse  joints,  along  some  of  which  slips  have 
evidently  occurred,  with  local  flexing  of  the  slate  ;  and  some  of 
these  are  marked,  as  in  the  White's  Neck  cut,  by  veinlets 
of  quartz. 

In  the  widening  of  the  cut  the  slate  was,  for  about  250  feet 
at  the  west  end,  removed  from  the  granite,  which  forms  the 
entire  hill  to  the  south;  but  a  considerable  part  of  the  granite 
wall  thus  exposed  was  left  intact.  Advancing  from  the  west, 
this  wall  trends  N.  60°-65°  E.  for  100  feet,  and  is  vertical  or 
slightly  overhanging.  In  this  part  it  is  also  strikingly  undu- 
lating or  warped  and  strongly  slickensided  and  furrowed  in  a 
vertical  direction,  as  shown  in  PL  17,  which  is  taken  looking 
in  a  southeast  direction.  At  the  east  end  of  this  view  the  wall 
has  been  blasted  away  for  about  100  feet;  and  then,  with  a 
trend  of  N.  80°  E.,  it  is  exposed  for  60  feet  farther,  as  shown 
in  PI.  18,  looking  in  the  same  direction  as  PI.  17.  A  com- 
parison of  the  two  views  shows  that  the  break  in  the  wall 
marks  not  only  a  change  of  trend  but  also  of  character.  East 
of  the  break  it'has  a  decided  northerly  hade  (28°),  correspond- 
ing to  the  dip  of  the  slate,  which  is  seen  lying  upon  the  granite 
in  the  left-hand  or  eastern  half  of  PI.  18  ;  and  it  is  remarkably 
flat  and  smooth,  and  even  polished,  but  still  showing  slight 
undulations  and  vertical  slickensides,  —  an  ideal  fault  plane. 
Against  this  sloping  wall  of  polished  granite  reclines  the  slate, 
with  apparent  conformity,  but  much  warped,  sheared  and 
crushed. 

The  entire  northern  slope  of  Mt.  Pleasant  above  the  slate 
consists,  as  previously  noted,  of  fine  granite  with  traces  oi 
diorite,  and  merging  southward  into  the  normal  granite.     Near 


398 


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the  slate  the  granite  is  in  part  micro- 
crystalline  to  felsitic  in  appearance  ; 
and  Mr.  White  describes  it  as  a  heavy 
greenish  or  reddish  rock,  becoming  in 
its  extreme  phase  a  compact  felsite 
without  phenocrysts,  and  presenting 
under  the  microscope  a  dusty,  iron 
stained  appearance  ;  this  aspect  of  the 
rock  being  consistent  with  the  view  that 
it  has  been  much  altered  by  shearing 
and  crushing  along  the  original  igneous 
contact.  The  probable  relation  of  the 
fine  granite  to  the  fault  is  shown  in 
the  general  section  (Fig.  25)  ;  and  the 
strong;  vertical  scoring  and  fluting 
which  the  granite  exhibits  need  not  be 
attributed  to  the  action  of  the  soft 
and  yielding  slates,  but  rather  to  the 
opposing  granite  wall  which  now  lies 
beneath  the  slate,  on  the  down-throw 
side  of  the  fault. 

The  entire  length  of  the  railroad 
cut  is  about  1050  feet,  giving  an 
oblique  section  of  the  slates,  and  ex- 
posing the  fault  contact  with  the 
Pore  River  grauitc  Only  at  the  west  end.  Fi'om 
the  point  where  the  granite  disappears 
behind  the  slate  in  PI.  18,  the  con- 
tact, although  not  again  actually  ex- 
posed, may  be  traced  eastward  across 
the  lower  slope  of  Mt.  Pleasant  to 
Webb  Street  as  a  straight  and  defi- 
nite line  marking  a  break  in  the 
topography,  with  numerous  ledges  of 

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from  Mt.  Pleasant  to  Hay-       fine  granite  and  cliorite  on  the  south, 

ward's  Creek. 


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and  on  the  north  a  smooth  field  sloping  gently  to  the  raili-oad, 
with  an  abundance  of  slate  debris  in  the  soil.  Two  to  tin-ee 
hundred  feet  east  of  Webb  Street  on  the  same  direct  line, 
which  is  here  about  500  feet  south  of  Commercial  Street, 
and  immediately  east  of  the  little  brook  draining  the  artificial 
pond,  the  granite  outci'ops  prominently,  as  before:  and  imme- 
diately north  of  it  the  slate,  with  a  nearly  vertical  northerly 
dip,  is  exposed  for  a  breadth  of  50  feet,  and  again  for  about  130 
feet  at  the  barn  midway  between  the  granite  and  Commercial 
Street.  The  granite  ledges  continue  to  the  eastward,  but  the 
slate  is  not  exposed  again  on  this  line.  Instead,  we  find  that 
on  the  east  side  of  the  brook  flowing  directly  north  from  the 
artificial  pond  the  ledges  of  granite  and  diorite  extend  north  to 
Commercial  Street,  and  thence  northeasterly  across  the  field 
to  the  railroad  cut  on  White's  Neck. 

Undoubtedly  the  slate  abuts  rather  abruptly  eastward  against 
a  wall  of  granite.  On  the  map  this  interruption  has  been  inter- 
preted as  a  transverse  fault,  which  would  thus  be  compensating 
with  reference  to  the  Mill  Cove  fault.  But  this  Fore  River 
'fault  is  not  so  urgently  demanded  as  the  Mill  Cove  fault ;  for 
the  actual  outcrops  are  not  inconsistent  with  the  view  that  the 
great  boundary  fault  is  not  broken  here,  but  simply  takes  a 
diagonal  course,  as  is,  in  fact,  indicated  by  the  oblique  way  in 
which  the  granite  approaches  and  enters  the  White's  Neck 
section.  The  Fore  Eiver  fault  is,  therefore,  proposed  with 
some  hesitation  and  with  a  desire  for  uniformity  in  the  geolog- 
ical interpretation.  The  topographic  indications  are  entirely 
consistent  with  either  view.  It  may  be  added,  however,  that, 
as  we  shall  presently  see,  the  Fore  River  section,  like  the 
White's  Neck  section,  passes  northward  from  black  slates  to 
green  and  red  slates,  the  two  sections  being  practically  identi- 
cal in  this  respect ;  and  this  important  geological  fact  certainly 
harmonizes  best  with  the  theory  of  a  transverse  fault,  so  that 
it  may  be  regarded  as  determining  a  controlling  probability  in 
favor  of  the  map. 


400 

The  ledge  between  the  mouth  of  Smelt  Brook  and  Quincy 
Avenue  and  that  projecting  from  the  north  bank  of  Weymouth 
Fore  River  directly  opposite  the  brook,  are  green  and  highly 
fissile  slate  with  east-west  strike  and  nearly  vertical  northerly 
dip,  agreeing  closely  with  the  outcrops  along  the  Monatiquot 
River  to  the  westward.  The  only  remaining  outcrop  of 
slate  in  the  Fore  River  section  is  that  on  the  north  side  of 
Fore  River  where  it  bends  to  the  northward,  about  one 
third  of  a  mile  east  of  Quincy  Avenue.  This  ledge,  whicli  is 
on  the  land  of  Mr.  T.  A.  Watson,  is  about  300  feet  long  in  a 
northeast  and  southwest  direction  and  100  feet  wide.  It  is  in 
the  main  a  very  massive  greenish  gray  rock  which  is  in  part 
banded  and  mottled  with  reddish  brown  and  purple.  The 
stratification  is  especially  marked  by  lenticular  chloritic  and 
micaceous  streaks,  which  are  often  partially  cavernous,  owing 
to  the  solution  of  calcareous  matter.  The  calcite,  where  undis- 
solved, is  highly  crystalline,  and  presents  distinctly  prismatic 
shapes  six  to  twelve  inches  long  and  one  fourth  to  one  half 
inch  in  diameter,  lying  longitudinally  in  the  dark  chloritic 
streaks.  The  slate  has  a  high  dip  to  the  southeast  and  is  very 
much  broken  by  transverse  rifts  and  joints.  Some  blasting 
has  been  done  on  the  south  side  of  this  ledge  ;  and  Mr.  Watson 
was  thus  enabled  to  find,  in  1892,  the  fragment  of  a  large 
Paradoxides  which  Mr.  Grabau  has  described.  It  shows  por- 
tions of  nine  segments  and  represents  an  unusually  large  indi- 
vidual. Through  the  generosity  of  Mr.  Watson  this  specimen 
is  now  the  property  of  the  Society.  Ledges  of  fine  granite  are 
near  by  on  the  west  and  northwest.  In  fact,  at  the  southwest 
end  of  the  ledge  the  slate  and  granite  are  seen  nearly  in  contact, 
and  at  the  northeast  end  the  actual  contact  is  exposed,  but  not 
clearly  enough  for  details.  The  nortliwest  side  of  the  slate 
ledge  thus  marks,  approximately  at  least,  the  contact  of  the 
granite  and  slate,  and  that  it  is  an  igneous  contact  is  plainly 
indicated  by  the  dikelets  or  apophyses  of  fine  granite  which 
may  be  observed  in  the  slate  toward  either  end  of  the  ledge 


401 


(Fig.  26),  as  well  as  by  the  massive  and  metamorpliic  charac- 
ter of  the  slate  itself.  Westward,  on  and  close  north  of  this  line 
of  contact,  which  must  follow  closely  the  junction  of  the  fiat 
land  and  marsh  Avith  the  upland,  the  fine  granite  is  exposed  at 
short  intervals  nearly  all  the  way  to  Mr.  Watson's  house.  Here 
the  line  is  abruptly  shifted  several  hundred  feet  to  the  south,  to 
correspond  with  the  outcrops  west  of  Quincy  Avenue,  along 
the  north  side  of  Allen  Street.  The  salient  angle  of  the 
gi-anite  due  to  this  shift  is  marked  by  the  bold  ledge  of  granite 
south  of  Mr.  Watson's  house  ;  and  the  shift  itself  is  a'  very 
probable  fault  line,  as  will  be  noted  later. 


Fig.  26.  —  Dikelet  of  flue  granite  in  tlie  Cambrian  slate  on  Weymouth  Fore 
River,  Braintree.     Scale  :  1  inch  =  2  feet. 


The  Monatiquot  JRiver  Area. — This  important  area  em- 
braces all  the  Cambrian  strata  of  East  Braintree  and  the 
Monatiquot  valley  west  of  Smelt  Brook.  The  actual  outcrops 
are,  however,  wholly  confined  to  the  district  less  than  a  mile 
in  length  between  Smelt  Brook  and  the  millpond  of  the  Jenkins 
rubber  works,   as  shown  on   the  special  map    (PI.    19).      As 


OCCAS,  PAPEIIS  B.  S.  N.  H. 


402 

before,  the  slates  are  dark  gray  to  black  on  the  south  side  of 
the  valley,  but  change  gradually  northward  to  greenish,  banded 
and  mottled  with  red,  and  then  to  brownish  red,  streaked  with 
green,  in  the  most  northerly  outcrops  ;  and  the  phenomena  of 
special  interest  are  found  chiefly  along  the  contacts  of  the  slate 
and  fine  granite  —  an  original  igneous  contact  on  the  north 
and  a  fault  contact  on  the  south.  Except  near  the  igneous 
contact,  the  slate,  of  whatever  color,  but  more  especially  the 
black  and  green,  is  highly  fissile,  the  cleavage  conforming 
perfectly  with   the  bedding. 

The  fine  granite  is  well  exposed  on  both  sides  of  Quincy 
Avenue  north  of  the  river,  and  especially  in  the  angle  between 
Quincy  Avenue  and  Allen  Street.  Going  west  on  Allen  Street 
the  granite  passes  behind  the  massive,  cross-jointed  slate  on  the 
north  side  of  the  street ;  but  about  800  feet  from  the  avenue 
it  is  brought  abruptly  out  to  the  line  of  the  street  again  by  a 
transverse  fault,  as  shown  on  the  special  map.  The  fault  is 
parallel  with  the  joint-structure  of  the  slate  and  hades  S.  E. 
10°.  The  normal  strike  of  the  slate,  as  revealed  by  occasional 
purplish  lines  in  the  dark  greenish  gray,  is  E.— W.  ;  and  dip 
S.  80°— 85°;  but  south  of  this  jutting  angle  of  granite  it  is 
deflected  to  a  south  of  east  strike.  This  salient  of  the  granite 
is  of  special  interest  because  its  contact  with  the  slate  is  a 
typical  fiiult  on  the  southeast  and  typically  igneous  on  the 
southwest.  The  igneous  contact  conforms  closely  with  the 
bedding  of  the  slate,  and  the  two  rocks  are  firmly  welded 
together,  the  granite  being  microcrystalline  and  almost  felsitic 
near  the  slate.  About  65  feet  west  of  the  granite  salient,  the 
slate  is  traversed  in  a  breadth  of  6  feet,  and  very  obliquely,  by 
half  a  dozen  or  more  vertical  dikelets  of  granite  varying  from 
one  fourth  inch  to  four  inches  in  width  and  hading  S.  W. 
15°- 20°;  and  it  is  probable  that  by  removing  the  soil  these 
apophyses  might  be  traced  back  to  their  source  in  the  main 
body  of  granite.  They  appear,  as  do  the  apophyses  generally, 
to  be   independent  of  the  joint-structure   of  the  slate.      The 


403 

massive,  dark  greenish  gray  and  brownish  slate  continues  with- 
out fartlier  interruption  to  Shaw  Street,  where,  immediately 
north  of  the  slate,  the  granite  outcrops  obscurely  in  the  private 
road.  Although  the  slate  bordering  the  granite  along  the 
north  side  of  Allen  Street  is  of  a  massive  and  more  or  less 
metamorphic  character,  it  becomes  gradually  but  rapidly  less 
metamorphic  away  from  the  granite ;  and  the  slate  between 
the  street  and  river,  some  fifty  feet  in  breadth,  is  entirely  nor- 
mal—  greenish  gray,  streaked  with  brown,  thin-bedded  and 
shaly,  and  dipping  S.  80°  — 90°. 

Farther  west  on  this  line,  the  fissile  slates,  chiefly  of  brown- 
ish red  color,  and  conformably  enclosing  two  small  trap  dikes, 
are  well  exposed  about  East  Braintree  Station  and  along  Front 
Street;  but  the  nearest  outcrops  of  fine  granite,  as  the  special 
map  shows,  are  several  hundred  feet  distant ;  and  the  contact 
cannot,  therefore,  be  definitely  located.  Through  the  middle 
of  the  valley  the  highly  fissile  green  slates,  with  streaks  and 
bands  of  red,  are  well  exposed.  At  the  Jenkins  Rubber  Mills 
the  outcrop  has  a  breadth  of  about  200  feet ;  strike  N.  82°  E. ,  dip 
N.  87°  ;  and  the  slate  again  encloses  conformably  a  narrow  dike 
of  diabase.  Along  the  river,  both  above  and  below  Com- 
mercial Street,  as  far  as  the  railroad  and  Shaw  Street,  the 
outcrops  are  almost  continuous,  with  a  total  breadth  of  fully 
300  feet,  the  strike  and  dip  remaining  unchanged.  On  the 
north  side  of  the  river  is  a  conformable  dike  one  foot  or  less  in 
width,  possibly  the  same  as  at  the  Jenkins  Mills,  but  highly 
chloritized  and  largely  replaced  by  vein  quartz. 

It  was  on  the  south  side  of  the  river,  just  east  of  the  junction 
of  Commercial  and  Union  Streets,  that  Mr.  Albert  Hobart 
found  in  the  green  fissile  slate,  during  the  excavation  of  the 
ledge  for  a  cistern  under  his  barn,  in  1872,  the  fragment  of  a 
Paradoxides  which  first  came  to  the  notice  of  Mr.  Watson  in 
1896  and  was  through  him  contributed  to  the  Museum  of  the 
Society.  It  shows  portions  of  seven  thoracic  segments  ;  and 
like  the  specimen  from  Mr.  Watson's  own  ledge,  a  mile  to  the 


404 

northeast,  it  represents  a  large  individual.  Mr.  Hobart  reports 
finding  in  the  coarse  of  the  excavation  numerous  similar  frag- 
ments ;  but  only  this  one  was  preserved.  It  is  identical  in 
lithological  character  with  the  adjacent  ledges  ;  but,  although 
I  regard  the  account  of  its  finding  as  absolutely  trustworthy, 
Mr.  Grabau  and  I  were  unable  by  careful  search  to  bring  to 
light  any  additional  paleontological  material  in  this  locality. 

South  of  the  line  of  Commercial  Street  we  pass  to  the  dark 
gray  and  black  slates  which  normally  overlie  the  green  slates 
in  the  middle  of  the  valley,  as  these  do  the  red  slates  on  the 
northern  border  of  the  valley.  Advancing  from  the  west,  the 
first  outcrop  occurs  on  Union  Street,  southeast  of  the  Jenkins 
Rubber  Mills,  showing  the  thin  and  fissile  black  slate  of  the 
Weymouth  railroad  cut  striking  N.  80°  E.  and  dipping  N.  88°. 
Turning  up  Liberty  Street  some  three  hundred  feet,  the  same 
black  thin  slate  is  found  exposed  in  the  gutter  for  a  breadth  of 
one  hundred  feet,  with  the  strike  unchanged  and  dip  vertical. 
It  was  also  extensively  exposed  during  the  construction  of  the 
Jonas  Perkins  school-house  on  the  east  side  of  Liberty  Street ; 
and  in  the  fields  to  the  southeast,  as  shown  on  the  map  (PI.  19) , 
the  outcrops  extend  nearly  six  hundred  feet  south  of  Commer- 
cial Street. 

The  chief  outcrop  on  this  line,  however,  is  that  half  a  mile 
farther  east,  at  the  junction  of  Commercial  Street  and  Quincy 
Avenue.  This  shows,  on  the  east  side  of  the  street,  from  the 
south: — (1)  Fine  pink  granite  south  of  Smelt  Brook:  (2) 
concealed,  75  feet;  (3)  trap  dike,  inclosing  a  large  body  of 
slate  and  granite,  225  feet ;  (4)  slate  north  of  the  trap,  nearly 
to  the  railroad,  120  feet.  The  slate  is  dark  gray  to  black,  and 
indistinguishable  from  tliat  in  the  railroad  cut  east  of  Smelt 
Brook,  except  that  near  the  great  dike  it  is  rather  more  mas- 
sive. The  strike  is  N.  70°-80°  E.  and  the  dip  N.  85°.  The 
contact  witli  the  large  body  of  compact  grecnisli  trap  is  a})prox- 
imately  conformable  with  the  bedding,  but  distinctly  igneous 
and  not  due  to  faulting,   the   two  rocks   being  firndy  welded 


405 

together  and  the  trap  becoming  appreciably  more  compact  and 
tlie  slate  less  fissile  near  the  contact.  In  the  slate,  fifty  feet 
north  of  the  great  dike,  are  two  vertical,  east-west  dikes  of 
gray  compact  trap  six  to  twelve  inches  wide  and  separated  by  a 
foot  of  slate.  The  inclusion  in  the  great  dike  consists  of  80  to  50 
or  60  feet  of  slate  (soft,  gray  and  visibly  stratified)  bordered  on 
the  south  by  a  gray,  slaty,  bastard  sort  of  granite.  Under  the 
blacksmith  shop  on  the  edge  of  the  marsh  is  trap,  which  seems 
to  divide  the  inclosed  mass.  The  whole  appearance  is  as  if 
this  great  dike  225  feet  wide  had  come  up  along  the  contact  of 
the  granite  and  slate  and  had  inclosed  a  large  block  of  the 
contact,  the  inclusion  showing  rather  more  slate  tlian  granite. 
Westward  from  Quincy  Avenue,  along  the  south  side  of 
Commercial  Street,  the  same  conditions  continue.  The  great 
dike  is  exposed  continuously  for  nearly  a  thousand  feet,  with 
an  average  breadth  of  at  least  three  hundred  feet.  Ledges  of 
fine  granite  are  quite  near  it  on  the  south,  and  the  included 
mass  or  partition  of  slate  and  granite,  with  several  detached 
masses  of  granite,  can  be  traced  as  far  as  the  first  lane  running 
south  from  Commercial  Street,  about  six  hundred  feet  west  of 
Quincy  Avenue  ;  so  that  in  this  part  of  its  course  the  dike  is 
virtually  double.  Farther  west  on  this  line  the  rocks  are  largely 
concealed  by  an  embryo  drumlin  ;  but  on  the  west  side  of  the 
lane  midway  between  Quincy  Avenue  and  Liberty  Street, 
the  trap  and  granite  are  well  exposed,  showing  that  the  dike  is 
still  at  least  two  hundred  feet  wide,  but  apparently  without 
included  slate,  although  inclosing  fragments  of  granite.  This 
outcrop  lies  to  the  south  of  the  main  line  of  the  dike,  suggesting 
a  transverse  fault  with  a  horizontal  displacement  of  about  two 
hundred  feet,  or  approximately  equal  to  the  breadth  of  the  dike  ; 
and  it  is  interesting  to  note  that  this  fault  corresponds  closely 
in  position  and  direction  of  throw  with  the  fault  breaking  the 
northern  contact,  on  Allen  Street.  Farther  west  on  this  line  the 
country  is  heavily  drift-clad  ;  the  great  dike  does  not  outcrop 
again  ;  and  it  cannot  be    confidently  traced    by    its    erratics ; 


406 

although  it  is  altogether  probable  that  it  continues  at  least 
as  far  to  the  westward  as  the  main  line  of  the  Old  Colony 
Railroad. 

Returning  to  the  section  on  Quincy  Avenue,  and  proceeding 
thence  eastward,  we  find  on  the  east  side  of  Smelt  Brook,  in 
the  trend  of  the  great  dike  and  the  slate,  a  continuous  and  abrupt 
wall  of  fine  granite,  extending  north  to  the  west  end  of  the 
railroad  cut  at  Weymouth  Station.  Smelt  Brook  appears, 
thus,  to  mark  the  position  of  a  transverse  fault,  with  a  lateral 
shift  of  four  or  five  hundred  feet.  The  topographic  indica- 
tions are  very  favorable  to  its  extension  northward  across  the 
river,  passing  in  the  rear  of  Mr.  Watson's  house,  where  the 
northern  wall  of  o-ranite  is  ioo:<j;ed  to  the  north  several  hundred 
feet  [antea,  p.  401)  ;  and  this  important  displacement  may  be 
accepted  as  the  true  boundary  between  the  Fore  River  and 
Monatiquot  River  ai'cas.  The  great  boundary  dike  is  not  only 
broken,  but  it  is  practically  interrupted  by  the  Smelt  Brook 
fault.  Between  the  granite  and  slate,  and  in  the  granite  near 
the  contact,  at  Weymouth  Station,  is  an  irregular  develojj- 
ment  of  trap,  from  a  few  feet  to  a  few  yards  in  breadth,  and 
of  a  similar  character  to  that  of  the  o-reat  dike.  Ascendino-  the 
hill  (Mt.  Pleasant),  two  large  dikes  are  found  in  the  granite  on 
Keith  Street,  trending  southeasterly  and  converging  in  that 
direction.  On  the  north  slope  of  Mt.  Pleasant,  midway 
between  Keith  and  Webb  Streets,  an  irregular,  branching 
trap  dike  40  to  50  feet  wide  runs  from  the  boundary  fault 
southeasterly  and  easterly  through  the  fine  granite  for  several 
hundred  feet  at  least.  The  trap  is  exposed  on  Webb  Street 
about  five  hundred  feet  south  of  Commercial  Street ;  and  east  of 
the  street  outcrops  are  scattering,  the  trap  appearing  to  be  still 
considerably  mixed  up  with  the  granite  and  somewhat  inter- 
rupted. But  east  of  the  brook  separating  the  Fore  River  and 
White's  Neck  areas,  and  south  of  Commercial  Street,  the  great 
dike  is  again  strongly  developed,  with  a  breadth  of  at  least 
two  hundred  feet ;  and  cast  from  this  point  it  follows  the  line  of 


407 

Commercial  Street,  with  numerous  outcrops  and,  apparently,  a 
great  breadth,  to  the  Mill  Cove  brook  and  fault.  Farther  cast 
it  is  lost  beneath  the  drift ;  but  it  may  be  regarded  as  con- 
tinued in  the  dikes  cutting  the  granite  on  the  northeast  side  of 
King  Oak  Hill.  This  great  dike,  whether  following  the 
boundary  fault  closely  or  branching  in  the  adjacent  granite,  is 
undoubtedly  related  to  the  fault  in  origin,  and  may  be  properly 
designated  the  boundary  dike. 

Westward  from  East  Brainti-ee  the  Monatiquot  Valley  is  an 
almost  complete  blank,  so  far  as  the  geology  of  the  hard  rocks 
is  concerned.  The  fine  granite  is  exposed  on  the  north  side  of 
the  valley  at  the  corner  of  Commercial  and  Adams  Streets  ; 
but  west  of  Commercial  Street  and  the  Jenkins  Mill  Pond 
reliable  outcrops  are  wholly  wanting  as  far  as  the  main  line  of 
the  Old  Colony  Railroad.  Immediately  west  of  the  mill  pond, 
and  bordered  on  the  north  and  west  by  the  Monatiquot  River, 
there  rises  to  a  height  of  from  fifty  to  nearly  one  hundred  feet 
above  the  valley  a  broad,  gently-sloping  mass  of  till  (a  group  of 
thi-ee  more  or  less  distinct  drumlins)  extending  south  to  Union 
Street  and  crossed  obliquely  by  Middle  Street.  The  contours 
suggest  a  moderate  depth  of  drift  —  immature  drumlins  ;  and 
the  bowlders  indicate  that  the  bed  rock  is  chiefly  fine  gi'anite. 
In  fact,  on  the  extreme  northei'n  edge  of  this  area,  east  of 
Middle  Street  and  immediately  south  of  the  railroad,  a  slight 
excavation  shows  such  large  and  angular  blocks  of  fine  granite 
as  to  indicate  that  the  rock  is  practically  in  place  here  ;  and  it 
is  probable,  judged  by  the  character  of  the  drift,  that  the 
granite  extends  a  short  distance  north  of  the  railroad,  perhaps 
as  far  north  as  the  northern  edge  of  the  slate  in  East  Braintree. 

It  thus  appears  probable  that  the  slate  is  practically  if  not 
entirely  cut  ofi",  in  its  direct  westward  extension,  in  the  vicinity 
of  the  Jenkins  Mill  Pond.  The  topographic  indication,  how- 
ever, is  favorable  to  the  view  that  the  slate  bends  to  the  north 
with  the  river  and  resumes  its  westward  course  along  a  more 
northern  line  ;   and  this  view  finds  satisfactory  confirmation  in 


408 

the  evidence  afforded  by  a  well  drilled  ten  years  ago  on  the 
north  side  of  the  valley,  at  the  tannery  of  Col.  A.  C.  Drink- 
water  in  the  angle  between  Adams  and  Elm  Streets.  Accord- 
ing to  Col.  Drinkwater,  the  well  is  151  feet  deep,  and  the  drill 
passed  through  gravel  and  quicksand  to  a  depth  of  16  feet  and 
then  135  feet  into  soft,  dark  slate ;  and  this  evidence  is 
corroborated  by  the  well  of  Mr.  Morrison,  drilled  about  the 
same  time,  on  the  opposite  (south)  side  of  Adams  Street, 
the  drill  having  passed  through  gravel  and  granite  bowlders 
into  slate.  More  recently  (April,  1899),  and  since  the 
preceding  sentences  were  written,  another  well  has  been  bored 
on  the  south  side  of  Adams  Street,  at  the  absorbent  cotton 
factory  about  midway  between  Elm  Street  and  Commercial 
Street.  According  to  data  kindly  collected  by  Mr.  Watson, 
the  section  for  this  well  is  :  sand,  gravel,  and  cobbles,  about 
20  feet  ;  fine  granite,  15  feet ;  massive  gray  slate,  83  feet ; 
fine  granite,  in  which  they  drilled  for  two  days  (probably  8  to 
10  feet)  without  getting  through  it,  the  well  ending  in  this 
rock,  with  a  total  depth  between  125  and  130  feet.  The 
upper  fine  granite  is  much  oxidized  and  weathered,  and  prob- 
ably consists  of  one  or  more  bowlders,  as  in  the  Morrison  well, 
although  it  possibly  is  an  oblique  dike  in  the  slate.  This  well 
confirms  the  records  of  the  other  wells  and  affords  the  further 
important  information  that  the  slate,  although  probably  highly 
inclined,  as  usual,  is  underlain  at  no  great  depth  by  the  fine 
granite,  against  which  it  probably  ends  abruptly  and  with 
entire  uncomformity  as  the  result  of  fusion  and  absorption  by 
the  molten  granite  magma.  The  testimony  of  these  wells  could 
not  be  moi-e  opportune  as  to  location,  and  it  may  be  accepted 
with  confidence. 

To  carry  the  slate  outcropping  on  Liberty  and  Union  Streets 
northward  a  full  third  of  a  mile  around  the  northern  edge  of 
the  granite  would  require  a  sharp  fiexure,  of  which  we  have  no 
indications  in  the  strike  of  the  slate  east  of  the  Jenkins  Mill 
Pond  ;   and  therefore,  following  the  analogv  of  the  structure  to 


409 

the  eastward  on  this  belt,  a  transverse  fault  appears  to  oiler  the 
best  interpretation  of  the  meagre  facts  ;  and  the  map  has  been 
drawn  in  accordance  with  this  view.  This  fault,  it  will  be 
observed,  is  complementary  with  reference  to  the  Smelt  Brook 
fault.  At  no  point  in  this  boundary  belt  of  slate  is  there 
evidence  of  any  important  flexure  affecting  the  strike  of  the 
beds ;  and  hence,  although  the  transverse  faults  shown  on 
the  map  are  not,  in  every  case,  fully  demonstrated,  they  are 
clearly  the  most  probable  form  of  displacement. 

The  Monatiquot  Heights  and  Quincy  Reservoir  Area. — 
This  area  embraces  that  part  of  the  southern  boundary  belt  of 
Cambrian  strata  west  of  the  main  line  of  the  Old  Colony  Rail- 
road. The  only  actual  outcrops  of  the  sedimentary  rocks  are 
on  the  eastern  shore  of  the  Quincy  Reservoir,  which  lies  in  the 
a,ngle  between  West  and  Granite  Streets,  near  Five  Corners. 
Beginning  near  the  outlet  of  the  reservoir  and  extending  south 
along  the  eastern  shore,  the  slate  is  obscurely  exposed  at 
intervals  for  a  total  breadth  of  nearly  a  thousand  feet.  About 
one  fourth  of  a  mile  east  of  the  reservoir,  on  Monatiquot 
Heights,  the  slate  was  reached  at  depths  of  11  and  12  feet  in 
two  wells  in  the  rear  of  the  house  of  Thomas  Hollinshead,  on 
Hollis  Avenue.  In  both  localities  the  slate  is  a  dark  gray  to 
black,  fissile  and  distinctly  stratified  variety,  with  numerous 
lenticules  and  streaks  of  finely  arenaceous  material,  some  of 
which  are  highly  pyritiferous.  The  strike,  as  observed  in  the 
wells,  is  east-west,  approximately,  and  the  dip  nearly  vertical. 
To  the  northward  of  these  wells  for  several  hundred  feet  the 
ground,  as  shown  by  street  excavations,  is  full  of  angular  frag- 
ments of  the  same  slate,  indicating  that  it  is  in  place  at  a 
slight  depth  ;  and  it  doubtless  extends  at  least  as  far  north  as 
the  bottom  of  the  hill,  v^^hich  is  in  about  the  same  latitude  as  the 
outlet  of  the  reservoir.  Southward  the  evidence  is  equally 
satisfactory  as  far  as  West  Street,  south  of  which  the  slate 
appears  to  give  way  to  the  fine  granite.  Monatiquot  Heights 
is  evidently  a  thinly  drift-clad  ridge  composed  of  slate  to  the 


410 

north  and  fine  granite  to  the  south,  the  common  boundary  of 
the  two  formations  following  approximately  the  line  of  West 
Street,  as  indicated  by  the  character  of  the  drift  and  by  the 
ledges  of  fine  granite  on  AV^est  Street  near  Five  Corners. 

So  far  as  known,  all  the  slate  of  this  area  is  essentially 
similar  to  that  in  the  Weymouth  railroad  cuts  and  along  the 
south  side  of  the  Monatiquot  Valley  in  East  Braintree,  and 
may  be  referred  to  the  Middle  Cambrian.  West  of  the  reser- 
voir, about  Five  Corners  and  northward  along  Granite  Street, 
the  drift  contains  more  or  less  slate.  In  part,  however,  it  is  very 
massive,  gray,  indistinctly  stratified,  and  somewhat  micaceous, 
one  specimen  being  also  garnetiferous,  closely  resembling  the 
massive  and  metamorphic  slate  of  the  North  Common  Hill  and 
West  Quincy  areas,  and  these  localities  may  be  its  source. 
West  of  Granite  Street  is  a  long  narrow  drumlin,  and  west  of 
that  a  broken  ridge  of  the  fluidal  felsite,  followed  by  widely 
scattered  outcrops  of  the  same  rock  both  east  and  west  of  Pur- 
gatory Road.  The  drumlin  referred  to  is  probably  underlain 
by  the  felsite,  and  possibly,  also,  the  small  drumlin  partly 
encircled  by  the  reservoir.  Knowing  the  felsite  to  be  eiFusive 
and  younger  than  the  Cambrian  slates,  it  appears  reasonable 
to  suppose  that  the  slate  extends  westward  beneath  the  felsite  : 
and  the  valley  of  Town  Brook,  to  the  northward,  according  to 
this  view,  is  where  erosion  has  cut  through  the  felsite  into  the 
underlying  slate.  Following  up  this  idea,  and  the  topographic 
indications  being  extremely  favorable,  I  have  ventured,  on  the 
map,  to  extend  tJie  slate  northward  from  the  Quincy  Reservoir 
and  Monatiquot  Heights  to  the  southern  base  of  Pine  Hill, 
and  westward  across  Granite  Street  and  Purgatory  Road. 
From  the  great  dike  or  vent  of  fluidal  aporhyolite  cutting  the 
granite  and  quartz  porphyry  of  Pine,  Rattlesnake  and  Wam- 
patuck  Hills,  a  sheet  of  this  acid  lava  flowed  southward  across 
a  broad  belt  of  Cambrian  slate  inclosed  in  the  deeply  denuded 
granitic  rocks  ;  and  erosion  is  now  slowly  removing  this  pro- 
tecting cover  of  hard  and   flinty  aporhyolite.      That  the  slate 


411 

cannot  be  regarded  as  extending  very  far  into  or  beneath  the 
igneous  rocks  of  the  Blue  Hills  is  evident  from  the  fact  that 
in  the  longitude  of  Chickatawbut  and  Hawk  Hills  and  farther 
west  the  quartz  porph}ay  of  the  contact  zone  is  continuous 
from  north  to  south  across  its  course. 

South  of  Pine  Hill,  according  to  the  interpretation  of  the 
structure  here  proposed,  the  breadth  of  the  slate  can  hai'dly  be 
less  than  three  fourths  of  a  mile,  its  southern  boundary  show- 
ing a  displacement,  as  compared  with  the  pi'obable  southern 
boundary  of  the  slate  east  of  the  railroad,  of  fully  half  a  mile. 
The  nature  of  this  break  is  doubtful ;  but  its  seeming  abrupt- 
ness and  the  fact  that  it  coincides  with  a  strongly  marked  and 
continuous  depression  extending  in  a  north-south  direction 
entirely  across  the  Blue  Hills  Complex  along  the  line  of  the 
Old  Colony  Railroad,  breaking  the  northern  boundary  belt  of 
slate  in  the  same  manner  as  the  southern,  is  certainly  sugges- 
tive of  a  fault  with  the  upthrow  to  the  east  —  a  profound  dis- 
placement, breaking  and  contrasting  the  rocks  of  the  contact 
zone  as  well  as  the  Cambrian  strata.  The  quartz  porphyry 
occurs  wholly  to  the  west  of  this  line,  and  the  fine  granite  mainly 
to  the  east  of  it.  This  contrast  in  the  contact  zone  suggests, 
also,  that  this  line  may  mark  an  original  structural  feature  of 
the  batholite,  — a  more  or  less  definite  line  west  of  which  the 
magma  rose  to  a  greater  height  in  the  Cambrian  strata  than  to 
the  eastward. 

The  question  has  been  raised  by  Mr.  Watson  and  myself 
as  to  whether  slate  might  not  be  advantageously  mapped  as 
extending  northward  from  Braintree,  under  the  broad  and 
level  valley  of  Town  Brook,  between  Payne's  Hill  and  Pine 
Hill,  as  far,  perhaps,  as  Quincy-Adams.  The  chief  consider- 
ations which  have  finally  influenced  me  not  to  do  this  are  : 
first,  that  elsewhere  throughout  the  Blue  Hills  Complex  the 
slate  occurs  in  belts  having  a  pronounced  east-west  trend ; 
second,  ledges  of  normal  granite,  with  no  suggestion  of  the 
contact   zone   rocks,    occur  in   the   middle  of  the   valley  near 


412 

Quincy-Adams  ;  and,  third,  this  depression  is,  perhaps,  suffi- 
ciently accounted  for  by  tlie  great  north-south  fault  already 
postulated. 


THE  NORTHERN  BOUNDARY  BELT. 

This  belt  is  exposed  at  intervals  for  a  length  of  about  seven 
miles,  from  North  Weymouth  neai-ly  to  Randolph  Avenue  in 
Milton.  On  account  of  the  widespread  and  well-nigh  continu- 
ous deposits  of  modified  drift  bordering  the  Complex  on  the 
north,  this  belt  is,  on  the  whole,  less  clearly  and  sharply 
defined  than  the  southern  boundary  belt.  This  is  especially 
true  of  the  northern  or  outer  edge  of  the  belt.  This 
margin,  like  the  corresponding  or  southern  margin  of  the 
southern  belt,  is  probably  determined  by  an  important  fault 
line  —  a  great  displacement  which  appears  to  be  indispensable 
in  explaining  the  relations  of  the  Carboniferous  strata  to  the 
Complex,  But  we  miss  on  this  line  the  clear  and  tangible 
proofs  of  faulting,  such  as  are  afforded,  for  the  southern  belt, 
by  the  railroad  cuts  in  Weymouth.  It  may  be  added,  how- 
ever, that  the  inner  margin,  facing  the  granite,  is  ranch  more 
extensively  and  satisfactorily  exhibited  than  for  the  southern 
belt ;  and  of  the  igneous  nature  of  this  contact  throughout  its 
entire  length  there  can  be  no  question.  This  belt,  like  the 
southern,  consists,  chiefly  at  least,  of  the  red  and  calcareous 
Lower  Cambrian  strata  in  the  eastern  half,  and  the  dark  gray 
to  black  non-calcareous  slates  of  probably  Middle  Cambrian 
age  to  the  westward;  and  like  the  southern  belt,  also,  this 
belt  may  be  divided  into  several  sections  for  convenience  of 
description  ;  but  the  transverse  faults  are  much  less  in  evidence 
than  along  the  southern  xifiargin  of  the  Complex. 

The  map  shows  a  strong  eastward  convergence  of  the  north- 
ern and  southern  boundaiy  belts,  the  distance  between  them 
ranofino:  from  more  than  two  miles  in  the  lono-itude  of  West 


Quincy  to  less  than  half  a  mile  in  North  Weymouth  :  and  it 
is  not  improbable  that  they  are  actually  united  before  reaching 
Back  River.  In  the  absence  of  evidence  to  the  contrary,  the 
map  has  been  constructed  in  accordance  with  this  hypothesis. 

The  Mill  Cove  Area.  — The  mantle  of  modified  drift  over 
North  Weymouth  is  almost  continuous  from  Fore  River  to 
Back  River,  and  from  the  northern  shore  and  Great  Hill 
to  King  Oak  Hill.  But  there  is  one  very  fortunate  hiatus, 
where  the  nearly  perfect  development  of  the  Quincy  plain 
(40  feet)  between  Fore  River  and  Old  Spain  fails  by  half  a 
mile,  in  the  latitude  of  Mill  Cove,  to  connect  with  the  higher 
Hingham  plain  to  the  south,  in  the  vicinity  of  King  Oak  Hill. 
(See  map  of  Lake  Bouve,  PI.  25.)  The  Quincy  plain 
terminates  on  the  southeast  in  an  almost  ideal  delta  front ;  and 
the  intervening  ground,  although  largely  of  a  swampy  character, 
discloses  a  considerable  body  of  fine  granite  and  north  of  the 
granite,  with  an  east-west  contact,  the  finest  development  of 
the  Lower  Cambrian  strata  to  be  found  in  the  region  of  the 
Blue  Hills  Complex. 

The  principal  and  most  interesting  of  the  Cambrian  outcrops 
are  on  the  wooded  promontory  lying  west  of  the  junction  of 
Pearl  and  Norton  Streets  and  projecting  southward  into  the 
cove  (PI.  20).  We  have  here  in  an  almost  continuous 
exposure  about  700  feet  of  strata,  disappearing  southward 
beneath  the  water  and  northward  beneath  the  drift.  The 
strike  is  N.  75°-80°  W.  ;  and  the  dip  S.  70°- 80°  ;  while  an 
indistinct  cleavage  structure  dips  N.  75°.  The  prevailing  rock 
is  a  deep  red  or  brownish  red  calcareous  slate,  occasionally 
streaked  and  striped  with  green,  and  enclosing  several  thin 
beds  of  limestone.  The  calcareous  matter  in  the  slates  occurs 
normally  in  the  form  of  flat,  lenticular  nodules  one  to  six 
inches  or  more  in  diameter,  lying  in  the  planes  of  stratification. 
These  may  be  regarded  as  fossil  claystones,  the  segregation  of 
the  calcium  carbonate  in  the  clay  having,  it  is  probable, 
occurred    soon    after    its    deposition,    and    before    the    general 


414 

lithification  of  the  deposit,  as  suggested  on  page  303.  Like 
modern  clay  stones,  they  are  intimate  admixtures,  in  variable 
proportions,  of  the  argillaceous  material  and  calcium  carbonate. 
They  appear  to  become,  as  a  rule,  more  calcareous  from  the 
surface  inwards  ;  and  where  not  aifected  by  epidotic  alteration 
they  are  inconspicuous  on  the  weathered  surface,  or  represented 
only  by  depressions  and  holes  through  the  solution  of  the 
calcareous  constituent.  As  indicated  on  the  map  (PL  20), 
which  does  not  embrace  the  most  northerly  ledges  of  slate,  the 
claystones  occur  mainly  in  lines  along  particular  bedding  planes 
of  the  slate  and  often  coalesce,  thus  tending  to  pass  into  con- 
tinuous streaks  of  calcareous  slate  or  more  or  less  impure 
limestone. 

Three  distinct  beds  of  limestone  have  been  observed  (PI.  20)  , 
varying  from  four  inches  to  two  feet  in  thickness.  The  most 
southerly  bed  outcrops  on  the  extreme  southern  end  of  the 
promontory,  with  a  thickness  of  one  foot  and  possibly  more. 
Unfortunately,  this  exposure  has  been  pretty  well  obliterated 
by  recent  blasting  of  the  ledge  for  road  metal.  On  the  shore, 
some  two  hundred  feet  west  and  fifty  feet  north  of  this  point, 
the  second  bed  outcrops,  with  a  total  thickness  of  about  two 
feet,  and  showing  a  displacement,  equal  to  its  own  thickness,  by 
a  transverse  fault.  One  hundred  feet  farther  north,  on  the 
extreme  west  side  of  the  promontory,  the  third  bed,  about  four 
inches  in  thickness,  and  also  broken  by  a  slight  fault,  is 
obscurely  exposed,  the  calcareous  matter  having  been  removed 
superficially  by  solution.  The  close  agreement  of  the  first  and 
second  beds  in  width  and  character  suggests  that  they  may  be, 
in  reality,  portions  of  the  same  bed  displaced  by  a  more 
important  but  undiscovered  fault.  The  dike  shown  on  the 
map  as  cutting  the  strata  between  these  two  outcrops  is 
certainly  not  on  a  fault  line,  for  the  lines  of  stratification  in  the 
slate,  and  especially  the  lines  of  epidotic  nodules,  are  not 
displaced  by  the  dike.  The  limestone  is  normally  a  white, 
finely   crystalline    rock,    with    some    accessory   and    secondaiy 


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415 

silicates    (epidote,    etc.)    which   give   it   exceptioiiiil   hardness. 
Selected  specimens  gave,  on  analysis  by  Dr.  W.  H.  Walker  :  — 

Insoluble  residue  27.80 

CaCO^  71.50 

MgCOg    >  by  difference      0.70 
FeCO.3     >  100.00 

Like  the  calcareous  concretions  described  above,  the  lime- 
stone bands  are  purest  in  the  middle  and  merge  rather  gradu- 
ally outw^ard  into  the  slate. 

One  of  the  most  puzzling  features  of  this  group  of  ledges  is 
a  band  of  massive  greenish  slate,  twenty  to  thirty  feet  in  width, 
crossing  the  formation  in  a  generally  north-south  direction  and 
interrupting  the  continuity  of  the  red  beds  along  the  strike. 
As  indicated  on  the  map,  it  commences  on  the  shore  west  of 
the  second  bed  of  limestone  and  extends  directly  north  across 
the  third  limestone  band;  and  then,  apparently,  it  is  bent 
abruptly  to  the  northeast,  reappearing  beyond  a  gap  in  the 
ledges  sixty  feet  to  the  eastward.  Still  farther  to  the  north- 
east, across  the  marsh,  the  same  or  a  similar  band  reappears 
and  branches  toward  Pearl  Street.  This  rock  is  so  massive  as 
to  suggest  a  dike  of  fine-grained  trap  ;  but  there  are  no  walls, 
and  in  both  color  and  structure  it  blends  both  east  and  west 
gradually  and  perfectly  with  the  red  slates.  The  bedding 
lines  and  calcareous  segregations  are  wholly  wanting  ;  but  the 
epidote,  which  occurs  both  in  a  diffused  form,  accentuating  the 
green  color  of  the  rock,  and  as  veinlets,  and  is  undoubtedly 
a  secondary  constituent,  is  a  plain  indication  that  the  massive 
slate  was  once  calcareous.  I  can  see  no  escape  from  the  con- 
clusion that  the  slate  along  this  line  was,  at  an  early  pei'iod 
and  probably  before  its  complete  consolidation  or  the  segre- 
gation of  the  calcareous  matter,  exposed  to  some  influence, 
such  as  faulting  or  compression  and  shearing  movements, 
which  has,  as  it  were,  shaken  it  down,  obliterated  its  original 
structures,    and    reduced    it  to   a   structureless    mass ;    giving 


416 

rise  to  what  is  virtually  a  slate  dike  of  the  same  type  as  the 
quartzite  dikes  described  by  Professor  J.  D.  Dana^  in  Dutchess 
County,  New  York. 

In  the  practical  absence  of  fossils,  the  geological  interest  of 
these  Lower  Cambrian  outcrops  culminates  in  the  alteration 
of  the  calcareous  portions  through  the  metamorphic  influence 
of  the  granite.  The  only  conspicuous  secondary  mineral  is 
ejoidote,  and  it  is  very  obvious  that  its  development  has  been 
confined  chiefly  to  the  impure  (aluminous  and  ferruginous) 
peripheral  portions  of  both  the  claystones  and  the  limestone 
bands,  so  that  each  calcareous  body  of  whatever  kind  has 
normally  an  epidotic  shell  or  outer  envelope,  which  for  the  clay- 
stones,  is  commonly  from  a  quarter  to  half  an  inch  in  thickness, 
and  for  the  limestone  strata  from  an  inch  to  five  or  six  inches. 
In  the  purer  central  portions,  alike  of  the  claystones  and  lime- 
stone strata,  the  aluminous  impurity  or  clay  was,  apparently, 
insufficient  for  the  formation  of  epidote  ;  and,  except  in  the 
case  of  the  thicker  limestone  layers,  the  residual  calcareous 
matter  has  very  generally  been  dissolved  out  by  meteoric 
waters,  leaving  within  the  epidotic  shells  only  the  insoluble 
silica,  iron  oxide,  etc.  I  have,  however,  observed  concretions 
in  which  the  calcareous  center  or  core  is  still  undissolved  ;  and 
in  a  few  instances  at  least  this  core  is  a  white  crystalline  lime- 
stone, like  the  centers  of  the  limestone  strata,  suggesting  that 
the  formation  of  the  epidote  may  have  been,  through  the 
removal  of  the  silica,  alumina  and  iron  oxide,  a  purifying  pro- 
cess for  the  calcareous  residue.  We  may,  perhaps,  suppose  that 
the  formation  of  epidote  began  toward  the  exterior  of  the  mass, 
where  the  various  constituents  occurred  most  nearly  in  the 
right  proportions ;  and  then  that  the  shell  thus  established 
thickened  both  externally  and  internally  until  the  supply  of 
certain  constituents  —  lime  for  the  outside  and  alumina,  silica, 
etc.,  for  the  inside  —  was  exhausted.  A  central  resithie  of 
calcium  carbonate  (calcite)  appears  the  more  probable  in  view 

1  Aiiier.  .journ,  sci.,  vol.  10;{,  ])]•.  181-182 ;  Bull.  Essex  Inst.,  vol.  27,  j).  145. 


417 

of  the  fcict  tliat  while  u  pure  epidotc  rcquh-es  only  2o  per  cent, 
of  lime,  claystones  may  be  more  than  half  calcium  carbonate, 
equivalent  to  at  least  30  per  cent,  of  lime  ;  and  the  fui'thcr 
fact  that  the  epidotic  shells  are  really  very  impure,  containiMi;' 
in  intimate  admixture  with  the  epidote  a  large  proportion  of 
free  silica.  Mr.  White  has  noted  the  impurity  of  the  epidotic 
shells  ;  and  he  also  mentions  peculiar  phenomena  of  rifting  in 
these  walls,  as  though  they  had  been  regions  of  special 
shrinkage  and  distortion,  and  states  that  the  same  tendency  is 
observed  in  thin  sections,  in  the  formation  of  minute  rifts. 
This  shrinkage  is  certainly  very  probable,  since  the  partial 
conversion  of  a  highly  hydrated  and  carbonated  clay  stone  to 
a  nearly  anhydrous  silicate  of  superior  hardness  and  density 
must  be  attended  by  condensation.  The  probability  of  a 
resultant  central  residue  of  calcium  carbonate  might  have  been 
tested  by  an  analysis  of  the  central  portion  of  a  claystone  which 
had  not  suffered  the  epidotic  alteration  ;  but  it  has  appeared 
])ractically  impossible  to  obtain  such  a  specimen  which  had  not 
had  some  of  its  calcium  carbonate  dissolved  out  by  meteoric 
waters.  Besides  the  epidote,  the  only  other  distinct  silicate 
recognized  among  the  alteration  products  is  chlorite,  and  this 
occurs  very  sparingly,  a  fact  which  harmonizes  well  with  the 
small  proportion  of  magnesia  among  the  original  carbonates, 
as  shown  by  the  analysis  of  the  limestone. 

The  metamorphism  described  in  the  preceding  paragraphs 
is  strictly  confined  to  the  calcareous  portions  of  the  formation, 
and  gradually  diminishes  in  intensity  northward,  so  that  in  the 
most  northerly  outcrops  the  secondary  minerals  are  wholly 
wanting.  Attention  is  thus  directed  to  the  belt  of  granite 
bordering  the  sedimentary  rocks  on  the  souths  as  the  probable 
source  of  the  metamorphic  influences  ;  the  only  other  -igneous 
mass  in  sight  being  the  small  trap  dike  cutting  the  slate  at  the 
southern  end  of  the  promontory,  and  this  evidently  was  formed 
after  the  epidote  and  has  exerted  no  appreciable  metamorphic 
influence  upon  the  inclosing  sediments. 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  26. 


418 

Eastward  from  the  shore,  these  red  slates  with  the  calcare- 
ous segregations  showing  increasing  epidotic  alteration  to  the 
southward,  outcrop  almost  continuously  for  half  a  mile.      On 
and  near  Pearl  Street  they  are  well  exposed  both  north  and 
south  of  the  brook  and  for  a  total  breadth  slightly  greater  than 
on  the  wooded  promontory.     Peai-1  Street   turns   to   the   east 
here,  following  the  northern  edge  of  tlie  granite  ;    and  between 
the  granite  and  the  brook,  north  of  and  in,  but  not  south  of, 
the  street,  the  slate  is  much  in  evidence.      On  and  near  North 
Street^  from  the  corner  of  Pearl   Street  northward,  and  in  the 
field  east  of  the  street,  the  slate  outcrops  are  almost  continuous 
for  a  breadth  of  400  feet.      The   most   easterly   outcrops   are 
fully  five  hundred  feet  east  of  the  street,  and  the  dip  is  practically 
vertical  throughout.      The  epidotic  shells   representing  original 
calcareous  segregations  ai'e  a  conspicuous  feature  to  the  south- 
ward, and  in  this  direction,  also,  the  color  is,  as  usual,   dark 
red  streaked  and  blotched  with  green  ;   but  to  the  northward 
the  color  proportions    are   gradually  changed,    until   we   have 
mainly  green  slate  streaked  with  red.     Farther  north  on  North 
Street,  after  an  interval   of  a  hundred  feet,  the  slate  outcrops 
obscurely  in  the  field  about  a  hundred  feet  west  of  the  street  and 
the  same  distance  south  of  the  southern  margin  of  the  sand  plain  ; 
and  just  in  line  with  the  south  side  of  Neck  Street.     The  out- 
crop shows  at  least  thirty  feet  in  breadth  of  greenish  gray,  fis- 
sile and  non-calcareous  slate,  with  east- west  strike  and  vertical 
diD.     This   is   the   most   northerly  outcrop   in   the   Mill   Cove 
area  ;  and  the  total  exposed  breadth  of  the  Lower  Cambrian 
strata  is  quite  certainly  not  less  than  eight  hundred  feet.     Modi- 
fied   drift    conceals    everything    from  the  North   Street  ledges 
northward  to  the  shore  and  eastward  to  Back  River. 

The  granite  is  not  exposed  east  of  North  Street  ;  but  it  out- 
crops boldly  at  the  corner  of  Nortii  and  Pearl  Streets,  near 
the  slate,  and  forms  a  broad  ridge  extending  west  and  south- 
west, parallel  with  Pearl  Street,  to  the  shore.  The  most 
southerly  outcrop  of  granite  in  this-  area  is  on  tlie  south  side 


419 

of  the  high  sandy  inclge  or  esker  on  which  the  cemetery  standB, 
and  several  hundred  feet  west  of  Norton  Street,  at  the  point 
where  it  cuts  through  the  ridge  and  turns  to  the  southeast. 
This  outcrop  is  of  medium  coarse  texture  —  distinctly  coarser 
than  any  of  the  outcrops  north  of  the  cemetery  I'idge  ;  and  as 
we  approach  the  slate  the  granite,  which  is  everywhere  very 
dark-colored  below  the  zone  of  surface  oxidation,  becomes 
gradually  finer-grained,  and  is  essentially  microcrystalline  along 
the  northern  margin  of  the  belt,  where  it  faces  the  slate. 
Westward,  the  granite  may  be  regarded  as  ending  against  the 
north-south  Mill  Cpve  fault,  which  farther  south  forms  the 
common  boundary  of  the  King  Oak  Hill  and  White's  Neck 
areas  of  the  southern  boundary  belt. 

Unfortunately,  the  contact  of  the  granite  and  slate  is 
nowhere  clearly  exposed ;  and  on  Norton  Street  the  nearest 
outcrops  of  the  two  rocks  are  about  150  feet  distant.  But 
there  can  be  no  doubt  that  it  is  an  original  igneous  contact. 
The  increasing  fineness  of  the  granite  and  raetamorphism  of 
the  slate  toward  their  common  boundary  require  this  explana- 
tion. We  have,  however,  still  more  conclusive  proof  of  the 
invasion  of  the  slate  by  the  granite  along  this  line.  The 
granite  ledges  end  on  the  west  in  Burying  Island,  a  low  emi- 
nence projecting  into  Mill  Cove  some  three  hundred  feet  south 
of  the  wooded  promontory  first  described,  and  connected  with 
the  mainland  by  the  salt  max'sh.  On  the  north  side  of  the 
island  the  fine  dark  OTanite  or  granodiorite  cuts  and  incloses 
isolated  masses  of  a  dark  gray  and  extremely  tough  and  mas- 
sive slate.  The  slate  is  decidedly  metamorphic,  showing  con- 
siderable secondary  mica  (sericite),  and  is  firmly  welded  to 
the  ffranite.  On  account  of  its  dark  and  massive  character  it 
is  easily  mistaken  for  trap  :  but  it  is  also  faintly  stratified  in  a 
vertical  east-west  direction.  It  is  clearly  cut  and  injected  b}^ 
the  granodiorite  ;  and  beautiful  veinlets  of  pegmatite  (red  feld- 
spar and  quartz)  from  a  line  to  one  fourth  of  an  inch  wide 
•extend  from  the  granodiorite  through  the  slate.      The  princi- 


420 

pal  mass  of  slate  is  three  to  six  feet  wide,  and  is  exposed  for  a 
length  (east-west)  of  over  forty  feet,  but  it  may  be  much 
longer.  One  foot  north  of  this  main  body  of  slate  is  the 
smaller  mass  shown  in  Fio-.  27. 


Fig.  27.  —  Showing  tlie  relations  of  the  massive  gray  slate  to  the  grano- 
diorite  on  Burying  Island,  Mill  Cove.     Scale  :  1  inch  =  1  foot. 

The  relations  of  this  massive  gray  slate  to  the  red  calcareous 
and  epidotic  slate  are  not  quite  clear  ;  but  that  they  belong  to 
the  same  conformable  series  is  indicated  by  the  ledge  on  Norton 
Street  immediately  south  of  Pearl  Street.  This  outcrop 
shows,  on  the  north,  twenty-five  feet  or  more  of  the  typical  red 
slates  dipping  south  as  usual  at  a  high  angle  —  about  85°; 
and  this  is  closely  followed  on  tlie  south  by  a  dark  gray,  mas- 
sive and  semi-metamorphic  slate  similar  to  that  on  Burying 
Island,  but  containing  less  sericite.  The  contact  of  the  two 
slates  is  not  clearly  exposed,  but  appears  to  be  quite  conforma- 
ble. The  bedding  cannot  be  certainly  made  out  in  the  gray 
slate,  except  in  partings  two  to  four  feet  apart.  These  are 
parallel  to  the  bedding  of  the  purple  slate.  The  exposed  thick- 
ness of  the  gray  slate  is  22  feet ;  and  the  granite  is  probably 
quite  near  it  on  the  south. 

Up  to  the  time  when  the  preceding  paragraphs  were  written, 
paleontological  proof  of  tlie  Lower  Cambrian  age  of  the  Mill 
Cove    slates    was,    as    stated,    practically    wanting:     altliough 


421 

Foerste  ^  had  noted  in  the  limestone  bands  obscure  indications 
of  Hyolithes  ;  and  the  vvritei-  had  found  many  years  ago,  in 
the  red  shite,  a  cross  section  of  a  trilobite,  wliich,  unfortunately, 
was  lost  before  it  could  be  described.  But  all  this  has  been 
changed  by  the  recent  (December,  1899)  brilliant  discovery  by 
Mr.  Henry  T.  Burr,^  in  the  dark  reddish  slates  in  the  southern 
part  of  the  field,  of  an  important  Lower  Cambrian  fauna.  The 
precise  locality  is  the  slate  ledge  on  the  north  side  of  Pearl 
Street,  about  midway  between  Norton  and  North  Streets,  on 
the  land  of  Mr.  Richard  Ash.  The  fossiliferous  bed,  which 
has  been  traced  for  only  about  thirty  feet  on  the  strike,  is  about 
one  hundred  feet  north  of  the  granite,  and,  apparently,  lies 
wholly  south  of  or  stratigraphically  above  the  calcareous  poi'- 
tion  of  the  series.  By  diligent  search,  aided  by  blasting,  the 
expense  of  which  was  generously  assumed  by  Mr.  Watson, 
ten  species  have  been  identified  up  to  the  present  time,  all  of 
them  typical  Lower  Cambrian  forms.  These  have  been  briefly 
described  by  Mr.  Burr  and  more  fully,  with  illustrations,  by 
Mr.  Grabau  in  his  monograph  on  the  Cambrian  faunas  of  the 
Boston  Basin  published  herewith.  Altogether,  this  exceed- 
ingly opportune  discovery  is  unquestionably  the  most  important 
contribution  to  the  paleontology  of  the  Boston  Basin  since  the 
discovery  of  Paradoxides  by  Professor  W.  B.  Rogers. 

Concerning  the  northern  limits  of  the  Mill  Cove  slates  we 
have  no  definite  knowledge.  They  give  no  indication  north- 
ward of  another  body  of  granite,  nor  do  they  exhibit  the 
marked  shaly  structure  suggestive  of  proximity  to  an  important 
longitudinal  fault.  We  have  noted  a  tendency  to  a  change 
northward  from  red  calcareous  to  green  non-calcareous  slates, 
which  suggests  a  passage  in  that  direction  from  the  Lower 
Cambrian  to  the  Middle  Cambrian.  This  would  reverse  the 
topographic  order  of  succession  observed  in  the  eastern  part 
of  the  southern  boundary  belt,  where  we  pass  southivard  from 

iProc.  Boston  soc.  nat.  hist.,  vol.  24, 1889,  pp.  261-263. 
-  American  geologist,  vol.  25,  Jan.,  1900,  pp.  41-50. 


422 

Lower  to  Middle  Cambrian,  and  would  put  the  two  boundary 
belts  in  anticlinal  relation  to  each  other  —  a  slightly  fan- 
shaped  or  constricted  anticline,  since  the  prevailing  dips  are 
northerly  in  the  southern  belt  and  southerly  in  the  northern 
belt. 

The  composition  of  the  North  Weymouth  sand-plain  gives  us 
no  definite  or  trustworthy  clue  to  the  character  of  the  under- 
lying rocks.  It  may  be  stated,  however,  that  there  is  no  indi- 
cation whatever  of  the  occurrence  of  the  fine  granite  or  any 
plutonic  rock  north  of  the  Mill  Cove  slates  ;  but  that  the 
drift  is  full  of  slate  debris,  —  mostly  black  and  fissile  and 
partly  gray  or  greenish,  with  ■  little  or  no  red  slate.  The 
vague  testimony  of  the  drift  is,  therefore,  confirmatory  of  that 
of  the  ledges,  and  we  may  fairly  conclude  that  the  Lower 
Cambrian  slates  are  probably  followed  northward  by  the  Mid- 
dle Cambrian. 

On  Neck  Street,  nearly  opposite  the  northern  end  of  Green 
Street,  I  observed,  in  the  sidewalk,  some  large  and  angular 
fragments  of  coarse  arkose  sandstone  identical  with  that  which, 
farther  west,  in  Milton,  immediately  borders  the  Complex 
and  the  Cambrian  strata  on  the  north.  Assuming  this  mate- 
rial to  be  approximately  in  sitzi  and  to  have  the  same  strati- 
graphic  significance  here  as  in  Milton,  Avould  double  the 
breadth  of  the  Mill  Cove  section  and  establish  its  northern 
boundary  a  little  south  of  Bridge  Street,  in  Old  Spain.  In 
the  absence  of  any  more  positive  or  trustworthy  evidence,  the 
map  has  been  drawn  to  accord  with  this  vicAv. 

The  Washington  Sti'eet  Area.  —  Crossing  Weymouth 
Fore  River  to  Quincy  Point,  we  find  again  a  very  perfect 
development  of  the  Quincy  plain,  extending  west  along  Wash- 
ington Street,  between  the  shore  of  Town  River  Bay  on  the 
north  and  the  ledg-es  of  fine  o:ranite  on  the  south.  The  ffran- 
ite  is  exposed  northward  to  a  fairly  definite  east-west  line  mid- 
way between  South  Street  and  Washington  Street.  The 
structural  value  of  this  line  is  apparent  to  the  westward,   in 


423 

the  vicinity  of  Union  Street,  where  it  is  seen  to  mark  tlie 
boundary  between  the  granite  and  the  Cambrian  strata ;  and 
we  may  safely  assume  that  tlie  broad  level  plain  north  of  the 
granite  and  east  of  Union  Street,  which  is  unbroken  by  a  single 
outcrop  of  any  kind,  is  underlain  continuously  by  sedimentary 
rocks. 

This  Washington   Street   area,   which  may  be  regarded   as 
extending  west  to  the  Old  Colony  Railroad,  embraces  no  actual 
outcrops   excejit  those   already   referred  to   in   the  vicinity  of 
Union   Street.     The  most  northerly  outcrop  is   on   the   south 
side  of  Washington  Street  just  east  of  Pond  Street,  where  the 
smoothly  glaciated  slate  ledge  has  been  uncovered  for  twenty 
feet  in  laying  the  electric  car  track.      The  slate  is  of  a  massive, 
gray,   semi-metamorphic  character,  with  a  nearly  vertical  dip 
and  east-west  strike.      Turning  south  through  Union    Street, 
scattering  outcrops  may  be  observed  on  either  side   as  far   as 
Baxter  Street,  where  we  pass  onto  the  fine  granite,  giving  a 
total  breadth  of  slate,  from  the  outcrop  on  Washington  Street 
to   the  ffranite,   of  nearlv  eio-ht  hundred  feet.     The   strike   is 
N.  80°  E.,  and  the  dip  S.  or  towards  the  granite,  70° -80°. 
Eastward    from    Union    Street,    along    Newcomb     Place,    the 
slate,  closely  bordered  on  the  south  by  the  granite,  is  exposed 
for   about  five   hundred    feet ;    and    the    granite    ledges    cross 
Newcomb  Place  between  six  and  seven  hundred  feet  east  of 
Union  Street.     The  exact  contact  of  the  slate  and  granite  is 
not  exposed  at  any  point ;   but  it  evidently  conforms  approxi- 
mately with  the  strike  of  the  slate  and  is  undoubtedly  igneous. 
The   granite  is   very  fine-grained  near  the   slate,   and  in   part 
microcrystalline.      On  the  main  line  of  contact  no  apophyses 
of  granite  or  inclusions  of  slate  have  been  observed  ;  but  on 
the  west  side  of  Union  Street,  between  three  and  four  hundred 
feet   south   of  Baxter  Street   and  one   hundred   feet  north    of 
South  Walnut  Street,  on  the  line  of  the  coarse  and  fine  gran- 
ite, is  an  outcrop  which  throws  clear  light  upon  the  relations 
of  the  granite  and  slate.      It  shows  from  south  to  riorth  :    (1) 


424 

coarse  granite  ;  (2)  about  twenty  feet  concealed  ;  (3)  a  band 
of  hai'd  siliceous  slate,  fifteen  to  twenty  feet  wide  ;  and  (4)  the 
typical  fine  granite  fifteen  feet,  and  probably  extending  much 
further.  An  undoubted  igneous  contact  between  the  slate  and 
fine  granite  is  clearly  exposed  ;  and  this  is  evidently  a  large 
mass  of  slate  enclosed  in  the  fine  granite  near  its  junction  with 
the  coarse  granite,  several  hundred  feet  south  of  the  main  body 
of  slate.  The  attitude  of  the  slate  is  normal —  east-v\^est  strike 
and  high  southerly  dip. 

In  all  these  Union  Street  outcrops  the  slate  is  dark  gray, 
with  usually  a  reddish  tone.  In  structure  it  is  identical  with 
the  slate  at  Mill  Cove,  —  elongated  epidotic  nodules,  often 
hollow,  forming  linear  groups  and  continuous  lines  or  bands 
in  the  bedding  planes.  In  the  outcrop  nearest  Washington 
Street,  on  the  west  side  of  Union  Street,  the  slate  encloses  a 
layer  of  white  crystalline  limestone,  six  to  eight  inches  thick, 
and  several  thinner  layers,  down  to  half  an  inch.  The  lime- 
stone is  indistinguishable  from  that  at  Mill  Cove.  Elsewhere 
the  presence  of  lime  is  indicated  by  the  epidotic  lenses  and 
shells  ;  and  igneous  metamorphism  is  shown  also,  near  the 
granite,  by  the  hard  and  flinty  character  of  the  slate.  Between 
Edwards  and  Baxter  Streets,  west  of  Union  Street,  according 
to  information  obtained  from  a  cellar  and  well  digger,  the  slate 
is  covered  by  only  from  four  to  six  feet  of  drift. 

Extending  the  boundary  between  the  granite  and  slate  east- 
ward from  Newcomb  Place,  without  change  of  direction, 
brings  us  to  Weymouth  Fore  River  at  a  point,  a  little  south  of 
Washington  Street ;  and  this  is  so  far  to  the  north  of  the  Mill 
Cove  section,  with  which,  at  least  in  a  general  Avay,  the 
Washington  Street  beds  must  be  correlated,  that  it  has 
appeared  best  to  recognize  a  transverse  fault  between  the  two 
areas  ;  and  this,  in  turn,  is  conveniently  regarded  as  a  north- 
ward extension  of  the  Fore  Kiver  fault  ah-cady  postulated  to 
explain  a  break  in  the  southern  boundary  belt. 

The  A.dams  Street  Area. — Goinij  westward  from  Union 


425 

Street  on  the  line  of  the  granite-slate  contact,  across  Town 
Brook  and  the  raih'oad,  brings  us  to  the  southeast  end  of 
President's  Hill,  —  a  long,  narrow,  l)ut  very  typical  and 
shapely  dnunlin.  lletween  Town  Brook  and  the  rail)'()ad  is 
a  very  perfect  development  of  the  Quincy  plain,  on  which  the 
oldest  and  densest  part  of  the  city  stands.  So  far  as  I  can 
learn  by  diligent  inquiry,  no  excavations  for  sewers,  wells,  and 
the  like  have  ever  reached  bed-rock  in  this  area ;  but  follow- 
ing the  topographic  indication  we  may  fairly  assume  that  the 
bed-rock  is  slate  north  of  the  line  of  the  granite-slate  contact. 
The  President's  Hill  drumlin,  on  the  other  hand,  is  quite  cer- 
tainly built  upon  ledges  of  granite,  as  may,  indeed,  be  clearly 
seen  along  its  southwestern  base.  In  the  valley  between  Presi- 
dent's Hill  and  Cranch  Hill,  its  larger  companion  drumlin,  the 
normal  granite  is  exposed  northward  nearly  to  the  northern  end 
of  the  hill.  Some  five  hundred  feet  farther  north,  across  the 
northern  end  of  the  hill,  the  massive  gray  Cambrian  slate  is 
well  exposed  with  the  normal  east-west  strike.  The  metamor- 
j)hic  character  of  the  slate  and  the  facts  observed  elsewhere 
along  this  boundary,  both  to  the  east  and  to  the  west,  indi- 
cate that  the  gap  between  the  ledges  of  granite  and  slate  is 
occupied,  mainly  at  least,  by  fine  granite ;  and  the  actual 
contact  of  the  slate  and  granite  is  probably  near  the  most 
southerly  outcrops  of  slate.  It  thus  becomes  reasonably  cer- 
tain that  the  granite-slate  contact  is,  in  the  vicinity  of  the  Old 
Colony  Railroad,  shifted  to  the  north  approximately  half  a 
mile  ;  and  since  there  is  in  the  strike  of  the  slates  no  sugges- 
tion of  a  flexure,  it  appears  necessary  to  postulate  .a  north- 
south  fault,  which  thus  falls  in  line  with  and  may,  perhaps,  be 
connected  with  the  very  similar  displacement  of  the  southern 
boundary  belt  on  the  line  of  the  railroad  in  North  Braintree. 

Tlie  Adams  Street  area  embraces  that  part  of  the  northern 
boundary  belt  between  the  main  line  of  the  Old  Colony  Rail- 
road and  the  West  Quincy  Branch  ;  but  the  outcrops  of  sedi- 
mentary rocks  are  wholly  confined  to  the  eastern  half  of  the 


426 

area,  closely  bordering  the  part  of  Adams  Street  east  and 
south  of  Furnace  Brook.  We  find  here  two  quite  distinct 
groups  of  Cambrian  outcrops,  —  one  north  of  the  drumlins 
(President's  and  Cranch  Hills)  and  the  other  on  the  northern 
slope  of  Mt.  Ararat,  the  high  northern  end  of  North  Common 
Hill.  The  first  group  afibrds  the  best  general  section,  and 
the  second  reveals  very  perfectly  the  relations  of  the  sedimen- 
tary rocks  and  the  granite.  In  the  first  or  eastern  group  the 
outcrops  of  Cambrian  slate  begin,  as  previously  noted,  at  the 
north  end  of  President's  Hill,  in  the  angle  between  Adams 
and  Goffe  Streets.  The  most  southerly  exposure  is  on  the 
west  side  of  GofFe  Street,  just,  south  of  Glendale  Road,  and 
some  six  hundred  feet  south  of  Adams  Street.  From  this 
point  the  ledges,  separated  onl)^  by  short  intervals,  extend  in 
a  west  and  northwest  direction  almost  to  Adams  Street ;  and 
north  of  Adams  Street,  west  of  the  abrupt  bend,  the  slate, 
essentially  continuous  with  the  ledges  south  of  the  street,  is 
exposed  for  a  breadth  of  420  feet,  the  total  breadth  of  the 
Cambrian  outcrops  being,  apparently,  nearly  one  thousand 
feet. 

The  slate  is  throughout  a  dark  gray  variety,  without  a  trace 
of  red  or  brown,  and  with  no  distinct  calcareous  segregations, 
to  suggest  that  it  may  possibly  be  Lower  Cambrian.  It 
resembles  the  slate  on  Washington  Street  near  Pond  Street, 
but  not  that  on  Union  Street ;  and  in  the  absence  of  all  more 
decisive  evidence  may  be  referred  provisionally  to  the  Middle 
Cambrian.  Southward,  or  toward  the  granite,  it  is  hard,  mas- 
sive, semi-metaniorphic  and  obscurely  bedded ;  but  toward 
Adams  Street  the  bedding  is  very  plain,  showing  a  fine  and 
even  banding.  The  strike  is  N.  65°- 75°  E.  and  the  dip 
steadily  to  the  south,  80°- 85°.  The  metamorphic  character 
of  the  slate  certainly  increases  toward  the  granite,  suggesting 
an  igneous  contact ;  and  one  doubtful  apophysis  of  granite  was 
observed,  although  the  nearest  undoubted  outcrop  of  granite 
is  about  four  hundred  feet  distant. 


427 

Northward,  in  the  vicinity  of  Furnace  Brook,  the  slate  is 
abruptly  succeeded  by  a  heavy  bed  of  normal  conglomerate  or 
pudding  stone,  composed  of  water-worn  pebbles,  one  to  several 
inches  in  diameter,  of  quartzite,  felsite,  normal  and  fine  granite 
and  slate.  The  conglomerate  has  a  total  breadth,  on  both 
sides  of  the  brook,  of  450  feet;  and  it  is  succeeded  northward, 
wdth  apparent  conformity,  by  the  distinctly  banded,  vertical, 
dark  gray  slates  of  the  newer  or  Carboniferous  series,  with  an 
exposed  thickness  of  300  feet,  and  to  be,  with  the  conglom- 
erate, more  fully  described  in  a  later  section. 

To  the  westward,  the  conglomerate  iDasses  wholly  south  of 
Furnace  Brook,  and,  still  bordered  by  the  newer  slate  on  the 
north,  can  be  traced  across  the  northern  base  of  North  Com- 
mon Hill,  nearly  to  the  junction  of  Adams  and  Common 
Streets.  It  is  here  largely  composed  of  rather  angular  frag- 
ments of  fine  granite  in  a  very  firm  and  flinty  paste,  and 
appears  to  be  closely  bordered  on  the  south  by  fine  granite, 
the  thousand  feet  of  Cambrian  slate  intervening  half  a  mile 
to  the  eastward  being,  apparently,  wholly  wanting  here. 

Going  north  from  Adams  Street,  midway  betw^een  Common 
and  Whitwell  Streets,  we  have  fine  granite  outcropping 
obscurely  for  three  hundred  to  four  hundred  feet,  followed 
by  two  hundred  to  over  four  hundred  feet  of  conglomerate, 
narrowing  rapidly  w^estward,  and  about  a  hundred  feet  of 
slate,  disappearing  northward  beneath  the  swampy  margin 
of  the  brook.  The  slate  strikes  N.  60°- 70°  E.,  with  ver- 
tical dip.  The  most  uncertain  factor  in  the  section  is  the 
contact  between  the  conglomerate  and  granite.  It  is  practi- 
cally impossible  to  locate  this  satisfactorily,  partly  because  of 
insufficient  outcrops,  and  partly  because  ,the  conglomerate  is 
largely  composed  of  granite  debris  in  an  almost  crystalline  paste, 
but  mainly  because  the  granite  itself  has,  near  the  contact,  a 
more  or  less  marked  fragmental  structure,  having  been  locally 
reduced  to  a  crush  breccia  which  is  almost  indistinguish- 
able in  these  weathered  and  lichen-covered  outcrops  from  the 


428 

true  cons^lomerate.  These  facts  ^ — the  cuttino;  out  of  a  o-reat 
body  of  Cambrian  slate,  the  brecciation  of  the  granite,  the 
rapid  narrowing  of  the  conglomerate,  and  the  exceptional 
induration  of  the  conglomerate,  as  if  by  hydrothermal  action,  — 
seem  to  be  most  readily  and  satisfactorily  explained  by  an 
important  longitudinal  fault  between  the  granite  and  conglom- 
erate, with  the  down-throw,  of  course,  to  the  north,  and 
increasing  to  the  westward.  This  is  the  northern  boundary 
fault  of  the  Blue  Hills  Complex  :  and  eastward  it  may  be 
regarded  as  passing  between  the  Cambrian  slates  and  the 
conglomerate. 

Where  the  fault  crosses  the  northern  slope  of  North  Com- 
mon Hill,  the  Cambrian  beds  were  not  lifted  wholly  above  the 
present  plane  of  erosion,  but  numerous  remnants  of  them  are 
still  left  sticking  in  the  granite,  —  roots  of  a  once  continuous 
body  of  slate.  The  brushy  and  ledgy  slope  between  Adams 
Street  and  the  quarries  at  the  top  of  the  hill  (Mt.  Ararat),  and 
between  Common  Street  on  the  west  and  Whitwell  Street  on  the 
east,  is  in  this  respect  of  very  special  interest,  affording  in  the 
most  lavish  profusion  the  essential  facts  concerning  the  original 
relations  of  the  Cambi'ian  sediments  and  the  granite,  which 
elsewhere  are  exposed  so  meagerly  or  wholly  concealed.  The 
special  outcrop  map  of  this  area  (PL  21)  has  been  worked 
out  with  great  care  ;  and  it  shows  as  accurately  as  the  scale  per- 
mits the  position  and  outline  of  every  ledge  and  the  structural 
relations  of  the  granite  and  slate.  A  glance  at  this  map  suf- 
fices to  show  that  this  area  is,  on  a  small  scale,  an  ideal  com- 
j)lex,  the  granite,  which  is  in  the  main  of  the  normal  variety 
and  of  medium  coarse  texture,  liaving  eaten  its  way  upward 
with  extreme  irregularity  into  the  vertical  Cambrian  beds. 

The  slate  is  throughout  extremely  massive,  obscurely  strati- 
fied, and  semi-metamorphic  ;  but  the  bedding  can  be  made  out 
in  almost  every  outcrop.  The  strike  is  N.  80°  E.,  and  the  dip 
S.  80°- 85°.  The  perfect  and  constant  orientation  of  the 
slate,  even  where  occurring  in  quite  small  and  isolated  masses. 


0«as.  PjpEf^  S.i5lon  Soc.  Nat.  Hist..  V.il.  IV. 


OUTCROP  MAP  SHOWING   THE   UKLATIONS  OF   THI£   liRANITE   AND  SLATt   ON    NORTH   (JjMMON    HILL,  QUINCY. 


429 


tells  strongly  in  favor  of  the  view  proposed  here  that  tlie  exist- 
ing relation  of  the  granite  and  slate  is  not  due  to  the  mechani- 
cal intrusion  of  the  former  into  the  latter,  but  rather  to  a  slow 
and  gradual  melting  and  absorption  of  the  slate  by  the  granite. 
Mr.  White,  in  describing  this  slate,  which  he  regards  as  analo- 
gous to  the  German  "  hornfels  "  or  hornstone,  says  the  feldspar 
grains  are  often  elongated  in  lath-shaped  forms,  intermino-led 
with  large  flakes  of  partially  altered  and  frayed  out  biotite, 
some  brown  fibrous  alteration  product,  and  grains  of  magne- 
tite. Single  large-sized  fragments  of  milky  feldspars,  par- 
tially colored  by  iron,  are  present  throughout.     The  mica  is 


1  mcli  =  6  feet. 


1  inch  =  6  feet. 


1  inch  =  2.5  feet. 


1  inch  =  5  feet. 


Figs, 
slate  in 


28-32.— Showing  in  detail  the  relations  of  the  granite  and  Cambrian 
the  ledges  on  the  northern  slope  of  North  Common  Hill,  Quiucy. 


430 


at  times  quite  a  conspicuous  feature  of  the  slate,  and  the  other 
secondary  minerals  include,  besides  the  lath-shaped  feldspars, 
red  garnets  in  very  sharp  and  perfect  dodecahedrons  up  to 
one  eighth  of  an  inch  in  diameter  ;  and  tourmaline,  as  dark 
brown  crystals  in  cavities  throughout  the  hornfels.  Although 
the  lines  of  stratification  are  often  distinctly  lenticular  in  form, 
this  slate,  uniformly  of  a  dark  gray  color,  is  never  appreciably 


1  inch  =  5  feet. 


# 


■A 

aW. 

v5^^ 

\                     r^ 

#:'*'" 

•; 

■tev"-'' 

■■• 

1  incli  = 

=  10  feet, 

1  inch  =8  feet. 

Figs.  33-35. —  Showing  in  detail  the  relations  of  the  granite  and  Camhriau 
slate  in  the  ledges  on  the  northern  slope  of  North  Common  Hill,  Quincy. 

calcareous  or  epidotic,  and  the  best  conclusion  seems  to  be,  as 
before,  that  it  is  a  Middle  Cambrian  sediment  profoundly 
altered  by  igneous  influences.  The  ledges  of  greatest  struct- 
in-al  interest  are  shown  in  greater  detail  in  figures  28  to  35, 
which  require  no  additional  explanation. 

According  to  the  interpretation  of  the  general  structure  of 


431 

the  Adams  Street  area  accepted  by  the  writer,  we  .should  find, 
if  tlie  rocks  were  exposed  continuously  from  North  Common 
Hill  to  President's  Plill  and  Goffe  Street,  that  eastward  the 
slate  patches  become  rather  gradually  larger  and  more  con- 
tinuous, until  the  granite  disappears  beneath  a  continuous 
cover  of  slate,  the  contact  being  everywhere  highly  irregular, 
with  the  bedding  planes  of  the  slate  ending  abruptly  downward 
against  it.  It  is  a  necessary  corollary  of  this  view  that  the 
slate  north  of  President's  Hill,  notwithstanding  its  continuity 
and  nearly  vertical  dip,  has,  probably,  no  great  depth  or  vei'- 
tical  thickness  near  its  southern  margin,  the  depth  of  the 
granite-slate  contact,  we  may  suppose,  increasing  northward 
until  both  rocks  are  cut  off  by  the  boundary  fault  and  the 
conglomerate. 

2^he  Quincy -Miltoyi  A.rea.  —  Between  North  Common 
Hill  and  the  Granite  Branch  of  the  Old  Colony  Railroad, 
about  three  fourths  of  a  mile  farther  west,  the  Cambrian 
strata  and  the  granite-Cambrian  contact  are  entirely  con- 
cealed by  the  drift  deposits  and  the  swampy  tracts  in  the 
vicinity  of  Furnace  Brook.  But  immediately  west  of  the 
railroad,  between  Furnace  Brook  and  Grove  Street,  and  in 
the  direct  line  of  the  contact  belt  on  North  Common  Hill, 
a  precisely  similar  complex  of  granite  and  slate  is  exposed 
in  numerous  outcrops,  as  shown  in  PI.  22. 

On  the  southwest,  as  this  special  map  shows,  a  prominent 
wooded  drumlin  partially  conceals  the  belt ;  but  ledges  of 
granite  and  slate,  with  frequent  exposures  of  the  contact,  occur 
at  short  intervals  along  the  valley  of  Furnace  Brook  to  Pleasant 
Street  in  Milton,  and  thence  westward  along  Pleasant  Street 
nearly  to  Randolph  Avenue,  a  total  distance  of  one  and  a  half 
miles.  Thei^e  is  no  evidence  of  important  transverse  faulting 
along  this  line,  and  the  character  of  the  slate  and  its  relations 
to  the  granite  are  so  uniform  throughout  that  this  may  properly 
be  considered  as  one  homogeneous  area. 

The  slate  in  the  group  of  ledges  i-epresented  in  PL  22  is  iden- 


432 

tical  in  character  with  that  in  the  North  Common  Hill  ledges. 
It  is,  as  a  rule,  dark  gray,  massive,  hard,  and  obscurely  bed- 
ded ;  usually  micaceous  or  semi-metamorphic ;  and  it  even 
incloses  at  one  point  small  red  garnets  indistinguishable  from 
those  on  North  Common  Hill.  The  dip  is  unchanged — S. 
80°- 85°  and  sometimes  vertical :  but  the  strike  is  more  north- 
westerly, varying  usually  between  N.  70°  and  80°  W.,  the 
extreme  range  being  N.   60°   and  90°  W.      As  before,   these 


I^ig.  3G.— Groiip  of  ledges  in  the  Quincy-Milton  area,  between  Grove 
Street  and  Euruace  Brook,  showing  in  detail  the  relations  of  the  granite  and 
Cambrian  slate.     Scale  :  1  inch  =10  feet. 


433 


masses  of  slate  isolated  in  the  granite  are  remarkably  constant 
in  the  orientation  of  the  bedding  planes  ;  and  the  contacts 
usually  conform  approximately  with  the  bedding,  showing 
that  the  structure  of  the  slate  has  been  sufficiently  marked  to 
influence  the  intrusion  of  the  granite.  The  details  of  some  of 
the  more  interesting  ledges  are  shown  in  figures  36  to  38  ;  and 
one  of  these  (36)  shows  an  apparent  slight  departure  from  the. 
parallel  orientation  of  the  slate. 

As  the  outcrop  map  (PL  22)  shows,  the  granite  is  normal,. 


Figp.  S7  aid  £8.  —  Showing  in  detail  the  relations  of  the  granite  and  Cam- 
brian slate  in  the  ledges  between  Grove  Street  and  Furnace  Brook,  in  the 
Quincy-Milton  area.     Scale  :  1  inch  =  4  feet. 

except  in  the  more  northern  ledges,  along  the  line  of  Furnace 
Brook,  where  are  considerable  masses  of  fine  granite,  holding 
precisely  the  same  relation  to  the  slate.  The  dike-like  mass  of 
fine  granite  on  the  north  side  of  the  brook,  near  the  west  end 
of  the  map,  lying  between  slate  on  the  south  and  normal 
granite  on  the  north,  is  twenty  to  thirty  feet  broad.  The 
contact  with  the  normal  granite  can  be  traced,  but  it  is  not  a 
sharp  line,  the  two  granites  appearing  to  be  blended  through 

OCCAS.  I'APEBS   B.  S.  N.  H.         IV.  28. 


434 

an  inch  or  two.  The  extreme  nortliern  ang^le  of  this  lecl^e  of 
normal  granite  shows  a  little  slate,  and  sixty  feet  farther  north, 
on  the  north  side  of  the  road,  the  slate  is  seen  passing  under 
the  south  ano-le  of  another  ledo-e  of  o-ranite  ;  while  the  lar^e 
ledge  of  granite  west  of  this,  touching  the  margin  of  the  map, 
is  bounded  on  the  southeast  by  slate  which  is  exposed  for  a 
breadth  of  fifteen  feet.  It  is  evident  that  these  exposures  give 
us  the  margins  only  of  a  body  of  slate  at  least  one  hundred 
feet  wide. 

Where  the  brook  turns  to  the  northward,  near  the  south Avest 
corner  of  the  map,  the  granite  crosses  the  brook  and  is  exposed 
for  over  two  hundred  feet  along  its  southern  bank  with  a 
breadth  of  fifty  feet ;  and  the  granite  is  closely  bordered  on 
the  south  for  the  entire  length  of  the  ledge  by  the  cotitinuation, 
with  an  exposed  breadth  of  twenty  feet  and  a  southerlj^  dip  of 
80°  to  90°,  of  the  slate  seen  south  of  the  fine  granite  where 
the  road  crosses  the  brook  ;  but  the  fine  granite  is  wanting  in 
the  more  westerly  outci'op;  The  actual  contact  of  slate  and 
granite,  which  is  at  this  point  about  two  hundred  and  fifty 
feet  north  of  the  town  line,  is  not  exposed  in  this  ledge  ;  but 
it  evidently  conforms  approximately  with  the  bedding  of  the 
slate.  About  five  hundred  feet  farther  west  the  slate, 
unchanged  in  character,  and  with  its  usual  strike  and  dip, 
outcrops  again  in  several  low,  broken,  moss -covered  ledges 
scattered  over  the  swampy  floor  of  the  valley,  on  either  side 
of  the  brook,  with  a  total  exposed  breadth  of  340  feet.  Near 
the  northern  edge  of  this  group  of  outcrops  the  slate  is  evi- 
dently much  cut  by  coarse  granite  with,  apparently,  some 
laro;e  dikes  of  granite.  A  part  of  one  of  the  small  dikes  is 
shown  in  figure  39  ;  and  the  orientation  of  the  slate  has  evi- 
dently been  disturbed  in  this  instance,  the  strike  being  N.  60° 
E.  Near  the  southern  border  of  the  slate,  also,  I  observed 
several  small  irregular  granite  dikes  (one  six  inches  wide), 
but  none  clear  enough  for  sketching.  A  large  trap  dike 
appears  to  run  in  a  general  east-west  course  through  tlie  mid- 
dle of  the  group  of  ledges  south  of  the  brook. 


435 

Some  three  liiindred  feet  west  of  these  outcrops  is  the  private 
road  of  the  Cuuniiighani  estate,  and  the  abrupt  eastern  end  of 
the  beautiful  esker  on  which  the  house  stands.  The  swampy 
floor  of  the  valley,  probably  still  underlain  by  slate,  has  a 
breadth  north  of  the  esker  of  nearly  five  hundred  feet.      On 


Fig.  39. —  Showing  in  detail  the  relations  of  the  granite  and  Cambrian 
slate  in  the  valley  of  Furnace  Brook,  on  the  Cunningham  estate,  Milton. 
Scale  :  1  inch  =4  feet. 

the  northern  edge  of  the  swamp,  probably  in  situ,  is  an 
obscure  exposure  of  granite  ;  and  a  little  farther  west  is  the 
private  road  passing  the  large  barn  between  the  swamp  and 
Edge  Hill  Road.  Immediately  north  of  the  swamp  this  road 
crosses  the  west  side  of  a  prominent  ledge  which  shows,  north- 
ward, 10  to  15  feet  of  granite  followed  by  120  feet  of  slate 
dipping  S.  80°— 85°,  and  indistinguishable  from  the  slates  pre- 
viously described.  The  contact  is  seen  only  at  one  point ;  but 
it  must  be  irregular,  as  the  granite  clearly  crosses  the  strike  of 
the  slate.  The  northei"n  edge  of  the  outcrop  is  350  feet  from 
Edge  Hill  Street,  along  the  private  road.  Crossing  the  ledge 
diagonally  (Fig.  40)  is  an  irregular  dike  of  rather  coarse 
granite,  4  to  10  feet  wide,  with  no  appreciable  gradation  in 
the  granite  near  the  clearly  exposed  contact. 

From  this  point  the  course  of  Furnace  Brook  is  southwest- 
ward,  toward  the  Blue  Hills  :  but  the  greater  part  of  the  swamp 
turns  to  the  northward  and  crosses  Edge  Hill  Road,  whence 
the  drainage  is  noi'thward  into  Unkety  Brook  and  the  Nepon- 
set.  Westward,  followino-  the  o-eneral  trend  of  the  contact 
belt,  the  outcrops  are  interrupted  for  half  a  mile  ;  but  half  that 


436 


distance  to  the  northwest,  across  the  small  tributary  of  Unkety 
Brook  just  mentioned,  brings  us  to  a  Cambrian  outcrop  which 


Fig.  40. —  Showing  dikes  of  granite  cutting  the  Cambrian  slate  between 
Furnace  Brook  and  Edge  Hill  Road,  Milton.     Scale  :  1  inch=  65  feet. 

is  approximately  nine  hundred  feet  north  of  Edge  Hill  Road, 
one  hundred  and  forty  feet  southeast  of  Pleasant  Street,  and 
five  hundred  and  fifty  feet  east  of  the  bend  in  Pleasant  Street, 
where  it  turns  due  south.  The  outcrop  is  75  feet  long,  15 
feet  wide,  trends  east- west,  and  consists  of  a  massive  gray 
slate  of  quite  uniform  character.  The  slate  is  somewhat  shaly 
(imperfect  cleavage),  but  otherwise  the  stratification  cannot 
be  made  out.  The  cleavage  strikes  E.— W.,  and  dips  N.  75° 
approximately.  No  granite  or  other  igneous  rock  is  exposed 
here. 

Between  two  hundred  and  three  hundred  feet  east  of  this 
ledge,  and  about  one  hundred  and  fifty  feet  further  north,  is 
a  small  outcrop  of  the  arkose  sandstone  of  the  Carboniferous 
series,  which  outcrops  again  on  the  east  side  of  the  little  north- 
ward flowino-  brook  :   and  some  two    luindred  feet  northeast  of 


437 


this  l.'ist  exposure,  where  the  brook  erosses  the  private  road, 
near  Pleasant  Street,  is  purple  and  green  slate  of  the  newer 
(Carboniferous)  series,  dipping;  north,  with  an  exposed  breadth 
of  about  one  hundred  feet.  These  are  the  only  outcrops  I 
have  been  able  to  find  north  of  the  Cambrian-Granite  belt, 
between  the  Granite  Branch  Railroad  and  Pleasant  Street ;  but 
bowlders  of  arkose  and  conglomerate  are  thickly  scattered  over 
the  entire  tract,  both  north  and  south  of  Edge  Hill  Road. 
The  boundary  fault  undoubtedly  lies  between  the  northern- 
most Cambrian  outcrops  and  the  arkose,  and  it  is,  therefore, 
quite  definitely  determined  in  the  vicinity  of  Pleasant  Street. 
Turning  west  on  Pleasant  Street  at  its  junction  with  Edge 


Pleasant 


Street 


Fig.  41.  —  Relations  of  granite    and  Cambrian  slate  on  Pleasant  Street, 
Milton.     Scale:  1  inch  =100  feet. 


Hill  Road,  we  come,  after  about  nine  hundred  feet,  or  just 
beyond  the  first  bend,  to  a  small  ledge,  on  the-  north  side 
of  the  street,  of  the  typical,  dark  gray,  massive  Cambrian 
slate.  Between  the  second  and  third  bends  in  Pleasant 
Street,  and  opposite  the  two  short  streets  running  off  from 
Pleasant  Street  on  the  south,  is  the  group  of  ledges  shown  in 
figure  41.  It  is  apparent  at  a  glance  that  the  complex  of 
granite  and  slate  still  continues,  and  is  nowhere  more  typically 


438 

developed  than  here.  The  ledges  at  the  noi'theast  corner  of 
the  enclosure  are  a  perfect  tangle  of  granite  and  slate  ;  and  in 
the  large  ledge  east  of  the  enclosure  are  two  distinct  dikes  of 
granite  cutting  the  slate  obliquely,  one  two  to  four  feet  wide, 
and  the  other  three  to  five  inches.  The  slate  in  these  ledo;es  is 
of  the  usual  character,  and  dips  S.  80°-  90°. 

Some  three  hundred  to  four  hundred  feet  farther  west  a  dike 
of  trap  (diabase)  fully  40  feet  wide,  and  trending  N.  60°  to 
70°  W.,  cuts  both  the  slate  and  granite,  the  normal  granite 
being  exposed  for  a  width  of  20  feet  on  the  north  side  of  the 
dike.  It  formerly  cropped  very  boldly,  but  has  been  levelled 
by  blasting  during  the  recent  widening  of  Pleasant  Street. 
The  same  improvement  has  shown  that  the  complex  of  slate  and 
granite  underlies  the  street  for  a  considerable  distance  :  and  both 
rocks  are  now  well  exposed,  but  not  in  contact,  in  the  bank  along 
the  south  side  of  the  street,  the  massive  gray  slate  showing  for 
about  135  feet.  Some  350  feet  south  of  Pleasant  Street,  just 
before  we  reach  the  school-house  at  the  corner  of  Gun  Hill  Road, 
is  a  small  quarry,  in  the  northern  edge  of  which  the  granite 
incloses  a  mass  of  slate  6  to  8  feet  wide  and  45  feet  long  east- 
west,  with  sharply  defined  contacts,  the  coarse,  normal  granite 
showing  absolutely  no  gradation  in  texture  or  composition 
against  the  slate. 

This  is  the  last  Cambrian  exposure  along  the  north  side  of 
the  Blue  Hills.  Westward  on  this  line  for  a  distance  of  one 
and  a  half  miles  both  the  granite  and  slate  are  wholly  con- 
cealed by  the  drift  deposits.  The  outcrops  of  arkose  on  Gun 
Hill  Road,  Randolph  Avenue,  and  Reedsdale  Road,  to  be 
described  in  the  section  on  the  Carboniferous  strata,  enable  us, 
however,  to  trace  the  approximate  northern  margin  of  the 
Complex  half  way  across  this  gap. 

On  the  west  side  of  Asylum  or  Town  Farm  Lane,  which 
runs  south  from  Canton  Avenue  east  of  Pine  Tree  Brook,  and 
one  eighth  to  one  fourth  mile  south  of  Canton  Avenue,  is 
plenty  of  granite,    and  some   unmistakable  ledges  with  large 


439 

blocks  of  conglomerate  resting  upon  them.  West  of  Pine  Tree 
Brook,  the  fine  granite  characteristic  of  the  extreme  margin  of 
the  complex  is  exposed  on  the  south  side  of  Canton  Avenue, 
just  west  of  Robbins  Street,  where  a  wood  road  runs  south  ; 
and  two  hundred  to  three  hundred  feet  farther  west,  the  fine  red 
o;ranite  shows  under  the  sidewalk  on  the  north  side  of  Canton 
Avenue,  apparently  overlain  by  arkose  ;  while  450  feet  north 
of  the  avenue,  on  the  west  side  of  Robbins  Street,  the  arkose 
crops  prominently.  About  five  hundred  feet  soutliwest  of 
these  ledges  of  fine  granite  the  coarse  normal  granite  rises 
boldly  and  forms  an  extensive,  elevated,  ledgy  tract,  closely 
bordering  Canton  Avenue  on  the  south  for  more  than  half  a 
mile.  Beyond  this  the  granite,  also,  is  hopelessly  concealed 
beneath  the  drift ;  and  the  boundary  of  the  Complex  would  be 
entirely  lost  but  for  the  fact  that  slight  traces  of  the  arkose,  on 
Milton  Street,  north  of  Blue  Hill  Avenue,  probably  crossing 
Brush  Hill  Avenue  midway  between  Blue  Hill  Avenue  and 
Paul's  Bridge,  indicate  that  it  still  pursues  its  westward  course. 
Taking  a  general  view  of  this  boundary  from  the  Old  Colony 
Kailroad  to  the  Neponset,  its  course  is  seen  to  be  nearly  due 
west  as  far  as  Pleasant  Street,  in  Milton,  and  then  it  begins 
to  curve  to  the  southward,  gently  at  first,  and  more  rapidly 
beyond  Robbins  Street.  Although  the  relations  of  the  gran- 
ite and  Cambrian  slate  are  throughout,  so  far  as  exposed, 
exceedingly  intricate,  —  a  typical  complex,  —  the  boundary 
of  the  Complex  itself  appears  to  be  a  remarkably  simple  and 
nearly  straight  line ;  a  fact  which  harmonizes  well  with  the 
theory  that  it  is  a  fault  line.  The  facts  sustaining  this  theory 
will  be  more  fully  discussed  in  the  section  on  the  Carboniferous 
series. 

THE   INTERMEDIATE    AEEAS   OE  CAMBRIAN   STRATA. 

Besides  the  main  down-foldings  of  the  Cambrian  strata  along 
east-west  lines,  which,  becoming  thus  lines  of  weakness  and 


440 

consequent  faulting,  have  determined  the  general  outlines  or 
boundaries  of  the  Blue  Hills  Complex  as  differentiated  from  the 
rest  of  the  great  batholite  of  Eastern  Massachusetts  and  isolated 
by  the  bordering  Carboniferous  strata  still  connecting  the  tracts 
depressed  by  faulting,  there  are  several  intermediate  and  minor 
belts  conforming  in  trend  with  the  boundary  belts,  and  like 
them  attended  by  more  or  less  important  longitudinal  displace- 
ments. These  isolated  Cambrian  remnants  differentiate  the 
orographic  block  determined  by  the  boundary  faults,  and  make 
of  it  a  true  complex  of  plutonic  and  sedimentary  rocks.  Al- 
though it  is  convenient  to  speak  of  all  these  belts  as  downfold- 
ings  or  deep,  closed  synclines  of  the  Cambrian  strata,  they  are 
really  of  less  definite  structural  value,  being  simply  the  portions 
of  the  once  continuous  cover  of  closely  folded  sediments  pene- 
trating most  deeply  into  the  batholite.  In  other  words,  they 
represent  the  rugosities  or  most  prominent  inequalities  (com- 
parable with  the  teeth  of  a  comb)  of  the  under  surface  of  the 
batholite  cover  ;  and  they  are  residuary  with  reference  to  both 
surface  erosion,  and  the  original  corrosion  and  solvent  action 
of  the  plutonic  magma.  This  absoi'ption  of  the  covering 
sediments  by  the  magma  must  have  been  highly  differential  in 
a  chemical  sense,  and  the  fact  that  the  roots  of  the  Cambrian 
series  now  belong  chiefly,  apparently,  to  the  Middle  Cam- 
brian, is  probably  not  due  wholly  to  the  inferior  and  hence 
more  exposed  position  of  the  Lower  Cambrian,  but  partly  to 
the  fact  that  its  mixture  of  calcareous  and  siliceous  sediments 
rendered  it  more  fusible. 

The  TIayward  Oreeh  and  Eldridge  Hill  Area.  —  This 
area  embraces  the  mouth  of  Haywai-d  Creek,  on  the  line 
between  Quincy  and  Braintree,  and,  extending  westward  across 
Howard  Street  and  over  Eldridge  Hill,  it  tapers  to  a  blunt 
point  on  the  western  brow  of  the  hill,  the  exposed  length  of 
the  belt  being  approximately  half  a  mile,  and  its  maximum 
bi'eadth,  east  of  Howard  Street,  about  one  thousand  feet. 
The  northern  boundary  is  straight  (a  very  probable  fault  line)  ; 


441 

it  trends  east-west,  parallel  with  the  strike  of  the  slate  ;  and  the 
slate  is  separated  from  the  fine  granite  along  this  line  by  a 
prominent  dike.  The  southern  boundary  cuts  obliquely  across 
the  slate,  with  a  south-southeast  trend,  and  is  throughout  a 
typically  igneous  contact. 

This  limited  area  is,  more  than  any  other,  classic  ground 
for  the  Cambrian  strata  of  the  Boston  Basin,  since  it  was  in 
the  little  quarry  at  the  mouth  of  Hay  ward  Creek,  on  the 
southern  or  Braintree  bank,  that,  as  noted  in  detail  by  Mr. 
Grabau  in  the  section  on  the  paleontology  of  the  Cambrian 
terranes,  fossils  were  first  found  in  situ,  and  brought  to  the 
notice  of  the  scientific  world  by  Professor  William  B.  Rogers 
in  1856,  and  during  the  last  forty  years  this  locality  has  prob- 
ably received  more  attention  from  students  of  geology  than 
all  the  other  Cambrian  outcrops  of  the  Boston  Basin  taken 
together.  It  was  in  this  area,  also,  and  within  a  few  yards 
of  the  Paradoxides  Quarry,  that  Dr.  M.  E.  Wadswoi'th,^  in 
1881,  brought  to  light,  for  the  first  time,  incontestable  proof 
that  the  granite  is  younger  than  and  intrusive  in  the  undoubted 
Cambrian  slates. 

The  Trilobite  or  Paradoxides  Quarry  (PI.  23)  is  about 
three  hundred  feet  from  the  southern  margin  of  the  area,  and 
shows  to  orood  advantaoje  the  massive  structure  characteristic 
of  the  Cambrian  slate  near  an  io-neous  contact  with  the  o-ranite. 
The  slate  is  here  a  gray  or  greenish  gray,  compact  and  flinty 
variety  of  remarkably  uniform  character.  Minute  grains  of 
pyrite  are  jDi'etty  generally  diflfused,  with,  very  rarely,  more 
conspicuous  developments  of  cubic  crystals.  In  the  quarry 
the  slate  is  apparently  only  coarsely  jointed  ;  but  the  weathered 
surface  reveals  much  finer  division  by  this  means.  The  strati- 
fication is  very  obscure,  and  can  be  determined  with  certainty 
only  by  means  of  the  trilobite  remains.  These  may  be  supposed 
to  lie  in  the  plane  of  the  bedding ;  and  according  to  this  indi- 
cation the  strike,   as  determined    by  the  writer   twenty  years 

1  Proc.  Boston  soc.  nat.  hist.,  vol.  21,  pp.  274-278. 


442 

ago,  when  the  quarry  was  being  worked  and  examples  of 
Paradoxides  were  clearly  exposed  in  its  walls,  is  E.— W.,  and 
the  dip  S.  80°— 85°.  The  higher  dip  accords  best  with  the 
other  outcrops  of  this  area,  as  on-Eldridge  Hill  and  in  adja- 
cent areas.  In  fact,  so  extreme  was  the  plication  of  the  Cam- 
brian sediments  of  this  region  that  an  undoubted  dip  below 
sixty  degrees  has  nowhere  come  under  my  observation  among- 
the  sedimentary  areas  of  the  Blue  Hills  Complex.  The  maxi- 
mum breadth  of  slate  south  of  the  creek  is  350  feet. 

On  the  edge  of  the  salt  marsh,  some  three  hundred  feet 
south  and  about  one  hundred  feet  east  of  the  Paradoxides 
quarry,  is  the  obscure  outcrop  where  Dr.  Wadsworth  noted 
the  contact  of  the  slate  and  granite.  He  says  (/.  c,  p.  275), — 
' '  The  slate  near  the  junction  is  greatly  indurated  and  changed 
in  color,  while  the  granite  is  compact  and  has  lost  its  distinc- 
tive characters,  being  transformed  into  a  spherulitic  quartz- 
porphyry.  The  modification  of  the  granite,  the  induration 
and  alteration  of  the  argillite,  the  complete  welding  of  the 
two  formations,  and  the  irregularity  of  the  line  of  junction, 
prove  that  the  granite  is  an  eruptive  rock,  whose  period  of 
eruption  is  of  later  date  than  the  Primordial  argillite." 

Westward  from  this  ledge  as  far  as  Hay  ward  Creek,  the 
contact  is  effectually  concealed,  with  no  outcrops  very  near  on 
either  side ;  but  the  topographic  indications  are  that  the  line 
runs  first  N.  78°  W.,  then  67°  W.,  and  finally  N.  58°  W.,  to 
Hayward  Creek,  where,  on  the  south  bank,  eight  hundred  feet 
northeast  of  Quincy  Avenue,  the  contact  is  nearly,  but  not 
quite,  exposed  a  second  time.  From  this  point  the  contact  of 
the  slate  and  fine  granite  evidently  crosses  Hayward  Creek  in 
a  northerly  direction  ;  and  it  is  readily  located  on  the  northwest 
side  of  How:ard  Street,  seven  hundred  feet  from  Quincy  Avenue  ; 
but,  again,  it  is  not  clearly  exposed  for  observation.  Wadsworth 
states,  however,  that  he  found  the  contact  by  digging,  and 
that  it  presents  the  same  characters  as  before.  Continuing  in 
the  same  general  direction,  the  contact   can  be  traced  by  infre- 


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443 

qucnt  outcroj)s  of"  slate  and  granite  to  the  top  of  Eldridgc 
Hill  ;  and  thence,  by  frequent  and  satisfactory  exposures  of 
the  actual  junction,  westward  across  the  level  summit  about 
five  hundred  feet  to  the  somewhat  abrupt  termination  of  the 
slate  on  the  western  brow  of  the  hill.  Near  the  west  end  the 
contact  is  exposed  continuously  for  ten  to  fifteen  feet.  It  is, 
throughout,  a  good,  typical  igneous  contact.  The  two  rocks 
are  firmly  welded,  the  slate  is  hard,  massive,  and  very  obscurely 
bedded,  and  the  granite,  along  the  immediate  contact,  is 
almost  felsitic.  Several  hundred  feet  east  of  the  western  ter- 
mination of  the  slate,  a  distinct  dike  of  fine  granite  from  two 
to  three  inches  wide  runs  into  the  slate  ;  and  a  specimen  col- 
lected for  the  Museum  shows  a  dikelet  of  micro-granite  from 
one  eighth  to  one  half  inch  wide  traversing  the  massive  green- 
ish gray  slate. 

At  the  west  end  of  the  belt,  the  granite  ledges  advance 
northward  and  clearly  cut  off  the  slate.  The  breadth  of  the 
slate  is  here  about  90  feet  to  its  straight  and  simple  northern 
margin,  which  can  be  traced  in  a  due  east-west  direction  by 
abundant  and  more  or  less  continuous  exposures  to  Howard 
Street,  at  the  corner  of  Howard  Avenue.  Along  this  line  the 
slate  and  fine  granite,  commencing  at  the  west  end,  are  sepa- 
rated by  a  vertical  dike  of  fine-grained  greenish  diabase  20  to 
40  feet  wide  (Fig.  42) .  About  five  hundred  feet  west  of  How- 
ard Street  the  dike  seems  to  leave  the  contact  and  run  very 
obliquely  northward  into  the  granite,  although  there  is  a 
smaller  dike  wholly  in  the  slate  near  this  point.  On  Howard 
Street  the  slate  and  granite,  as  I  had  an  opportunity  to  observe 
several  years  ago  in  an  excavation  for  a  water  main,  are  in 
close  contact,  with  no  trap  intervening.  The  trap  is  un- 
doubtedly newer  than  the  granite  ;  and  the  main  dike  incloses, 
on  the  northeastern  corner  of  Eldridge  Hill,  a  mass  of  the  fine 
granite  some  three  feet  in  diameter. 

The  outcrops  of  slate  in  this  acutely  triangular  area  cross- 
ing the  summit  and   eastern   slope  of  Eldridge  Hill   are  broad 


,444 

and  numerous  ;  and  in  the  vicinity  of  Howard  Street  we  have 
a  nearly  continuous  section  across  the  belt  for  a  breadth  of 
over  five  hundred  feet.  The  slate  is  throughout  of  a  massive, 
obscurely  bedded,  hard  and  flinty  character.  At  some  points 
it  is  semi-metamorphic  in  character,  being  perceptibly  crystal- 
line or  micaceous.  It  varies  in  color  from  greenish  gray  to  a 
dark  but  distinct  red  or  purplish  red.  The  colors  are  more  or 
less  alternating  or  interstratified  ;  but  the  gray  is  found  chiefly 
to  the  south  and  the  red  tints  chiefly  to  the  north.  The  strike 
is  throughout  approximately  east-west,  pai-allel  with  the 
straight  northern  margin  of  the  belt ;  and  that  this  margin  is 
a  fault  is  quite  plainly  indicated  by  its  rectilinear  character 
and  also  by  the  occurrence  along  it  of  the  large  trap  dike, 
which  may  be  regarded  as  occupying  a  fault  fissure  (Fig.  42). 
Certainly  the  dike,  which  is  so  clearly  newer  than  both  the 
slate  and  granite,  would  be  most  unlikely  to  follow  so  faith- 
fully an  igneous  contact,  where  the  slate  and  granite  are  inva- 
riably firmly  welded  and  all  tendency  in  the  slate  to  cleave 
with  the  bedding  is  completely  obliterated.  The  dip,  at  all 
points  where  it  can  be  made  out,  is  approximately  vertical, 
varying  perhaps  five  degrees  both  north  and  south  of  the 
vertical. 

East  of  Howard  Street  the  slate  outcrops  obscurely  in  Howard 
Avenue  for  nearly  three  hundred  feet ;  but  beyond  this  both  the 
slate  and  granite  are  lost  beneath  the  level  sand  plain  of  Quincy 
Neck.  South  of  Hayward  Creek,  the  granite-slate  contact 
follows,  perhaps  fortuitously,  a  depression ;  and  this  topo- 
graphic expression  of  the  contact  suggests  the  possibility  that 
it  may  follow  the  shore  of  Weymouth  Fore  River,  which  trends 
east-southeast  toward  White's  Neck  in  Weymouth  for  more  than 
half  a  mile  before  turning  due  south  toward  Weymouth  Landing. 
The  shore  skirts  the  base  of  Wyman  Hill  quite  closely,  all  the 
way  from  Hayward  Creek,  southeast,  south  and  southwest,  to 
the  Monatiquot  River  ;  and  the  hill,  except  for  the  drumlin 
which  crowns  it,  is  undoubtedly  wholly  composed  of  fine  granite, 


445 

which  is  exposed  at  several  points  nciivly  or  quite  to  the  water's 
edge.  Soutlieastward  from  Hayward  Creek,  the  fine  granite 
outcrops  more  or  less  obscurely  at  several  points  on  and  near 
the  shore,  whicii  is  thickly  paved  with  angular  fragments  of  the 
granite  :  but  tlie  slate  seemed  at  first  to  be  wholly  wanting. 
Mr.  Watson,  however,  has  called  my  attention  to  an  obscure 
but  indubitable  occurrence  of  the  slate  in  situ  at  the  north- 
eastern base  of  the  hill.  The  angle  of  the  shore  where  it  turns 
southward  is  due  to  a  small  drift-clad  hill  of  fine  granite  ;  but 
immediately  west  of  this  we  pass  northward  from  the  granite 
to  a  much  broken  half-tide  ledge  of  slate.  Several  hundred 
feet  west  of  this  the  granite  crops  again  on  the  same  line,  but 
the  slate  is  not  exposed.  These  ledges  of  slate  and  granite  are 
on  the  general  line  of  the  contact,  as  traced  from  the  top  of 
Eldridge  Hill  to  the  mouth  of  Hayward  Creek  ;  and  enable 
us,  with  confidence,  to  extend  the  Hayward  Creek  and  Eldridge 
Hill  Cambrian  area  eastward  under  a  considerable  breadth  of 
the  Weymouth  Fore  River. 

About  two  thousand  feet  of  fine  granite  separate  this  slate 
ledge  on  the  northern  bend  of  the  shore  from  the  ledo;e  on  the 
southern  bend,  where  the  shore  turns  westward  again,  and 
where  Mr.  Watson  found  the  fragment  of  a  large  Paradoxides 
in  the  massive  gray  slate.  This  ledge  shows  a  northeasterly 
strike  ;  and  the  slate  belts  north  and  south  of  Wyman  Hill  are 
thus  seen  to  converge  eastward.  There  is,  independently  of 
this  consideration  (see  p.  396),  a  strong  probability  that  the 
bed-rock  of  White's  Neck  is  wholly  Cambrian  ;  and  hence  we 
might,  perhaps,  fairly  conclude  that  the  original  igneous  con- 
tact of  the  slate  and  granite  follows  the  western  shore  of  the 
river,  at  least  approximately,  around  the  eastern  base  of 
Wyman  Hill.  It  might  be  better,  however,  in  view  of  the  pro- 
nounced east-west  trend  of  the  Cambrian  belts  throughout  the 
Blue  Hills  Complex,  to  let  the  granite  terminate  under  the 
river,  against  the  north-south  fault  which  we  have  already 
introduced  to  explain  the  apparent  dislocation  of  the  White's 
Neck  and  Fore  River  areas  of  the  southern  boundary  belt. 


446 

This  Fore  River  fault  is  complementary  with  reference  to 
the  Mill  Cove  fault,  separating  the  King  Oak  Hill  and  White's 
Neck  areas,  against  which  we  have  terminated  the  fine  granite 
of  North  Weymouth.  The  White's  Neck  fault  block  is  on  the 
downthrow  side  of  both  displacements  ;  and  it  has  thus  carried 
the  granite  below  the  present  plane  of  erosion.  This  view 
o-ives  exceptional  value  to  the  part  of  the  great  southern  bound- 
ary fault  bounding  the  White's  Neck  area  on  the  south  ;  and 
thus,  perhaps,  may  be  explained  the  exceptional  alteration  of 
both  the  slate  and  granite  along  this  line  and  the  marked 
hydrothermal  action  evidenced  by  the  extensive  development  of 
vein  quartz  in  the  slate  adjacent  to  the  fault. 

The  slate  immediately  bordering  the  fine  granite  of  Wyman's 
Hill,  on  both  the  north  and  the  south,  is  shown  by  unquestion- 
able paleontologic  evidence  to  be  Middle  Cambrian  ;  and  that  is 
the  most  probable  age  of  the  dark  gray  massive  slate  on  Norton 
Street  in  North  Weymouth,  and  inclosed  in  the  granite  on 
Burying  Island  (see  p.  419).  Hence  it  is  reasonably  certain 
that  the  northern  end  of  White's  Neck  is  also  Middle  Cam- 
brian ;  and  since  the  southern  end  of  the  Neck  is  quite  clearly 
Middle  Cambrian,  an  anticlinal  structure  is  suggested. 

The  strike  of  the  Lower  Cambrian  beds  of  the  Mill  Cove 
section  would  carry  them  somewhat  to  the  north  of  the  Hay- 
ward's  Creek  and  Eldridge  Hill  section  ;  and  the  calcareous 
layers  and  segregations  of  the  red  slate  of  both  the  Mill  Cove 
and  Washington  Street  sections  are  wholly  wanting  in  the  red 
slates  of  the  more  northern  outcrops  on  Eldridge  Hill.  I  con- 
clude, therefore,  that  the  red  slate  of  Eldridge  Hill  marks  the 
passage  upward  from  the  calcai'eous  red  slate  of  the  Lower 
Cambrian  to  the  non-calcareous  greenish  gray  slate  carrying 
the  Middle  Cambrian  fiiuna  ;  and  this  is  in  accord  with  the 
conclusion  of  Aug.  F.  Foerste  '  that  the  Mill  Cove  beds  nor- 
mally underlie  the  Paradoxides  beds.  The  general  relations  of 
this  area  to  the  granite  are  shown  diagrammatically  in  figure  42. 

1  rroc.  Boston  soc.  nat.  hist.,  vol.  24,  p.  2G2. 


447 


The  Ruggles  Creeh  Area. — This  belt  of  slate  forms  the 
floor  of  the  straight  and  narrow  east-west  valley  of  Ruggles 
Creek,  extending  inland  a  mile,  from  the  tidal  portion  of  the 
creek,  with  a  maximum  breadth  of  less  than  five  hundred 
feet.  The  outline  is  severely  simple  and  regular,  and  the 
north-south  profile  presents  the  usual  structural  contrast,  the 
slate  meeting  the  bordering  fine  granite  in  an  original  igneous 
contact  on  the  south  and  a  secondary  fault  contact  on  the 
north  (Fig.  42).  This  displacement  is  remarkably  straight, 
unbroken  for  the  entire  length  of  the  valley,  and  developed 
topographically,  through  the  more  rapid  erosion  of  the  slate, 


Fig.  42.  —  Diagrammatic  section  from  Hayward  Creek  nortliward  across 
Eldridge  Hill  and  Ruggles  Creek,  Quincy. 

as  a  smooth  and  abrupt  wall  of  granite,  unquestionably  the 
most  ideal  fault  scarp  in  the  Boston  Basin  (PI.  24). 

Going  west  along  the  north  side  of  the  valley,  the  granite 
ledge  is  continuous,  and  the  fault  scarp,  commonly  ten  to 
twenty  feet  high,  with  a  steep  slope  to  the  south,  is  well  devel- 
oped west  of  Howard  Street.  Immediately  west  of  Chubbuck 
Street  the  slate  begins  to  appear  in  the  grassy,  sloping  floor  of 
the  valley  below  the  fault  scarp,  and  outcrops  almost  continu- 
ously for  half  a  mile.  The  slate  is  mainly  of  a  dark  red  or 
reddish  gray  color,  with  occasional  greenish  stripes,  resembling 
the  slate  of  Eldridge  Hill  ;  and,  as  in  that,  the  bedding  is,  on 
the  whole,  very  obscure.  The  strike  is  E.-W.,  parallel  with 
the  fault  and  the  valley,   and   the   dip  is   S.    70°- 80°.      The 


448 

cleavage  of  the  slate  is  quite  marked  at  most  points,  trending 
with  the  strike  and  hading  steeply  to  the  north. 

About  one  fourth  of  a  mile  west  of  ISTorth  Street  is  a  small 
road-metal  quarry  in  the  fine  granite,  showing  the  contact 
with  the  slate  directly  in  the  line  of  the  escarpment.  Fig.  43 
shows  the  contact  as  it  was  clearly  exposed  in  1891.  The 
slate  lies  flat  against  a  smooth,  straight  wall  of  granite 
hading  S.  about  20° ;  and  for  the  first  six  to  twelve  inches  the 
slate  is  very  much  sheared  and  comminuted.  There  are  also 
several  subordinate  shear  planes  parallel  to  the  fault  in  the  first 


Fig.  43.  — Fault  contact  of  the  fine  granite  and  Cambrian  slate,  on  the 
north  side  of  Ruggles  Creek,  Quincy. 

yard  or  two  of  the  slate,  and  a  very  distinct  one  in  the  granite 
nearly  three  feet  from  the  contact.  The  slate  is  gray  and  mas- 
sive, probably  by  leaching  near  the  contact,  but  becomes  fissile 
and  red  at  a  little  distance. 

Farther  west  the  slate  is  not  clearly  seen  on  the  north  side 
of  the  brook,  the  fault  scarp  being  closely  bordered  by  a  wet 
meadow.  At  the  west  end  of  the  slate  belt,  just  before  reacliing 
New  Road,  crossing  obliquely  from  South  Street  to  Quincy 
Avenue,  the  ledges  of  fine  granite  advance  a  little  to  the  south, 
cutting  ofi"  some  of  the  slate.  The  slate  ledges  come  to  an 
abrupt  and  definite  end  about  one  hundred  feet  east  of  New 
Road,  and  afford  a  practically  continuous  section  across  the 
belt  at  its  narrow  western  extremity.  Parallel  with  New 
Road  the  slate  measures  200  feet  from  the  fine  granite  on  the 
nortli  to  the  normal  granite  which  here  meets  it  on  the  soutli  ; 


Occas. Papers, BoslOTi  Soc.Nat.HivSt.,Vol.Iv: 


Plate  VA 


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THE     HELIOTYPE     PRINTING    CO         BUTTON 


FAULT-SCARP,  RUGGLES  CREEK,  QUINCY. 


449 

and  continuing  in  the  same  direction,  we  cross  300  feet  of  the 
coarse,  reddish,  normal  granite  before  reaching  Quincy  Avenue, 
The  normal  granite  outcrops  west  of  New  Road  and  again  to 
the  northward  on  Glencoe  Street,  where  it  meets  and  incloses 
the  fine  granite,  as  described  on  page  353.  In  the  direct  line 
of  the  slate  there  are  no  outcrops  for  a  long  distance  ;  but  the 
rapid  narrowing  of  the  slate  at  the  last,  and  the  relations  of 
the  fine  and  normal  granites  north  and  south  of  the  slate,  make 
it  very  improbable  that  the  slate  extends  much,  if  any,  beyond 
New  Road,  the  normal  granite  probably  being  continuous 
across  its  path. 

The  contact  of  the  slate  and  normal  granite  on  the  south  is 
not  clearly  exposed,  a  blank  of  five  feet  separating  the  nearest 
outcrops.  The  granite  shows  no  appreciable  gradation  in 
texture  toward  the  slate  ;  but  that  this  is  a  true  igneous  contact 
is  indicated  by  the  hard  and  massive  character  of  the  slate,  and 
more  especially  by  a  distinct  dike  of  granite  one  and  one  half 
inches  wide  which  Mr.  Watson  and  I  found  in  the  slate  near 
the  contact.  A  north-south  dike  of  diabase,  about  eight  feet 
wide,  cuts  across  the  slate  a  hundred  to  a  hundred  and  fifty 
feet  east  of  New  Road,  but  it  cannot  be  traced  south  into  the 
normal  granite,  nor  north  into  the  fine  granite,  seeming  to  be 
confined  to  the  slate,  and  suggesting  the  possibility  that  it 
belongs  to  the  pre-granitic  series  of  dikes. 

Eastward  from  New  Road  the  contact  of  the  slate  and  nor- 
mal granite  runs  obliquely  toward  Quincy  Avenue,  and  the 
fine  granite  begins  to  appear  in  its  normal  place,  between  the 
slate  and  normal  granite,  with  no  appreciable  gradation  be- 
tween the  two  granites.  The  fine  granite  widens  eastward, 
and  thus  the  rather  sharp  contact  of  the  normal  and  fine  gran- 
ites is  kept  near  Quincy  Avenue,  as  the  map  shows,  all  the 
way  to  Hayward  Creek.  East  of  the  little  brook  crossing 
Quincy  Avenue  some  five  hundred  feet  east  of  New  Road,  the 
slate  comes  within  about  one  hundred  and  twenty-five  feet  of 
the  avenue,  and  the  intervening  space  is  all  fine  granite.      A 

OCCAS.  PAPERS  B.  S.  N.  II.  IV.         29. 


450 

little  farther  east  the  normal  granite  appears  on  the  north  side 
of  the  avenue,  and  the  slate  is  150  feet  from  the  avenue,  with 
fine  granite  between.  Beyond  this  the  contact  of  the  slate  and 
fine  granite  is  jogged  one  hundred  and  fifty  to  two  hundred 
feet  by  two  transverse  and  complementary  faults,  as  shown  on 
the  map,  the  second  fault  coinciding  with  the  second  small 
brook  crossing'  the  avenue. 

A  good  breadth  of  slate  is  exposed  on  the  south  side  of  the 
valley  for  about  two  thousand  feet  east  of  New  Road,  or  to  a 
point  just  east  of  the  small  quarry  where  the  slate  outcrops 
begin  on  the  noi'th  side  of  the  brook.  East  of  this  point 
the  outcrops  are  wholly  wanting  on  the  south  side,  as  west 
of  it  they  are  wanting  on  the  north  side.  South  of  the  brook 
the  slate  and  fine  granite  are  seen  very  near  each  other  at 
several  points,  but  the  actual  contact  is  nowhere  clearly 
exposed.  That  it  is  an  igneous  contact  may  be  safely  inferred 
from  the  hard  and  massive  character  of  the  slate,  the  bedding 
being  so  obscure  as  to  make  strike  and  dip  determinations  quite 
unreliable  or  impossible. 

In  the  vicinity  of  South  Street  the  fine  granite  forming  the 
immediate  south  wall  of  the  valley  is  traversed  by  a  large  east- 
west  dike  of  diabase,  very  similar  to  the  dike  bounding  the 
Eldridge  Hill  slate  on  the  north. 

Immediately  east  of  Howard  Street  Ruggles  Creek  expands 
into  an  estuary,  and  the  bordering  ledges,  alike  of  slate  and 
o-ranite,  subside  beneath  the  sand  plains  of  Quincy  Neck  and 
Quincy  Point.  But  that  the  slate  belt  continues  essentially 
unchanged  at  least  as  far  as  Weymouth  Fore  River,  is  indi- 
cated by  an  obscure  but  undoubted  ledge  of  fine  granite  in 
the  frino-e  of  salt  marsh  on  the  north  side  of  the  estuary,  a 
quarter  of  a  mile  east  of  Howard  Street  and  exactly  in  line 
with  tlie  east-west  fiiult  scarp  west  of  Howard  Street. 
Beyond  this  we  can  only  speculate ;  but  it  may  be  noted 
in  passing  that  the  belt  of  fine  granite  separating  the  Ruggles 
Creek  and   Union    Street   Cambrian   areas    probably   narrows 


451 

eastward.  This  is  indicated  by  the  distribution  of  the  out- 
crops, and  by  the  gradual  narrowing  eastward  and  dying  out 
of  the  axis  of  normal  granite  dividinar  the  fine  granite  on  the 
west.  This  plainly  suggests  the  convergence  and  probable 
union  under  Weymouth  Fore  River  of  these  two  sedimentary 
belts.  Topographically,  both  Quincy  Neck  and  Quincy  Point, 
east  of  Howard  Street,  should  be  relegated  wholly  to  the  sedi- 
mentary rocks  ;  and  the  fact  that  these  low  and  level  tracts  must 
be  regarded  as  largely  underlain  by  granite,  is  one  of  the  puz- 
zles of  the  region. 

The  contact  of  normal  and  fine  granites  south  of  Ruggles 
Creek  belongs  to  the  modified  or  sub-normal  type,  as  explained 
on  page  356,  a  displacement,  with  the  downthrow  to  the 
north,  subsequent  to  the  solidification  of  the  fine  granite  and 
prior  to  the  solidification  of  the  normal  granite.  In  the 
vicinity  of  New  Road,  the  original  westward  extension  of 
the  Ruggles  Creek  slate  belt  and  the  fine  o-ranite  borderino- 
it  have  thus  been  lifted  above  the  present  plane  of  erosion, 
and  the  contact  of  the  slate  and  normal  granite  is  readily 
accounted  for.  The  irregularity  and  evidently  igneous  char- 
acter of  both  the  granite-granite  and  granite-slate  contacts 
south  of  Ruggles  Creek,  make  the  view  that  these  are  ordi- 
nary displacements  quite  untenable ;  while  the  modified  or 
sub-normal  theory  of  the  granite-granite  contact  affords  an 
easy  and  natural  interpretation  of  all  the  observed  facts.  The 
granite-slate  contact  on  the  north  side  of  Ruo^o-les  Creek  is 
an  unmistakable  fault ;  but  the  displacement  is  not,  apparently, 
very  great,  since  at  no  point  has  it  brought  the  normal  granite 
up  to  the  level  of  the  present  surface,  and  it  seems  to  die  out 
rapidly  as  it  approaches  New  Road. 

Evidence  of  true  faulting  wholly  subsequent  to  the  develop- 
ment of  the  granitic  series  is  not  entirely  wanting  on  the  line 
of  Quincy  Avenue,  although  it  is  confined  to  the  normal  gran- 
ite and  cannot  be  correlated  with  either  the  granite-granite  or 
srranite-slate  contacts.      Along  the  south  side  of  the  avenue. 


452 

west  of  Howard  Street,  the  normal  granite  forms  a  precipitous 
escarpment  30  to  50  feet  high  for  nearly  a  thousand  feet.  This 
is,  conceivably,  only  a  line  of  master-jointing  ;  but  in  appear- 
ance, certainly,  it  is  an  ideal  fault  scarp.  Furthermore,  Mr. 
Watson  has  found  that  some  of  the  granite  on  the  line  of  this 
topographic  break  is  comminuted  and  highly  oxidized,  and  that 
the  Eldridge  Hill  dike  is  quite  certainly  jogged  or  displaced  to 
the  northward  in  crossing  this  line.  That  the  Quincy  Avenue 
escarpment  corresponds  to  an  actual  displacement  or  fault  is 
most  probable  ;  but  the  facts  do  not  warrant  a  positive  conclu- 
sion as  to  the  magnitude  of  the  movement. 

West  of  the  Haggles  Creek  area  the  bed-rocks  are  concealed 
over  a  considerable  area  east  of  the  Old  Colony  Railroad  by 
the  sand  plain  (Quincy  plain)  in  the  valley  of  Town  Brook. 
Continuous  sewer  excavations  in  School  and  Water  Streets 
and  Quincy  Avenue,  twelve  feet  deep,  have  failed  to  reach 
the  bed-rock  at  any  point.  The  topographic  indications  are 
favorable  to  a  concealed  slate  area  here  ;  and  this  view  seemed 
to  be  confirmed  by  a  study  of  the  drift  on  the  northwest  slope 
of  Payne's  Hill.  Many  fragments  of  the  massive,  gray  Cam- 
brian slate  were  observed  in  the  drift ;  and  at  some  points  the 
slate  forms  fully  one  third  of  the  drift  bowlders.  But  the 
slate  is  so  similar  to  that  of  the  Adams  Street  area,  which  also 
is  on  the  line  of  glacial  movement  towai-d  Payne's  Hill  and 
but  a  mile  distant,  that  it  has  appeared  best,  on  the  whole,  to 
regard  the  Town  Brook  slate  area  as  not  })roved,  especially  as 
there  is  no  indication  of  the  fine  granite  border  which  a  normal 
occurr-ence  of  slate  would  demand. 

The  Pine  Hill  Area. — This  is  the  smallest  and  most 
completely  isolated  Cambrian  area  yet  discovered  in  the  Blue 
Hills  Complex.  My  attention  was  first  called  to  it  by  Mr. 
Watson  ;  and  it  has  proved  ujion  investigation  to  be  a  very 
com[)lete  area,  almost  ideal  in  its  relations  to  the  plutonic 
rocks,  and  contrasted  with  all  the  areas  previously  described 
by  occurring  in  that  j)art  of  the  complex  where  quartz  por- 


p!vyry  is  the  (loiiiinant  feature  of  the  eontact  zone  and  by  the 
absence  of  fault  boundaries.  It  is  in  a  brushy  wihlerness  in 
the  northeastern  part  of  the  Pine  Ilill  plateau,  near  the  rail- 
road between  Braintree  and  West  Quincy,  and  in  a  direct  line 
between  the  junction  of  Willard  and  West  Streets  on  the  West 
side  of  the  plateau  and  Liberty  Street  on  the  east  side.  The 
smallness  of  the  area  has  somewhat  interfered  with  its  accurate 
delineation  on  the  general  map,  while  the  brushy  and  imprac- 
ticable character  of  the  surface  and  the  lack  of  landmarks  have 
discouraged  me  from  attempting  a  special  map  of  the  Pine 
Hill  plateau. 

Briefly  described,  this  Cambrian  area  is  a  body  of  slate  of 
compact  and  simple  outline,  fifty  to  possibly  a  hundred  feet 
wide,  and  extending  from  near  the  northeastern  edge  of  the 
plateau  in  a  west-northwest  direction  for  more  than  five  hun- 
dred feet.  The  rock  is  wholly  massive  gray  slate,  like  that  of 
Hayward  Creek  and  the  northern  boundary  belt  west  of  the 
Old  Colony  Railroad,  and  may  provisionally  be  called  Middle 
Cambrian.  Although  very  firm  and  massive,  as  would  be 
expected  of  so  small  an  island  in  a  sea  of  plutonic  rocks,  it  is 
yet  distinctly  bedded  at  several  points,  the  strike  being  parallel 
with  the  belt  —  N.  70°-  80°  W.  —  and  the  dip  about  vertical. 
Much  of  the  slate  is  very  siliceous,  and  it  embraces  some  good 
slaty  quartzite. 

The  contact  of  the  slate  and  bordering  basic  porphyry  is 
exposed  at  several  points  and  is  typically  igneous,  the  two 
rocks  being  firmly  welded  together,  while  the  porphyry 
encloses  fragments  of  the  slate,  and  at  the  west  end  of  the  belt 
distinct  dikes  of  the  porphyry  cut  the  slate.  The  slate  is 
encircled  by  successive  zones  of  the  plutonic  rocks.  These  are 
most  distinct  on  the  south  side,  where  the  normal  gradation, 
as  described  on  page  365,  is  developed  with  almost  ideal  regu- 
larity. We  have  first  the  basic  porphyry  just  referred  to,  in 
one  of  its  most  perfect  and  continuous  developments.  This, 
with  a  total  breadth  of  one  hundred  to  two  hundred  feet,  grad- 


454 

ually  changes  outward  through  acid  or  quartz  porphyry  to  fine 
granite,  which  has  a  similar  breadth  and  blends  outward  with 
the  widely  extended  normal  granite.  The  fine  granite  encloses 
numerous  large  segregations  of  the  basic  porphyry  ;  and  these 
extend  outward  into  the  normal  granite,  becoming  gradually 
fiswer  and  smaller. 

About  one  hundred  and  fifty  feet  north  of  this  main  body  of 
slate,  near  its  eastern  end,  is  a  smaller  mass  of  slate  parallel 
with  the  main  belt  in  dip  and  strike  and  about  fifty  feet  long  in 
the  direction  of  the  strike.  This  also  o-ives  S'ood  io-neous  con- 
tacts,  and  is  cut  by  several  dikes  thi^ee  to  six  inches  wide.  The 
slate  also  incloses  some  trap  which  does  not  appear  to  cut  the 
granite  and  may  belong  to  the  pre-granitic  series.  As  would 
be  expected  with  so  small  a  mass,  the  zonal  arrangement  of 
the  inclosing  plutonics  is  less  distinct  than  for  the  main  belt ; 
although  with  the  latter  it  holds  only  for  the  sides  and  not  for 
the  ends  of  the  belt. 

From  the  west  end  of  the  main  belt  of  slate  the  basic  por- 
phyry extends  northwest  with  a  breadth  of  several  hundred  feet 
for  at  least  one  fourth  of  a  mile  to  the  east  end  of  the  small 
street  running  east  from  the  junction  of  Willard  and  West 
Streets.  This  band  of  basic  porphyry  is  bordered  on  both  sides 
by  the  continuation  of  the  fine  granite  zones,  fading  outward 
into  the  normal  granite.  In  other  words,  the  zonal  arrange- 
ment of  the  plutonics  extends  northwest  a  quarter  of  a  mile, 
and  possibly  farther  under  the  drift,  beyond  the  slate  in  which 
the  zonal  structure  has  its  origin.  As  previously  explained 
(page  365) ,  this  must  mean  a  former  extension  of  the  slate  itself 
far  beyond  its  present  limits.  Erosion  has  cut  here  below  the 
bottom  of  the  slate  ;  and  the  slate  that  remains  is  a  mere  rem- 
nant of  the  original  belt.  The  change  of  trend  is  real,  though 
perhaps  less  abrupt  than  the  map  shows  it. 

At  the  east  end  of  the  slate  the  conditions  appear  to  be 
essentially  similar,  but  the  exposures  are  less  continuous.  The 
basic  porpliyry  is  continuous  around  the  end  of  the  slate,  but 


455 

is  itself  soon  cut  off  by  a  great  dike  of  trap  which  here  forms 
the  margin  of  the  phiteau,  overlooking  the  railroad.  About 
one  fourth  of  a  mile  southeast  of  the  slate,  on  tiie  railroad, 
opposite  Liberty  Street,  is  a  magnificently  glaciated  ledge 
of  medium  to  normal  granite  crowded  with  lai'ge  segregations 
of  basic  porphyry.  It  is  an  exceptionally  good  exposure  of 
the  plutonics  south  of  the  slate  in  their  eastward  extension, 
and  testifies  to  the  former  extension  of  the  slate  at  least  thus 
far  to  the  eastward,  and  in  this  instance  without  appreciable 
change  in  the  trend  of  the  belt. 

The  Pine  Tree  Brook  Area. — This  is  much  the  largest 
of  the  insular  Cambrian  areas  and  the  one  lying  most  com- 
pletely within  the  area  of  the  Blue  Hills  proper.  It  forms  a 
highly  irregular  east-west  belt  at  least  two  miles  and  not  more 
than  two  and  a  half  miles  long.  It  is  crossed  midway  in  its 
length  by  Randolph  Avenue,  extending  about  a  mile  east  of 
the  avenue,  and  rather  more  than  a  mile  west  of  it,  ending  in 
the  latter  direction  under  the  swamp  west  of  Harland  Street. 
It  varies  in  breadth  from  about  five  hundred  feet  at  the  east 
end  to  fifteen  hundred  feet  near  the  avenue,  and  possibly  half 
a  mile  near  Harland  Street.  Yielding  more  readily  than  the 
granite  and  quartz  porphyry  to  the  agents  of  erosion,  it  is 
developed  topographically  as  a  longitudinal  valley  dividing 
the  Blue  Hills  into  two  ranges  and  occupied  for  almost  its 
entire  length  by  Pine  Tree  Brook,  which  finally  escapes  from 
the  hills  through  a  wide  notch  in  the  northern  range  at  the 
northwestern  corner  of  the  valley.  In  the  upper  part  of  the 
valley,  east  of  Randolph  Avenue,  the  outcrops  are  frequent 
and  extensive,  so  that  the  main  structural  features  are  readily 
and  satisfactorily  determined  (see  special  map,  PI.  26).  But 
west  of  Randolph  Avenue  the  outcrops  are  few ;  and  the 
lower  third  of  the  valley  is  a  continuous  swamp,  unrelieved  by 
a  single  exposure  of  the  underlying  rocks.  That  the  sedimen- 
tary rocks  continue  to  some  point  west  of  Harland  Street,  is 
indicated  by  the  topography  and  proved  by  the  abundant  occur- 
rence of  slate  in  the  drift  on  the  south  side  of  the  valley. 


456 

The  only  sedimentary  rock  of  which  we  have  any  trace  in  the 
valley  is  the  dull  gray  to  black  slate  similar  to  that  of  the  Pine 
Hill  area,  Hayward  Creek,  and  the  northern  boundary  belt, 
and  presumably  of  Middle  Cambrian  age.  When  not  too 
closely  involved  with  the  plutonic  rocks,  as  in  the  central 
portions  of  the  belt  near  Randolph  Avenue,  the  slate  is  usually 
distinctly  bedded,  and  even,  in  part,  more  or  less  fissile:  but 
near  the  granite  and  porphyry,  and  especially  in  the  narrow 
eastern  end  of  the  belt,  the  slate  is  often  very  obscurely 
bedded,  hard  and  brittle.  However,  it  exhibits  only  slightly 
the  micaceous  character  so  strongly  developed  in  the  northern 
boundary  belt :  and  no  garnets  or  other  secondary  minerals 
have  been  noted.  Its  relations  to  the  plutonics  are  so  intimate 
that,  apparently,  w^e  must  assume  that  this  contrast  in  meta- 
morphic  character  is  due  to  a  difference  in  composition. 

On  the  noi'th  side  of  the  valley,  the  outcrops  of  slate  begin 
about  midway  between  Harland  Street  and  Randolph  Avenue ; 
and  from  this  point  to  the  avenue,  and  for  fifteen  hundred  feet 
east  of  the  avenue,  the  strike  is  N.  80°— 90°  E.,  and  the  dip 
S.  70°- 90°.  Farther  east,  as  the  special  map  shows,  the  con- 
tinuity of  the  slate  is  interrupted  by  an  irregular  belt  of  fine 
granite  and  porphyry  three  hundred  to  five  hundred  feet  wide  : 
and  beyond  this  the  strike  is  southeasterly  for  about  eight 
hundred  feet  and  then  S.  30°  E.  for  five  hundred  feet.  The 
dip  for  these  two  sections  is  reversed,  being  northeasterly 
70°- 90°.  We  have  now  reached  the  hill  of  basic  porphyry 
known  as  Little  Dome,  about  two  thirds  of  a  mile  from 
Randolph  Avenue.  The  slate  here  regains,  with  apparent 
abruptness,  its  normal  strike  —  N.  80^—  90°  E.,  and  passes  east- 
ward along  the  north  side  of  Little  Dome,  with  a  possible  breadth 
of  six  hundred  feet.  North  of  Great  Dome  the  breadth  of  the 
slate  diminishes  from  five  hundred  feet  to  three  hundred  feet 
and  less,  as  it  approaches  its  rather  abrupt  termination  against 
an  oblique  (N.  W.— S.  E.)  wall  of  fine  granite  a  short  dis- 
tance south  of  the  Glencoe  granite  quarry.     As  indicated  on 


Occas.  Papers,  Boston  Soc,  Nat.  Hist.,  Vol.  IV. 


OUTCROP  MAP  OF   PINE  T 


Plate  26. 


.  ^  +  tL. 


^j,#?^^  .-^^f""^'^'   ''''''^'Kt\'':^^x\\ 


Ji  ♦//=Roc 


•-A  -A 


E   BROOK  AREA. 


457 

the  map,  the  slate  probably  extends  beyond  its  last  outcrop  to 
the  intersection  of  tlie  belts  of  fine  granite  on  the  south  and  on 
the  northeast,  at  a  point  between  Great  Dome  and  Sawcut 
Notch. 

In  the  vicinity  of  liandolph  Avenue  the  slate  is  traversed, 
pai;allel  with  the  strike  and,  mainly  at  least,  with  the  dip,  by 
the  quite  remarkable  series  of  pre-granitic  diabase  dikes 
described  on  page  388.  These  vary  in  width  up  to  forty  feet ; 
but  the  great  dike  along  the  southern  border  of  the  slate  must 
be  over  three  hundred  feet  wide,  if  all  the  exposures  are  to  be 
referred  to  one  dike  ;  although  two  dikes  one  hundred  and  fifty 
to  two  hundred  feet  wide  would  also  satisfy  the  observations. 
The  conformity  of  the  dikes  with  the  strike  of  the  slate  is 
especially  well  shown  in  their  trends  changing  as  the  strike 
changes.  The  dikes  are  strictly  limited  to  the  slate.  In  no 
case  do  they  extend  into  the  granite  ;  but,  as  the  map  shows  in 
several  instances,  they  abut  with  undiminished  breadth  against 
the  granite.  Even  the  great  dike,  the  slate  walls  of  which  are 
nowhere  exposed,  is  definitely  and  absolutely  cut  off  by  the 
granite  in  the  same  general  longitude  as  the  smaller  dikes. 
The  dikes  are  of  very  uniform  character  (see  page  388),  and 
more  numerous  than  represented  on  the  map,  almost  every 
good  exposure  of  slate  in  the  central  part  of  the  belt  showing 
one  or  more  dikes.  In  its  more  westerly  outcrops,  nearly  half 
a  mile  Avest  of  Randolph  Avenue,  the  slate  is  still  accompanied 
by  the  dikes  ;  but  toward  the  eastern  end  of  the  belt  the  dikes 
are  apparently  wholly  wanting,  although  this  may  be  due  in 
part  to  the  insufficiency  of  the  outcrops.  The  dikes  are 
undoubtedly  older  than  and  cut  off  by  the  granite  and  por- 
phyry ;  and  in  several  instances  fragments  of  similar  diabase, 
one  to  two  feet  in  diameter,  were  found  inclosed  in  the  granite. 
Also,  in  one  ledge  (Fig.  44)  in  the  central  part  of  the  group 
of  dike  outcrops,  Mr.  Watson  and  I  found  a  small  but  clear 
dike  of  o-ranite  traversino-  a  large  dike  of  diabase.  The  dikelet 
is  six  to  seven  inches  wide  at  the  laro'e  end  and,  with  a  curvino^ 


458 


diagonal  course,    tapers    to   nothing    in    a    distance    of   about 
twenty-five  feet.      One  of  the  simpler  of  the  many  intrusions 


Granite  DiKe 


Fig.  44.  —  Small  dike  of  granite  intersecting  a  large  dike  of  diabase  in  the 
Cambrian  slate  of  the  Pine  Tree  Brook  area. 


of  granite  in  the  slate  is  also  shown  in  this  figure.  Unlike  the 
diabase  intrusions  they  do  not  conform  with  the  strike  of  the 
slate,  except  occasionally  by  mere  chance  (Fig.   45). 

As  previously  described  (page  348),  and  as  the  map  plainly 
shows,  the  granite  bordering  and  intimately  associated  with  the 
slate  is  the  fine  granite  ;  but  it  changes  rapidly  outward  or 
away  from  the  slate  to  normal  granite.  At  some  points,  and 
notably  near  Randolph  Avenue,  and  in  Pine  Rock  and  Little 
Dome  farther  east,  the  coarsely  and  profusely  porphyritic  basic 
phorphyry  takes  the  place  of  the  fine  granite  as  the  immediate 
contact  zone  ;  and  throughout  the  field  the  fine  granite  is  more 
or  less  spotted  with  patches  and  segregations  of  the  basic  por- 
phyry. Almost  every  ledge  of  fine  granite  shows  some  por- 
phyry, and  many  ledges  ai^e  crowded  with   the  porpliyry  inclu- 


459 

sions,  which  are  numbered  by  thousands,  grading  into  the 
continuous  bodies  of  porphyry.  In  the  outcrop  of  basic  por- 
phyry nearest  to  Randolph  Avenue,  is  a  distinct  dike  of  fine 
granite  from  three  to  four  inches  wide,  and  I  observed  another 
of  about  the  same  size  in  the  porphyry  between  Pine  Rock  and 
Little  Dome. 

These  contact  zone  phenomena  are  equally  v/ell  developed 
along  the  north  and  south  sides  and  across  the  eastern  end  of 
the  slate  ;  and,  as  with  the  Pine  Hill  Area,  and  unlike  all  the 
other  Cambrian  areas  where  the  facts  are  exposed,  evidence  of 
a  fault  contact  along  any  margin  of  the  slate  is  entirely  wanting. 
On  the  other  hand,  the  proofs  of  the  intrusive  relation  of  the 
plutonic  rocks  to  the  slate  are,  in  every  direction,  manifold  and 
convincing.  They  include,  first,  the  zonal  arrangement  of  the 
plutonics  themselves  ;  second,  the  highly  irregular  general  out- 
lines of  the  slate  so  entirely  at  vai'iance  with  its  own  structure  ; 
third,  the  irregular  masses  and  dikes  of  fine  granite  and  basic 
porphyry  dividing  the  slate  in  every  direction,  and  mainly 
across  the  strike  ;  fourth,  the  numerous  dikelets  and  apophy- 
ses of  these  plutonics  penetrating  the  slate  and  ramifying 
through  it;  and  fifth,  the  fragments  of  slate  inclosed  in  the 
plutonics.  The  belt  of  granite  and  porphyry,  three  hundred 
to  five  hundred  feet  wide,  dividing  the  slate  about  one  third  of 
a  mile  east  of  Randolph  Avenue,  tapers  southward,  and  may 
be  likened  to  a  great  plutonic  wedge  splitting  the  slate  ;  or, 
coming  as  it  does  at  the  point  where  the  first  marked  change 
in  the  strike  occurs,  it  may  be  called  the  keystone  of  the  slate 
arch.  Doubtless,  if  the  outcrops  were  more  continuous,  this 
area  would  present  an  ideal  complex.  One  of  the  more  regu- 
lar dikelets  of  granite  is  shown  in  figure  45.  It  cuts  a  ledge 
of  slate  near  the  centre  of  the  widest  part  of  the  belt.  It  is 
ten  to  twelve  inches  wide,  and  is  exposed  for  fifty  feet, 
although  probably  much  longer.  At  the  west  end  it  is  parallel 
to  the  strike,  but  eastward  it  curves  to  the  north  across  the 
strike.      The  dike  is  nearly  vertical,  while  the  slate  dips  S.  70°, 


460 

appi^oximately.  The  actual  contact  of  the  slate  and  plutonics, 
wherever  exposed,  is  typically  igneous,  the  two  rocks  being 
firmly  welded. 


Eig.  45. — ■  A  dike  of  line  granite  cutting  the  Cambrian  slate  of  tlie  Pine 
Tree  Broolc  area.     Scale:  1  inch=  15  feet. 

Around  the  eastern  end  of  the  slate  the  actual  contact  with 
the  main  body  of  granite  does  not  seem  to  be  anywliere 
exposed,  although  nearly  so  at  many  points,  especially  across 
the  northern  slope  of  Great  Dome,  where  the  slate  reaches 
nearly  to  the  summit  of  the  liill.  Nevertheless,  the  relation  of 
the  two  rocks  is  not  uncertain,  for  the  slate  is  traversed  in 
every  direction  by  dikes  of  fine  granite  and  basic  porphyry. 
The  dikes  are  most  numerous  in  the  low  ledges  north  of  Great 
Dome  ;  but  several  very  distinct  ones  were  observed  near  the 
top  of  the  liill.  They  vary  in  size  from  a  fraction  of  an  inch 
to  a  yard  or  more,  are  usually  quite  irregular  and  branching, 
and  tlie  larger  dikes  sometimes  inclose  fragments  of  the  slate. 
The  dark  gray  slate  is  hard,  brittle,  and  very  massive.  In 
fact,  the  bedding  is  so  obscure  that  I  could  not  be  quite  sure 
of  the  dip  and  strike  at  any  point.  Although  the  breadth  of 
the  slate  is  considerable  (three  hundred  to  five  hundred  feet), 
it  is  evidently  shallow,  every  part  of  the  area  being  near  the 
underlying  or  surrounding  granite,  which  lias  injected  the 
downward-opening  cracks  of  the  slate.  This  shallowness  of 
the  vertical  strata  is  one  of  the  most  general  and  important 
facts  of  the  Cambrian  outcrops  of  the  Blue  Hills  Complex, 
and  points  more  distinctly,  perhaps,  than  any  other  to  the  gen- 
eral conclusion  that  a  great  volume  of  the  formation  was 
absorbed  by  the   plutonic  magma. 


461 


THE   BLUE   HILLS    COMPLEX    IN    CARBONIFER- 
OUS  TIMES. 

The  exact  geological  dates  of"  the  different  series  of  igneous 
rocks  formed  during  the  Cambro-Carboniferous  interval,  and 
their  relations  to  erosion,  must  remain,  apparently,  in  the 
absence  of  contemporaneous  sediments,  a  matter  of  conjecture, 
unless  we  incur  the  risk  of  correhation  with  the  similar  rocks  of 
the  Maritime  Provinces.  But  the  beginning  of  Carboniferous 
time  gives  the  student  of  our  local  geology  once  more  a  well- 
determined  hoi'izon  within  his  own  field,  —  a  new  base-line  or 
point  of  departure  for  the  study  of  the  later  periods  of  the 
geological  history  of  this  region.  Strictly  speaking,  the  Sub- 
carboniferous  formation,  including  the  Mountain  Limestone  of 
the  Mississippi  Valley,  the  Vespertine  series  of  the  Appalach- 
ian region,  and  the  Lower  Carboniferous  limestones,  shales 
and  gypsum  of  the  Maritime  Provinces,  is,  so  far  as  known, 
unrepi'esented  in  New  England  ;  and  the  great  conglomerate 
series  underlying  the  coal  measures  of  this  region  may,  appar- 
ently, be  best  referred  to  the  Millstone  Grit  epoch.  The 
great  thickness  of  the  Carboniferous  conglomerates,  alike  in 
the  Appalachian  region  and  in  New  England  and  the  Maritime 
Provinces,  indicates  that  this  was  a  period  of  subsidence,  when 
the  stable  marine  conditions,  with  flat  shores  and  stagnant 
lagoons,  of  Subcarboniferous  time  gave  way  to  an  encroach- 
ment of  the  sea  upon  the  land,  attended,  however,  by  marked 
and  oft-repeated  oscillations  of  level.  Each  advance  of  the 
sea  spreads  a  layer  of  conglomerate  over  the  invaded  area  ; 
and  during  the  subsequent  recession  of  the  shore-line  this 
newly  formed  and  unlithified  stratum  is,  in  large  part,  borne 
seaward  :  and  so,  as  tiie  process  is  repeated  time  after  time, 
there  is  gradually  built  uj),  delta-like,  a  great  formation  of 
conglomerate.      In    some  such   way  as  this,  we  may  suppose, 


462 

the    Carboniferous    conglomerate    was    spread     over    Eastern 
Massachusetts. 

The  conglomerate  is  composed  very  largely,  in  the  Boston 
Basin  almost  wholly,  of  the  Cambrian  quartzite,  the  different 
varieties  of  felsite,  and  the  granites.  The  abundance  of  felsite 
debi'is  in  the  conglomerate  is  particularly  noteworthy  ;  and, 
since  felsite  is  by  no  means  the  most  abundant  rock  in  Eastern 
Massachusetts  at  the  present  time,  it  is  a  legitimate  inference 
that,  as  before  noted,  it  covered  much  wider  areas  in  pre-Car- 
boniferous  times.  The  validity  of  this  point  is  especially  obvious 
when  we  note  the  extraordinarily  large  proportion  of  felsite  peb- 
bles in  the  conglomerate  in  some  districts,  such  as  Nantasket, 
where  ledges  of  felsite  are  almost  wholly  Avanting.  All  this 
harmonizes  well  with  the  effusive  and  superficial  origin  of  the 
felsites  ;  since,  so  far  as  we  know,  they  were  the  newest  rocks 
in  this  region  at  the  beginning  of  Carboniferous  time.  The 
granites,  on  the  other  hand,  although  covering  hundreds  of 
square  miles  in  the  vicinity  of  the  Carboniferous  basins,  have 
contributed  relatively  little  material  to  the  composition  of  the 
conglomerate.  Moreover,  the  granitic  pebbles  are  largely  of 
the  finer-grained  (contact-zone  and  intrusive)  varieties  ;  the 
coarse  (normal)  varieties,  which  largely  prevail  in  our  granite 
ledges  to-day,  being  of  somewhat  exceptional  occurrence,  save 
very  locally  where  Carboniferous  erosion  cut  most  deeply  ;  the 
exposure  of  the  coarse  or  normal  granites  requiring,  in  general, 
the  removal  of,  first,  the  felsites,  and,  second,  the  finer  granites 
of  the  contact  zone,  with,  possibly,  some  slate.  In  fact  the  bio- 
titic  normal  granite  and  its  contact  zone  of  basic  granite  and  dio- 
rite,  the  most  truly  abyssal  rocks  of  this  region,  are  also  the  most 
scantily  represented  in  the  conglomerate.  It  may  be,  liowever, 
that,  as  I  have  elsewhere  suggested, ^  the  paucity  of  the  diorite 
detritus  in  the  conglomerate  is  due  in  part  to  the  greater  sus- 
ceptibility of  this  rock  to  chemical  decay.  Next  to  the  felsites, 
in  the  order  of  abundance  in  the  conglomerate  of  the  Boston 

1  Occas.  papers  Koston  soc.  iiat.  hist.,  vdl.  3,  p.  28. 


463 

Basin,  come  the  Cambrian  quartzites,  so  extensively  developed 
in  the  country  north  and  west  of  Boston.  These  are  the  most 
resistant  and  truly  imperishable  rocks  of  the  region  ;  and  they 
probably  formed  salient  features  of  the  early  Carboniferous 
landscape  ;  in  fact,  the  topographic  prominence  of  the  quartz- 
ites in  still  earlier  times  may  have  prevented  their  being  entirely 
covered  by  the  outflows  of  felsite.  It  is  interesting  in  this  con- 
nection to  observe  that,  as  I  have  pointed  out,^  the  quartzite 
pebbles  are  most  abundant  and  largest  in  those  portions  of  the 
conglomerate  series  lying  nearest  to  the  existing  areas  of 
quartzite.  The  controverted  question  as  to  whether  the  Cam- 
brian slates  are  represented  in  the  Carboniferous  conglomerate 
must  now  be  answered  in  the  affirmative  ;  and  in  explanation  of 
their  rather  scanty  occurrence  in  this  way  it  may  be  noted  that 
such  soft  rocks  are  little  likely,  in  the  presence  of  the  much 
harder  felsites,  granites,  and  quartzite,  to  form  pebbles  ;  and 
that  the  slates,  like  the  granite^,  were  probably  buried  beneath 
the  flows  of  felsite. 

The  oscillations  of  level  during  the  formation  of  the  Carbonifer- 
ous conglomerate  are  marked  in  all  the  basins,  but  especially  in 
the  Narragansett  and  Norfolk  basins,  by  frequent  and  somewhat 
extensive  beds  of  sandstone  and  shale,  usually  of  reddish  or 
greenish  colors.  The  green  tints,  it  is  well  understood,  may  be 
referred  to  the  reduction  of  the  red  colorinaf  assent  —  ferric  ox- 
ide  —  by  organic  matter  ;  but  it  is  believed  that  the  red  color 
(and  through  that  the  green)  has  climatic  significance.  In 
other  words,  the  colors  of  these  sediments  lend  important  sup- 
port to  the  view  proposed  by  Mr.  T.  T.  Bouve^  that  during 
long  ages  preceding  the  Carboniferous  subsidence  the  rocks  of 
this  region  (felsites,  granites,  etc.)  were  subject  to  chemical  as 
well  as  mechanical  decay,  under  conditions  similar  to  those  which 
now  obtain  in  the  Southern  States  and  in  the  Tropics,  but  even 
more  favorable  to  the  production  of  important  results.      All  the 

iLoc.  cit. 

2Proc.  Boston  soc.  nat.  hist.,  vol.  23,  pp.  29-36. 


464 

rocks,  except,  perhaps,  the  quartzites,  were  rotted  to  a  great 
depth,  possibly  hundreds  of  feet,  and  the  residual  earths  were 
strongly  colored  with  ferric  oxide.  When  the  sea  finally  in- 
vaded this  area  it  found  an  immense  amount  of  detritus  ready 
to  be  washed  away  ;  and  the  deposition  of  the  Carboniferous 
strata  went  forward  at  a  comparatively  rapid  rate.  Where  the 
sea  and  tributary  streams  acted  directly  upon  the  still  undecom- 
posed  rocks,  the  pebbles  were  fasliioned  to  build  the  conglomer- 
ate beds  ;  but  the  alternating  beds  of  sandstone  and  shale  ai*e  in 
large  part  chemical  detritus  which  was  transported,  assorted  and 
deposited  by  the  water  without  farther  essential  change  ;  and  the 
proof  of  this  is  found,  as  indicated,  in  the  prevailing  red  color. 
One  of  the  earliest  and  most  important  applications  of  this  princi- 
ple was  made  by  RusselP  in  explaining  the  abundant  red  sediments 
of  the  Triassic  formation.  He  was  able  to  show  that  not  only 
is  the  argillaceous  deti-itus  strongly  colored,  but  that  it  closely 
invests  the  grains  and  fragments  of  quartz  and  other  materials, 
so  that  the  red  color  is  very  generally  imparted  to  the  coarser 
as  well  as  the  finer  sediments.  In  this  connection,  it  is  inter- 
esting to  note,  also,  that  the  red  beds  occur  chiefly  in  the  lower 
part  of  the  Carboniferous  series  of  Eastern  Massachusetts,  and 
in  general  where  we  may  suppose  that  the  material  has  not 
been  transported  far  from  its  source.  Some  of  the  seemingly 
exceptional  cases  are  found  on  examination  to  owe  their  red 
color  largely  to  an  entirely  distinct  cause,  viz.,  the  mechanical 
comminution  of  intrinsically  red  rocks,  such  as  the  red  felsite 
and  the  red  Lower  Cambrian  slates. 

At  this  period  (early  Carboniferous),  it  is  improbable  that 
the  existing  Carboniferous  basins  were  even  outlined  ;  but  the 
Carboniferous  sea,  subject  to  the  oscillations  of  level  already 
noted,  spread  its  sediment  far  and  wide  over  the  entire  region. 
Indications  are  not  wanting,  however,  that  the  subsidence  of 
the  sea-floor  was  most  marked  in  the  latitude  of  Boston,  one 
of  these  indications  being  the  almost  comj)lete  absence  of  ter- 

'  15nll.  no.  52,  U.  S.  geol.  survey,  Wasliinnton,  1889. 


465 

restrlal  plants  (Calamites,  etc.)  in  the  conglomerate  of  the 
Boston  Basin  ;  and  a  decided  differentiation  soon  manifested 
itself  in  the  breaking  out  within  the  present  limits  of  the  Bos- 
ton Basin  of  volcanic  activity.  The  eruptions  appear  to  have 
been  submarine,  and  numerous  sheets  of  neutral  and  basic 
lavas  (porphyrite  and  melaphyre,  chiefly  the  latter),  with 
occasional  beds  of  tuff,  were  interstratified  with  the  conglomer- 
ates and  sandstones.  In  this  way,  through  the  cooperation  of 
aqueous  and  igneous  agencies  and  the  alternation  over  the 
same  area  of  beds  of  conglomerate,  sandstone,  shale,  and  con- 
temporaneous lavas,  was  built  up  the  great  conglomerate  series 
of  the  Boston  Basin,  which  has  a  measured  thickness  in  the 
Nantasket  and  Hingham  sections  of  more  than  a  thousand  feet, 
and  which  differs  essentially  from  the  synchronous  series  of  the 
neighboring  Norfolk  Basin  only  in  the  volcanic  rocks.  The 
contemporaneous  lavas  are  well  developed  in  the  Neponset 
Valley,  on  Hough's  Neck,  in  Hingham,  and  in  the  Nantasket 
district;  but  south  of  the  Blue  Hills,  in  the  Norfolk  Basin, 
they  are  wholly  wanting.  Therefore  although  we  can  hardly 
suppose  that  the  Blue  Hills  were  in  existence  at  the  time  when 
these  strata  were  forming,  their  future  position  was  thus  early 
defined,  in  the  southern  margin  of  the  Carboniferous  volcanic 
area  ;  and  no  better  concise  definition  of  the  Boston  Basin  at 
this  period  can  be  framed  than  that  it  was  an  area  of  volcanic 
activity  on  the  floor  of  the  Carboniferous  sea.  Lest  the  vol- 
canic rocks  should  be  regarded  as  a  serious  bar  to  the  correla- 
tion of  the  Boston  and  Norfolk  Basins,  it  may  be  noted  that  a 
similar  localization  of  volcanic  phenomena  occurs  in  the  un- 
doubted Carboniferous  beds  of  the  Maritime  Provinces.  The 
vast  development  of  Carboniferous  strata  in  New  Brunswick, 
Nova  Scotia,  and  Cape  Breton  is  singularly  free  from  effusive 
igneous  rocks,  except  in  a  few  districts.  Dawson  says  of  the 
Pictou  district :  ' '  The  lowest  Carboniferous  rocks  seen  here  are 
conglomerates  interstratified  with  beds  of  amygdaloidal  trap, 
which  have  flowed  over  their  surfaces  as  lava  currents,  just  as 

OCCAS.  PAPERS   B.  S.  N.  H.  IV.  30. 


466 

the  trap  of  the  Bay  of  Funcly  has  flowed  over  the  red  sand- 
stone. Several  of  these  ancient  lava  streams  alternate  with 
beds  of  conglomerate  ;  and  while  their  lower  parts  have  by 
their  heat  slightly  altered  the  underlying  bed,  their  upper  parts, 
cooled  and  acted  on  by  the  waves,  have  contributed  fragments 
to  the  overlying  conglomerate.  Over  these  conglomerates  is 
a  great  series  of  reddish  and  gray  sandstones  and  shales."  ^ 

The  oscillations  of  level  during  the  formation  of  the  Carbonif- 
erous conglomerate  series  were  of  a  massive  character  for  the 
region  at  large,  but  attended  by  deformation  of  the  crust  in 
the  Boston  Basin  ;  and  so  it  happened  that,  while  the  Carbon- 
iferous land  was  approximately  base-levelled  and  the  main 
supply  of  coarse  sediments  thus  cut  oft',  the  extensive  flat  areas 
near  sea-level  and  favorable  for  the  formation  of  coal,  partly 
constructive  and  partly  destructive  in  origin,  were  established 
■over  the  Norfolk  and  Narragansett  Basins  ;  and  the  Boston 
Basin  became,  after  the  volcanic  fires  had  died  out,  a  trough 
too  deep  for  the  formation  of  coal-beds,  and  too  remote  from 
?iny  elevated  land  to  receive  coarse  sediments.  Thus  we  pass 
gradually  and  with  entire  conformity  from  the  conglomerate 
series  to  the  coal  measures  in  the  area  of  the  shallow  southern 
basins  and  to  the  synchronous  newer  slate  series  of  the  deeper 
Boston  Basin,  the  thickness  of  which  can  hardly  be  less  than 
a  thousand  feet.  Where  the  Blue  Hills  now  stand,  or  in  the 
region  immediately  to  the  southward,  the  two  sets  of  conditions 
—  deep  estuary  or  bay  and  marine  flats  and  marshes  — 
must  have  merged  ;  and  we  may  aftirm  with  a  good  degree  of 
confidence  that  the  true  coal  measures  never  extended  over  the 
Boston  Basin. 

The  Boston  and  Norfolk  Basins,  separated  by  the  Blue 
Hills  Complex,  three  miles  in  breadth,  are  overlapping  troughs, 
the  one  shallowing:  westward  and  formino-  the  entire  northern 
border  of  the  Complex,  and  the  other  shallowing  eastward  to 
its   termination  in  the   vicinity  of  Great   Pond  in   Braintree, 

1  Acadian  Geology,  p.  31G. 


467 

mi(lway  on  tlie  soutliern  border.  Although  it  cannot  be 
doubted  that  the  two  basins  were  once  continuous  across  this 
common  barrier,  the  fact  that  not  a  single  outlier  or  slightest 
vestige  of  the  Carboniferous  strata  has  been  discovered  within 
the  area  of  the  Complex  indicates  very  clearly  that  these  newer 
strata  have  never  been,  over  this  area,  folded  or  faulted  down 
so  as  to  become  in  any  sense  a  part  of  the  Complex  ;  but  they 
formed,  rather,  a  broad,  simple,  and,  doubtless,  much  broken, 
anticlinal  cover  of  the  Complex,  which  erosion  has  completely 
removed.  The  Carboniferous  strata  do  not,  however,  form  a 
complete  frame  for  the  Complex,  erosion  having,  for  five 
miles,  from  Weymouth  Back  River  to  Braintree  Great  Pond, 
cut  below  the  shallow  eastern  end  of  the  Norfolk  Basin. 


THE  CARBONIFEROUS  STRATA  ALONG  THE  SOUTHERN 
BORDER  OE  THE  BLUE  HILLS  COMPLEX. 

Since  the  Carboniferous  strata  south  of  the  Blue  Hills  belono;' 
to  the  Norfolk  Basin,  and  since  they  encroach  but  slightly 
upon  the  area  of  the  map  of  the  Boston  Basin  (Plate  13), 
their  chief  claims  to  consideration  here  are,  first,  their  struc- 
tural relations  to  the  Blue  Hills  Complex  and  the  southern 
boundary  fault,  and,  second,  their  lithological  relations  to  the 
corresponding  strata  of  the  Boston  Basin.  It  is  proposed, 
therefore,  to  give  particular  attention  only  to  that  part  of  the 
Norfolk  Basin  east  of  the  Neponset  River  ;  and  not  to  under- 
take a  systematic  or  exhaustive  study  of  even  this  limited 
area,  for  which,  indeed,  the  infrequent  outcrops  oiFer  little 
encouragement . 

The  Norfolk  Basin  is  approximately  two  miles  wide  where 
crossed  by  the  Boston  and  Providence  Railroad,  one  and  a  half 
miles  in  the  longitude  of  Ponkapog  Pond,  and  one  mile  on  the 
line  of  Randolph  Avenue.  Beyond  this  it  appears  to  narrow 
more  rapidly  ;   and  it  is  quite  certainly  not  more  than  half  a 


468 

mile  wide  in  the  vicinity  of  Braintree  Great  Pond.  The 
southern  margin  is  heavily  and  broadly  drift-covered,  and  its 
accurate  definition  is  impossible  with  the  facts  now  available  ; 
but  the  northern  boundary,  fortunately  for  our  present  pur- 
pose, may  be  readily  traced  by  contiguous  outcrops  of  con- 
glomerate and  quartz  porphyry,  besides  being,  at  most  points, 
strongly  expressed  in  the  topography.  The  most  complete 
and  satisfactory  section  of  this  or  any  part  of  the  Norfolk 
Basin  is  afforded  by  the  ledges  on  and  near  the  Boston  and 
Providence  Railroad.  It  begins  on  the  north  in  the  ledges  of 
very  coarse  or  giant  conglomerate  near  Back  Street  south  of 
Willow  Street  in  Canton ;  and  in  the  broad  rocky  ridge 
between  Back  Street  and  Ponkapog  Brook  the  section  is 
almost  continuous  southward  for  three  fourths  of  a  mile  to 
Pecunit  Street.  The  coarse  conglomerate,  which  is  undoubt- 
edly here,  as  elsewhere  throughout  the  Basin,  the  basal  mem- 
ber of  the  series,  is  almost  ideally  developed  in  broad  ledges 
and  great  bowlders,  consisting  largely  of  pebbles  from  six 
inches  to  a  foot  and  even  two  feet  in  diameter.  Southward 
the  coarse  conglomerate  repeatedly  alternates  with  and  gradu- 
ally merges  into  the  more  normal  type,  and  at  last  the  con- 
glomerate series,  which  has  a  total  breadth  of  about  a  quarter 
of  a  mile,  gives  way  in  like  gradual  manner  to  the  red  sand- 
stone series,  which,  with  many  layers  and  partings  of  red 
shale  and  occasional  bands  of  fine  conglomerate,  is  exposed 
for  half  a  mile  farther.  The  dip  is  steadily  to  the  south  and 
steep  —  70°— 90°  ;  it  can,  however,  rarely  be  observed  in  the 
coarse  conglomerate.  Ripple-marked  surfaces  in  the  sandstone 
show  that  the  strata  are  not  inverted. 

The  outcrops  east  of  Back  Street  probably  overlap  some- 
what, southward,  the  more  northerly  cut  on  the  railroad,  which 
embraces,  with  some  interruptions  and  an  approximately  verti- 
cal dip,  about  four  hundred  feet  of  strata.  Red  sandstone  and 
shale  predominate  througliout,  but  with  an  occasional  green 
layer  and  an  increasing  proportion  of  fine  conglomerate  south- 
ward.     Near  the  middle  of  the  section  are  three  bands  of  im- 


469 

pure  limestone  and  calcareous  slate  one  to  four  feet  wide  and 
a  dike  (diabase?)  eighteen  inches  wide.  The  rocks  all  show 
great  disturbance — crushing,  shearing,  etc.  Some  thirty- 
eiglit  hundred  feet  south  of  this  is  the  second  cut,  about  two 
hundred  feet  long,  of  essentially  similar  composition,  — alter- 
nating sandstones,  shales  and  conglomerates,  —  except  that  the 
prevailing  tone  is  gray  instead  of  red,  conglomerate  is  relatively 
more  abundant,  and  the  dip  is  S.  70°.  As  before,  the  rocks 
are  much  crushed  and  sheared.  About  a  hundred  feet  farther 
brings  us  to  a  ledge  on  the  east  side  of  the  track,  showing  sixty 
feet  of  gray  sandstone  and  shale  with  the  same  dip  as  the  last. 
It  was  in  the  greenish  gray  shales  of  these  highest  beds  that 
Woodworth  ^  found  the  compressed  stems  of  Calamites  and  Sig- 
illaria,  thus  confirming  and  supplementing  the  proof  of  the  Car- 
boniferous age  of  the  Norfolk  Basin  series  discovered  by  Pro- 
fessor George  H.  Barton  and  the  writer  at  Rockdale,  nine  miles 
to  the  southwest.^  More  recently  Miss  C.  M.  Endicott  has 
found  other  forms  of  Carboniferous  vegetation  in  these  Canton 
ledges.  A  blank  of  nearly  half  a  mile  (2400  feet)  separates 
this  exposure  from  the  granite  ledges  near  Railroad  Street  north 
of  Canton  Junction. 

Neither  here  nor  elsewhere  is  there  the  slightest  indication  of 
a  repetition  of  the  coarse  basal  conglomerate  on  the  southern 
margin  of  the  basin ;  but  all  the  facts  support  the  view  that  the 
general  structure  of  this  eastern  extremity  of  the  basin  is  mono- 
clinal.  Although  faulting  or  isoclinal  folding  may  have  re- 
peated the  beds  to  some  extent,  and  thereby  increased  the  ap- 
parent thickness  of  the  section,  the  great  basal  conglomerate, 
the  red  beds  and  the  gray  beds  must  be  regarded  as  essentially 
distinct  members  of  a  terrane  having  an  aggregate  thickness  of 
at  least  five  thousand  feet,  and  possibly  nearly  ten  thousand 
feet.  .An  important  displacement  along  the  southern  margin 
of  the  basin,  with  the  down-throw  to  the  north,  is  a  necessary 
concomitant  of  the  monoclinal  structure  ;  and  this  displacement, 

1  Amer.  journ.  sci.,  vol.  148,  pp.  145-148. 
=  A.mei'.  .iourn.  sci.,  vol.  120,  pp.  416-420. 


470 

hopelessly  concealed  though  it  be  by  the  drift,  is  best  regarded 
as  a  continuation  to  the  west  and  southwest  of  the  great  south- 
ern boundary  fault  of  the  Blue  Hills  Complex. 

Eastward  from  the  Neponset  Valley,  the  gray  beds,  through 
the  narrowing  of  the  basin  by  this  obliquely  longitudinal  fault, 
and  in  perfect  accordance  with  the  monoclinal  structure,  are  cut 
out  of  the  section  :  and  east  of  Ponkapog  the  red  beds,  in 
diminishing  proportion,  occupy  exclusively  the  southern  half  of 
the  basin.  The  ledges  of  red  sandstone  and  shale  on  Farm 
Street,  south  of  Ponkapog  Pond,  exhibit  in  part  a  low  dip  to 
the  north,  but  this  may  be  readily  explained  by  their  proximity 
to  the  boundary  fault,  the  di'ag  of  which  has  locally  reversed 
the  dip. 

In  contrast  with  the  faulted  and  narrowing^  southern  mar^'in 
of  the  basin,  the  northern  margin,  in  which  we  now  have  a 
more  special  interest,  consists  throughout,  from  the  Neponset 
to  Braintree  Great  Pond,  of  the  giant  conglomerate,  grading 
southward  throug-h  normal  conoiomerate  to  the  red  sandstone 
series,  with  persistent  high  southerly  dips.  East  of  Ponkapog 
Brook  the  giant  conglomerate  passes  beneath  the  high-level  sand 
plain  of  the  Neponset  Valley,  which  has  here,  south  of  great 
Blue  Hill,  one  of  its  most  perfect  developments,  and  reappears 
two  miles  to  the  eastward,  in  the  vicinity  of  Hoosickwhissick 
Pond,  protruding  from  the  level  surface  of  the  plain.  South- 
east of  the  pond  it  is  well  developed,  the  jjrincipal  outcrops  be- 
ing represented  on  the  topographic  map  (PL  14).  It  is  in  part 
very  coarse,  while  partings  of  sandstone  show  that  the  dip  is 
approximately  vertical.  The  most  northerly  outcrops  are  still 
about  five  hundred  feet  south  of  the  most  southerly  exposures 
of  the  quartz  porphyry,  which  forms  the  entire  southern  slope 
of  the  Blue  Hills  from  Green  Street  eastward.  But  the  inter- 
vening ground  is  quite  certainly  underlain  by  the  quartz  por- 
phyry ;  for  west  of  the  pond  and  in  line  with  its  southern  shore 
is  a  ledge  of  quartz  porpltyry,  and  other  ledges  of  this  rock 
reach  the  same  latitude  half  a  mile  or  so  east  of  the  pond. 
Crossing  Monatiquot  Stream  by  the  Ponkapog  Trail,  we  find 


471 

in  the  woods  north  and  northeast  of  Ponkapog  Pond  many 
large  and  extremely  picturesque  bowlders  of  conglomerate,  some 
of  which  appear  to  be  residuar}^  masses  or  approximately  in  situ  ; 
and  farther  east  on  this  line  are  some  undoubted  ledges  of  con- 
glomerate, the  exposed  breadth  of  the  conglomerate  belt  being 
somewhat  greater  than  near  the  Neponset. 

Following  the  southern  base  of  the  Blue  Hills  eastward,  the 
ledges  and  prominent  bowlders  and  groups  of  bowlders  so 
faithfully  indicated  on  the  topographic  map  (PL  14)  are  found 
to  be  all  quartz  porphyry  for  nearly  a  mile,  and  the  next 
exposure  of  the  conglomerate  ioi  situ  is  found  in  the  high 
ledge  rising  steeply  from  the  northern  bank  of  Monatiquot 
Stream  at  the  point  where  it  crowds  most  closely  against  the 
hills,  between  six  hundred  and  a  thousand  feet  west  of  the 
junction  of  Randolph  Avenue  and  High  Street.  The  coarse- 
ness of  the  conglomerate  is  impressive.  Rounded  bowlders  of 
the  typical  quartz  porphyry  of  the  Blue  Hills,  and  identical  in 
character  with  the  rock  in  the  neighboring  ledges,  from  one  to 
two  feet  in  diameter,  make  up  the  mass  of  the  rock,  while  the 
largest  are  three  and  even  four  feet  in  diameter  ;  and  nearly 
all  are  well  rounded  or  water- worn,  being  thus  readily  dis- 
tinguished, wdiere  weathered  out,  from  the  drift  bowlders. 
Besides  the  quartz  porphyry,  there  are  in  the  conglomerate 
many  bowlders  of  the  fine  granite  into  which  the  quartz  por- 
phyry natui'ally  grades,  and  a  surprising  abundance  of  dark 
Cambrian  slate,  mainly  but  not  wholly  in  small  fragments. 
The  most  western  exposure,  probably  in  situ,  shows,  in 
layers  of  sandstone,  a  southerly  dip  of  85°- 90°.  Immediately 
north  of  these  ledges  the  rocks  are  concealed  by  till,  and  the 
contact  with  the  quartz  porphyry  is  not  exposed.  But  the 
quartz  porphyry  ledges  between  the  conglomerate  and  Ran- 
dolph Avenue  are  so  nearly  in  line  with  the  northern  edge  of 
the  conglomerate  as  to  indicate  that  we  have  exposed  here 
the  very  base  of  the  conglomerate  series.  The  conglomerate 
is  clearly  exposed   again   on   this   line   in    High   Street  at  its 


472 

junction  with  Randolph  Avenue,  and  in  the  angle  between  the 
two  streets. 

Eight  in  the  ano;le  between  Hio;h  Street  and  the  avenue, 
and  resting  on  the  conglomerate,  is  a  bowlder,  about  five  feet 
in  diameter,  of  what  appears  at  first  sight  to  be  the  quartz 
porphyry  conglomerate,  distinguished  only  by  being  composed 
of  greenish  gray  masses  six  inches  to  a  foot  in  diameter,  in  a 
brownish  red  paste.  But  a  closer  examination  shows  that 
pebbles  and  paste  are  alike,  diifering  only  in  color ;  and  a 
comparison  with  the  true  conglomerate,  as  exposed  near  by  in 
High  Street,  makes  it  clear  that  this  bowlder  is  simply  a  mass 
of  quartz  porphyry  mottled  by  diffei-ential  oxidation,  through 
the  agency  of  meteoric  waters  following  the  joint  cracks  of  the 
rock.  On  re-examination  of  the  ledges  of  quartz  porphyry 
immediately  north  of  the  conglomerate,  they  are  found  to 
exhibit  the  same  red  and  green  mottling  ;  but  farther  north 
this  character  is  wholly  wanting;  and  it  may,  therefore,  be 
regarded,  provisionally,  as  a  contact  phase  of  the  quartz 
porphyry. 

Between  the  high  conglomerate  ledge  and  Monatiquot  Stream 
are  many  bowlders,  and  weathered-out  bowlder-like  pebbles,  of 
the  cono'lomerate.  South  of  the  stream  the  cono-lomerate  is 
well  developed  in  ledges  and  bowlders  for  a  breadth  of  about 
one  fourth  of  a  mile.  It  becomes  rapidly  finer  southward,  the 
main  body  being  of  rather  normal  character,  with  considerable 
beds  of  red  shale  and  sandstone  interstratified.  The  red  sand- 
stone series  begins  near  the  point,  west  of  High  Street,  where 
an  attempt  was  once  made  to  quarry  it,  and  with  many  alter- 
nations and  partings  of  red  shale,  and  bands  of  fine  conglomer- 
ate, extends  southward  along  the  line  of  Randolph  Avenue 
nearly  if  not  C[uite  to  West  Street.  Both  the  conglomerate 
and  the  sandstone  are  occasionally,  and  very  locally,  some- 
what calcareous,  as  indicated  by  a  cavernous  mode  of 
weathering. 

Following  the  basal  conglomerate  eastward  from  Randolph 


Avenue,  we  find  only  till  with  bowlders  of  quartz;  porphyry  for 
a  thousand  feet.  Beyond  this,  for  a  thousand  feet  more,  is  a 
steep  slope,  with  ledges  of  quartz  porphyry  above,  and  bowl- 
ders of  the  same  extending  down  to  the  swamp  and  river.  But 
in  all  of  this  two  thousand  feet  I  could  discover  no  trace  of  the 
conglomerate  or  the  mottled  porphyry.  Beyond  this  still  is  an 
irregularly  rounded  hill,  some  five  hundred  feet  in  diameter  and 
from  25  to  30  feet  high,  the  southern  margin  of  which  is  marked 
Streamside  Ledge  on  the  topographic  map  (PL  14)  ;  and  about 
seven  hundred  feet  due  east  of  this  is  a  very  similar  but  smaller 
hill.  These  little  hills,  rising  like  islands  in  the  swamp  between 
the  river  and  the  straight  southern  margin  of  the  Blue  Hills, 
are  united  by  a  very  perfect  little  sand  plain  and  separated  from 
the  high  land  to  the  north  by  a  depression  v^^hich  is  occupied  by 
a  chain  of  little  kettle  ponds,  as  shown  on  the  map  (PI.  14). 
These  elevations  are  entirely  composed  of  the  giant  conglomer- 
ate, just  like  that  west  of  Randolph  Avenue  ;  but  the  exposures 
are  far  more  extensive  and  satisfactory.  The  conglomerate  is 
chiefly  made  up  of  well-rounded  bowlders  of  quartz  porphyry 
and  fine  granite  porphyry,  from  six  inches  to  three  and  four 
feet  in  diameter.  No  normal  granite  or  fluidal  felsite  was  ob- 
served. There  is,  however,  a  large  amount  of  Cambrian  slate 
of  all  the  different  varieties  recog'nized  in  the  reoion  of  the  Blue 
Hills  Complex  —  black,  gray,  greenish,  reddish,  etc.,  and  some 
fine  brown  sandstone  or  quartzite  of  probably  Cambrian  age  ; 
but  no  banded  slate,  such  as  might  represent  the  newer  or  Car- 
boniferous slates  of  the  Boston  Basin.  Pebbles  of  the  green 
quartz  porphyry  are  not  uncommon  in  the  conglomerate  ;  but  no 
masses  of  the  mottled  i^ed  and  green  rock  were  observed,  al- 
though it  is  probable  that  they  occur ;  and  I  could  find  no  peb- 
bles of  melaphyre  or  trap.  Neither  could  I  discover  any  clue  to 
the  stratification  of  the  conglomerate,  or  any  variation  or  gra- 
dation in  the  character  of  the  conglomerate  from  north  to  south  ; 
but  it  is  just  as  coarse  and  free  from  visible  stratification  on  the 
south  side  of  these  little  hills  as  on  the  north  side,  suggesting 


474 

that  possibly  the  bedding  may  be  nearly  horizontal.  The  Cam- 
brian slate  debris,  as  we  should  naturally  expect,  is  mostly 
rather  fine,  frag-ments  more  than  six  inches  lono;  beino-  uncom- 
mon  ;  but  its  abundance  goes  far  to  prove  that  the  Blue  Hills 
were  formerly  covered  with  slate. 

Resting  on  the  east  side  of  Streamside  Ledge  are  several 
bowlders  of  the  mottled  quartz  porphyry  ;  and  north  of  this 
hill  and  the  row  of  kettle  ponds  is  a  fine  group  of  ledges  of  this 
rock.  It  begins  where  the  wood  road  running  north  (PL  14) 
passes  through  the  east-west  stone  wall,  about  two  hundred 
feet  north  of  the  most  northerly  exposure  of  the  conglomerate, 
and  extends  north  fully  two  hundred  and  possibly  nearly  three 
hundred  feet.  It  is  identical  in  character  with  the  bowlder 
and  ledges  near  Randolph  Avenue,  and  certainly  presents  a 
most  striking;  o-eneral  resemblance  to  a  cono-lomerate.  But 
that  it  must  be  a  greenish  quartz  porphyry  deeply  oxidized 
along  the  joints,  and  not  a  true  conglomerate,  is  proved  by  :  — 

(1)  The  remarkable  uniformity  of  material  —  the  apparent 
pebbles  being  all  of  the  normal  quartz  porphyry,  with  no 
granite  or  slate,  or  anything  foreign  to  the  porphyry,  and 
identical,  except  in  color,  with  the  apparent  paste  or  matrix. 
The  only  variation  noted  consists  of  two  angular  inclusions  of 
red  jasper   or    fine    quartzite,  from    one   to  two  inches  long. 

(2)  The  great  uniformity  in  size  of  the  pseudo-pebbles.  They 
correspond  closely  in  size  and  arrangement  with  the  joint 
blocks  of  the  normal  porphyry,  with  no  small  masses  two 
inches  in  diameter  and  less  filling  the  interstices.  (3)  The 
green  masses  or  pseudo-pebbles  rarely  touch  each  other  as  do 
the  bowlders  in  the  true  conglomerate. 

The  mottled  porphyry  is  probably  a  surface  and  contact 
phase,  representing,  we  may  suppose,  a  surface  altera- 
tion of  the  porphyry  dating  from  the  Carboniferous  age. 
The  quartz  porphyry  was  probably  all  greenish  originally. 
The  bowlders  in  the  conglomerate  are  normally  oxidized,  at 
least  externally,  as  is  also  the  quartz  porphyry  in  ledges  all 


475 

over  the  Blue  Hills  to  a  greater  depth  than  any  artificial  exca- 
vations, except  where  it  has  recently  (geologically)  been 
uncovered  by  the  erosion  of  the  conglomerate. 

From  these  ledges  eastward  are  only  ledges  and  bowlders  of 
normal  quartz  porphyry  for  fifteen  hundred  feet,  to  where  the 
road  cuts  the  wall  six  hundred  feet  northeast  of  the  eastern 
conglomerate  hill.  Here  is  a  high  ledge  of  normal  quartz 
porphyry  and  immediately  south  of  it,  on  the  road,  two  masses 
(possibly  bowlders)  of  the  mottled  rock,  with  a  little  fine  red 
slaty  material  between  the  greenish  nuclei,  making  the  whole 
seem  very  much  like  a  true  conglomerate  with  a  slaty  paste.  I 
concluded,  however,  that  it  marks  the  exact  contact  of  the 
conglomerate  and  quartz  porphyry,  with  the  paste  of  the 
former  penetrating  the  latter  a  little  way  where  erosion  had 
cut  out  the  oxidized  and  more  or  less  decomposed  material,  as 
it  seems  disposed  to  do.  On  the  same  surface  is  a  bowlder  18 
inches  in  diameter  of  the  normal  quartz  porphyry  half  imbedded 
among  the  green  nuclei.  From  what  precedes  we  may  con- 
clude that  befox'e  the  formation  of  the  conglomerate  the  quartz 
porphyry  was  deeply  oxidized,  —  wholly  oxidized  at  the  sur- 
face, and  only  along  joint  cracks  deeper  down.  The  super- 
ficial oxidized  portion  was  worn  away  to  provide  material  for 
the  conglomerate,  down  to  the  mottled  zone,  which  was  cov- 
ered by  the  conglomerate,  and  protected  from  further 
oxidation. 

Farther  east,  following  the  northern  edge  of  the  swamp 
(see  PI.  14),  are  many  bowlders  of  quartz  porphyry,  but  no 
ledges.  About  fifteen  hundred  feet  due  east  of  the  second 
little  hill  of  coarse  conglomerate,  however,  is  a  third  exposure 
of  this  rock,  in  the  form  of  a  ridge  about  five  hundred  feet 
long  east-west,  and  connected  with  the  northern  edge  of  the 
swamp  by  a  low,  wooded  isthmus  of  drift.  This  ledge  is  simi- 
lar to  the  preceding,  and  contains  many  rounded  bowlders  of 
quartz  porphyry  one  to  two  feet  in  diameter.  A  layer  of  red 
shale  shows  a  southerly  dip  of  about  60°  ;  and  this  is  confirmed 


476 

by  a  band  of  normal  conglomerate  south  of  the  main  mass  and 
almost  isolated  by  the  swamp.  Continuing  in  the  same  east- 
west  line  obliquely  across  the  swamp  and  Monatiquot  Stream, 
we  find  the  next  and  last  exposure  of  the  giant  conglomerate 
on  the  southeast  side  of  West  Street,  and  some  two  hundred 
feet  north  of  Great  Pond.  The  dip  cannot  be  made  out  here. 
North  of  the  ledge  are  rounded  bowlders  of  quartz  porphyry, 
probably  representing,  in  part  at  least,  a  disintegrated  mass  of 
the  conglomerate.  So  far  as  I  can  discover,  this  is  the  most 
easterly  outcrop  of  the  Norfolk  Basin  ;  and  it  is  of  interest  in 
this  connection  to  note  that  Mr.  Fuller's  study  *  of  the  distribu- 
tion of  the  Carboniferous  rocks  in  the  drift  points  very  plainly 
to  the  same  general  conclusion,  viz.,  that  the  Carboniferous 
strata  of  the  Norfolk  Basin  terminate  definitely  and  with  appar- 
ent abruptness  in  the  vicinity  of  Braintree  Great  Pond.  The 
quartz  porphyry  floor  of  the  Norfolk  Basin  also  comes  to  an 
end  in  this  longitude,  or,  if  it  extends  east  of  Cedar  Swamp,  is 
covered,  as  previously  suggested,  by  the  compact  and  fluidal 
(eflfusive)  forms  of  aporhyolite.  The  extensive  and  continu- 
ous swampy  tract  in  the  direct  course  of  the  Norfolk  Basin 
east  of  Great  Pond  would  ordinarily  be  regarded  as  a  suflficient 
explanation  of  the  absence  of  outcrops,  if  not  of  erratics  ;  and 
it  is  a  satisfaction,  therefore,  to  find  that  the  testimony  of  the 
quartz  porphyry  and  the  drift  is,  to  some  extent,  supplemented 
and  confirmed  by  the  stratigraphy  of  the  Carboniferous  beds. 

On  the  northwest  side  of  Great  Pond,  in  the  angle  between 
the  pond  and  Monatiquot  Stream,  we  have,  first,  large  bowl- 
ders of  the  coarse  conoiomerate,  followed  southward  bv 
bowlders  and  apparent  ledges  of  greenish  and  red,  normal  and 
fine  cong^lomerate  and  o-rit.  After  about  two  hundred  feet  of 
these  coarse  sediments,  we  come  to  bowlders  and  some  good 
ledges  of  the  red  sandstone  series,  which  is  quite  satisfactorily 
exposed  for  a  breadth  of  eight  hundred  feet,  with  many  alter- 
nations of  sandstone  and  bright  red  shale,  before  finally  disap- 

1  Proc.  Boston  soc.  nat.  hist.,  vol.  28,  pp.  251-264. 


477 

pearing  beneath  the  drift.  A  strongly  marked  cleavage  dips 
N.  80°— 85°,  but  the  true  dip  is  prevailingly  south  about  sixty 
degrees.  At  several  points  (probable  synclinal  axes),  how- 
ever, the  southerly  dip  completely  subsides,  and  we  note  instead 
a  westerly  dip  of  five  to  ten  degrees.  This  westward  pitch  is, 
apparently,  best  explained  as  due  to  an  endwise  tilting  of  the 
formation ;  and  for  that  we  may  find  a  simple  and  sufficient 
cause  in  the  drag  of  an  important  transverse  fault  with  the 
down-throw  to  the  west.  This  interpretation  of  the  structure 
satisfies  and  reconciles  all  the  latest  essential  facts,  including 
the  abrupt  termination  of  the  Carboniferous  strata  ;  and  it  has, 
accordingly,  been  observed  in  the  construction  of  the  map. 
According  to  this  view,  the  original  eastern  extension  and 
gradual  termination  of  the  Norfolk  Basin  was  lifted  above  the 
present  plane  of  erosion. 

In  a  general  view  of  this  part  of  the  Norfolk  Basin,  no  facts 
stand  out  more  clearly  than  that  the  giant  conglomerate  is  the 
true  basal  member  of  the  Carboniferous  series,  that  it  was 
deposited  over  the  partially  denuded  surface  of  the  Blue  Hills 
Complex,  that  this  original  floor  of  the  Carboniferous  sea  is 
preserved  in  the  contact  of  the  giant  conglomerate  and  the 
quartz  porphyry  against  which  it  rests,  and  that  above  this 
floor  the  general  structure  of  the  Norfolk  Basin  is  monoclinal 
through  a  great  thickness  of  overlying  strata,  including  both 
the  red  and  gray  sandstone  series  ;  and,  finally,  the  conclusion 
cannot  be  avoided  that  these  thousands  of  feet  of  coarse  sedi- 
ments once  arched  over  the  Blue  Hills,  areas  from  which  they 
have  recently  been  denuded  being  indicated  to  some  extent  by 
the  diflerential  oxidation  or  mottling  of  the  quartz  porphyry,  a 
surface  alteration  which  must,  apparently,  date  from  Carbon- 
iferous times.  One  instance  of  this  mottling  not  previously 
noted  occurs  on  the  west  side  of  Green  Street,  southwest  of 
Little  Blue  Hill  and  nearly  half  a  mile  north  of  the  ledges  of 
coarse  conglomerate  on  Back  Street  in  Canton,  If  this  occur- 
rence is  rightly  interpreted,  it  goes  far  to  prove  that  above  the 


478 

present  plane  of  erosion  the  Carboniferous  strata  arched  rather 
srentlv  over  the  now  denuded  axis  of  the  Blue  Hills.  At  the 
southern  margin  of  the  hills  they  plunge  steeply  down  to  a 
nearly  vertical  monocline,  recalling  most  strikingly  the  struc- 
ture of  the  Front  Range  of  Colorado,  in  the  vicinity  of  Manitou 
and  elsewhere  ;  and  it  is  clear  in  each  case  that  to  this  strati- 
graphic  break  is  due  the  strongly  accentuated  outlines  of  the 
relief. 


THE    CARBONIFEROUS   STRATA   ALONG   THE   NORTHERN 
BORDER   OF   THE    BLUE    HILLS   COMPLEX. 

The  heavy  drift  deposits  of  North  Quincy  and  Milton  form 
a  natural  boundary  between  the  Blue  Hills  Complex  and  the 
lower  Neponset  Valley,  and  offer  little  encouragement  for  the 
investigation  of  the  geological  conditions  along  one  of  the  most 
critical  lines  of  the  region  and  a  comparison  of  the  northern 
and  southern  margins  of  the  Complex.  In  fact  the  meager 
data  are  not  easily  correlated  ;  and  general  conclusions  are,  of 
necessity,  unduly  speculative.  The  outcrops  occur  in  three 
distinct  and  widely  separated  groups  or  areas  which  appear  to 
have  little  in  common,  and  may  conveniently  be  described 
separately. 

The  Milton  A.rea.  —  It  is  intended  to  include  here  all  the 
outcrops  adjacent  to  the  noi'thern  margin  of  the  Complex,  in 
Milton.  They  are  widely  scattered  along  a  line  three  miles  in 
length,  which  is  crossed  midway  by  Randolph  Avenue  ;  and 
the  chief  exposures  are  on  or  near  this  thoroughfare. 

Going  north  on  Randolph  Avenue  from  the  corner  of  Reeds- 
dale  Road  about  875  feet  across  a  rather  level  till-covered 
area  without  outcrops,  we  come  to  the  great  dike  of  diabase  pre- 
viously noted  (page  438)  as  cutting  the  Cambrian  slate  and 
ffr-anite  on  Pleasant  Street  east  of  Gun  Hill  Road.      On  Ran- 


479 

dolph  Avenue  the  exposed  bresidth  of  the  dike  is  125  feet ;  and 
it  here  probably  marks  the  boundary  between  the  complex  of 
granite  and  Cambrian  strata  on  the  south  and  the  newer  and 
presumably  Carboniferous  strata  on  the  north.  About  75  feet 
north  of  the  dike,  across  a  well-marked,  canal- like  depression, 
we  come  to  a  rock  cutting  which  affords  an  almost  continuous 
section  for  nearly  five  hundred  feet  northward  down  the  slope 
toward  Unkety  Brook,  terminating  at  a  point  about  750  feet 
south  of  the  intersection  of  Randolph  Avenue  and  Gun  Hill 
Road.  The  entire  leno-th  of  this  section  is  throuo;h  a  coarse, 
gritty,  white  sandstone  or  quartzite  with  frequent  thin  layers  or 
partings  of  purplish  brown  slate.  Viewed  more  closely,  the 
sandstone  is  seen  to  be,  in  the  main  at  least,  of  distinctly 
arkose  character.  The  feldspathic  element  is  now,  however, 
generally  kaolinized  ;  but  in  spite  of  this  some  of  the  rock  is 
quite  granitic  in  its  aspect,  and  there  can  be  no  doubt  that  the 
granitic  rocks  of  the  Blue  Hills  were  exposed  to  erosion  during 
the  accumulation  of  this  sediment.  The  slaty  partings  show 
the  dip  to  be  constantly  to  the  north,  but  varying  in  amount 
from  forty  to  seventy  degrees.  Irregular  veinlets  of  quartz  are 
a  very  characteristic  feature  of  the  arkose. 

The  great  dike  probably  coincides,  in  the  vicinity  of  Ran- 
dolph Avenue,  with  the  northern  boundary  fault  of  the  Blue 
Hills  Complex ;  and  the  northward  dip  of  the  arkose  beds 
is,  perhaps,  due  to  the  drag  of  the  fault.  The  depres- 
sion separating  the  dike  and  the  arkose  at  the  surface  is 
a  quite  remarkable  topographic  feature.  It  is  steep-walled, 
15  to  25  feet  deep,  50  feet  wide  at  bottom,  and  about  100 
feet  at  the  top  in  the  narrowest  places.  Eastward  it  deepens 
somewhat,  and  is  quite  straight,  following  the  strike  of  the 
formations.  It  holds  its  form  well  as  far  as  Gun  Hill  Road, 
and  there  widens  out  and  loses  its  distinctive  character.  The 
arkose  is  obscurely  exposed  on  Gun  Hill  Road,  north  of  the 
entrance  to  the  cemetery  ;  and  the  dike,  bearing  to  the  east- 
southeast,  enters  the   granite,  passing   about  five  hundred  feet 


480 

north  of  the  schoolhouse  at  the  corner  of  Gun  Hill  Road  and 
Pleasant  Street.  The  granite  extends  fully  seven  hundred 
feet  north  of  this  corner,  or  nearly  to  the  bottom  of  the  slope. 
The  boundary  fault  appears  thus,  regardless  of  the  deflection 
of  the  dike,  to  hold  to  its  normal  east-west  course.  One 
fourth  of  a  mile  east  of  Gun  Hill  Road,  in  the  southeastern 
corner  of  the  cemetery,  the  arkose  is  well  exposed  again, 
forming  the  steep  slope  on  the  north  side  of  the  brook.  It  is 
practically  in  situ  for  several  hundred  feet  farther,  and  is 
readily  traced  by  bowlders  to  the  point  where  the  brook  turns 
abruptly  to  the  north. 

Westward  from  Randolph  Avenue  the  depression  between 
the  dike  and  arkose  curves  to  the  southwest  slightly  and  dies 
out  before  reaching  Reedsdale  Road.  The  dike  continues, 
and  is  exposed  for  a  breadth  of  fifty  feet  on  Reedsdale  Road, 
closely  bordered  by  granite  in  obscure  outcrops  and  bowl- 
ders on  the  south,  while  from  two  hundred  to  four  hundred 
feet  south  of  the  trap  the  granite  outcrops  more  freely.  About 
two  hundred  feet  north  of  the  trap,  on  the  west  side  of  the 
road,  the  arkose  is  obscurely  exposed,  with,  apparently,  a 
high  dip  to  the  north  ;  and  before  the  recent  grading  of  the 
bank  the  total  breadth  of  the  outcrop  was  nearly  three  hundred 
feet. 

■  Farther  west,  across  Highland  Street,  the  mantle  of 
drift  is  unbroken  for  one  and  a  half  miles.  The  country  north 
and  south  of  Canton  Avenue,  between  Milton  Centre  and 
Pine  Tree  Brook,  is  strewn  with  numerous  bowlders  of  con- 
glomerate, some  of  which  are  of  considerable  size ;  and  it 
appears  in  the  highest  degree  probable  that  a  belt  of  conglomer- 
ate is  concealed  here,  not  far  to  the  north  of  the  arkose  series. 
The  arkose  itself  appears  again  on  Robbins  Street,  450  feet 
north  of  the  fine  red  granite  on  Canton  Avenue.  The  arkose 
is  exposed  for  a  breadth  of  about  seventy  feet,  but  without  the 
slaty  partings.  The  dip  is  N.  W.  70°.  Between  two  hundred 
and  three  hundred  feet  west  of  Robbins   Street,  on  the  north 


481 

side  of  Canton  Avenue,  the  arkose  appears  to  overlie  the  fine 
granite  under  the  sidewalk ;  which  would  make  the  total 
breadth  of  the  arkose  about  the  same  as  on  Randolph  Avenue. 
Farther  west  on  this  line  indubitable  outcrops  are  wholly 
wanting.  But  on  Milton  Street,  about  five  hundred  feet  north 
of  Blue  Hill  Avenue,  and  one  and  a  fourth  miles  west  of 
Robbing  Street,  the  arkose  occurs  in  the  drift  to  such  an 
extent  as  to  be  practically  equivalent  to  an  outcrop. 

Eastward  from  Randolph  Avenue  and  Gun  Hill  Road  and 
the  valley  of  Unkety  Brook,  the  arkose  series  is  next  exposed 
between  Pleasant  Street  and  Edge  Hill  Road,  immediately 
north  of  the  line  of  the  Cambrian  slate,  as  described  on  page 
436,  where  it  is  also  stated  that  north  of  the  arkose  about  a 
hundred  feet  of  purple  and  green  slate  are  exposed,  dipping 
north  at  a  hio;h  ansle. 

The  rather  meager  and  widely  separated  outcrops  of  the 
arkose  series  are  reinforced  by  the  drift  to  such  an  extent  as 
to  prove  a  continuous  belt  of  arkose  sandstone  extending  from 
Brush  Hill  Road  south  of  Paul's  Bridge  to  the  Granite  Branch 
Railway,  a  distance  of  over  four  miles  ;  and  it  is  highly  prob- 
able that  the  arkose  belt  is  paralleled  on  the  north  by  a  belt  of 
conglomerate,  with  an  intervening  belt  of  purple  and  green 
slate.  The  undoubted  persistence  of  the  unique  and  easily 
recognized  arkose  leaves  little  room  to  doubt  that  the  other 
rocks  are    equally  constant. 

The  Furnace  Brooh  Area.  —  Between  the  Granite  Branch 
Railway  and  North  Common  Hill  in  Quincy,  outcrops  are 
wholly  wanting  north  of  the  granite  :  and  the  modified  character 
of  the  drift  —  a  level  sand  plain  —  makes  its  indications  of  the 
underlying  rocks  of  little  value.  But  in  the  valley  of  Furnace 
Brook,  where  it  runs  north  of  and  parallel  with  Adams  Street, 
north  of  North  Common  and  President's  Hills,  the  Cambrian 
outcrops,  as  described  on  pages  426—428  are  matched  by 
equally  satisfactory  exposures  of  the  conglomerate  and  newer 
slate    series.      The    post-Cambrian   or  Carboniferous    outcrops 

OCCAS.  PAPEllS   B.  S.  N.  H.  IV.         31. 


482 


are  confined  to  the  immediate  valley  of  the  brook,  north  of 
Adams  Street,  and  between  the  point  where  Adams  Street 
crosses  the  brook  and  Hancock  Street,  the  extreme  length  of 
this  group  of  ledges  being  about  three  fourths  of  a  mile. 

The  outcrops  west  of  the  main  line  of  the  Old  Colony  Rail- 
road have  been  briefly  described,  as  noted  above  ;  and  a  little 
repetition  is  essential  to  the  completeness  of  this  section.  Going 
north  from  Adams  Street  between  Common  and  Whitwell 
Sti'eets,  we  have  the  fine  granite,  forming  the  northern  margin 
of  the  granite  and  Cambrian  complex,  outcropping  obscurely 
for  three  hundred  to  four  hundred  feet,  followed  by  two  hun- 
dred to  over  four  hundred  feet  of  conglomerate,  widenino; 
rapidly  eastward.  The  conglomerate  is  chiefly  composed  of 
felsite  and  of  fine  granite  like  that  of  the  adjacent  ledges,  and 
the  granite  pebbles  are  in  part  distinctly  angular.  The  essen- 
tially local  and  arkose  character  of  the  conglomerate  is  thus 
plainly  indicated.  The  true  arkose  of  Milton  is  wanting  in 
this  section  ;  but  in  the  absence  of  any  positive  evidence  that 
the  arkose  changes  to  conglomerate  eastward,  it  appeal's  best 
not  to  correlate  these  two  rocks,  but  rather  to  consider  that 
the  throw  of  the  boundary  fault  increases  eastward  until  it 
becomes  sufficient  to  conceal  both  the  arkose  series  and  the 
band  of  purple  and  green  slate  separating  it  from  the  con- 
glomerate, and  thus  to  bring  the  conglomerate  itself  down 
against  the  border  of  the  complex.  The  conglomerate  is  here 
unusually  hard  and  flinty,  the  paste  appearing  to  have  been 
completely  baked,  or,  perhaps,  silicified,  as  if  by  prolonged 
hydrothermal  action.  Mr.  White  '  has  specially  noted  the 
metamorphic  aspect  of  the  conglomerate,  stating  that  while  the 
prevailing  rounded  forms  of  the  pebbles  prove  that  it  is  a  true 
sedimentary  rock,  the  groundmass  or  paste  cementing  the 
pebbles  appears  in  thin  sections  exceedingly  like  that  of  a 
volcanic  tufl^,  consisting  largely  of  rather  large  lath-shaped 
plagioclases  and  short   stubby  hornblende  crystals  witli   scat- 

1  I'roc.  Boston  soc.  iiat.  hist.,  vol.  '2S,  p.  Vli. 


483 


tered  grains  of  magnetite  and  quartz.  As  previously  stated, 
it  is  practically  impossible  to  locate  satisffictorily  the  contact 
between  this  altered  conglomerate  and  the  granite,  partly 
because  of  insufficient  outcrops,  and  partly  because  of  the 
arkose  and  crystalline  character  of  the  conglomerate ;  but 
mainly  because  the  granite  itself  has,  near  the  contact,  a  more 
or  less  marked  fragmental  structure,  having  been  locally 
reduced  to  a  crush  breccia.  The  conglomerate  is  bordered  on 
the  north  by  dark  gray  slate  striking  N.  61°— 70°  E.,  with  a, 
vertical  dip,  and  an  extreme  exposed  thickness  of  125  feet. 
These  facts:  the  cutting  out  of  the  arkose  series,  the  inter- 
vening slate  and  a  part  of  the  conglomerate,  the  resulting 
oblique  strike  of  the  slate  north  of  the  conglomerate,  the 
crushing  of  the  granite  and  metamorphism  of  the  conglomerate, 
as  well  as  the  lifting  above  the  present  plane  of  erosion  of  the 
main  part  of  the  Cambrian  strata  north  of  North  Common 
Hill,  not  only  demand  a  fault  along  the  border  of  the  granite- 
Cambrian  complex,  but  a  fault  of  greatly  increased  displace- 
ment in  the  vicinity  of  North  Common  Hill,  accompanied  by 
crushing  of  the  granite,  and  probably  also  of  the  conglomerate, 
as  well  as  by  pronounced  hydrothermal  activit}^ 

Further  east  the  conglomerate  becomes  of  more  normal 
character,  and  attains  a  breadth  of  450  feet.  Northwest  of 
President's  Hill  it  is  crossed  obliquely  by  the  brook,  and  is 
closely  bordered  on  both  sides,  as  previously  described,  by 
nearly  vertical  slates.  The  structure  was  formerly  ^  inter- 
preted as  anticlinal,  the  conglomerate  being  regarded  as  a 
denuded  axis,  and  the  slate  on  the  north  as  a  repetition  of  that 
on  the  south.  The  southern  slate  is  in  part  at  least,  older  than 
the  granite;  and  since  no  appreciable  difference  nor  line  of 
demarcation  can  be  discovered  in  it,  the  only  alternative 
appears  to  be  to  refer  it  all  to  the  older  or  Cambrian  series, 
especially  as  its  lithological  characters  nve  entirely  in  harmony 
with  this  disposition.      Although  the  contact  of  this  slate  and 

1  Occas.  papers,  Boston  soc.  nat.  hist.,  vol.  3,  p.  209. 


484 

the  conglomerate  occupies  a  drift  hollow  and  is  nowhere 
exposed,  it  is,  apparently,  quite  abrupt,  and  may  therefore  be 
regarded  as  marking  the  position  of  the  boundary  fault.  The 
conglomerate  is  still  a  firm,  hai'd  rock,  as  indicated  by  its  bold 
exposures,  and  grades  northward  through  fifty  feet  or  so  of  alter- 
nating fine  conglomerate,  sandstone,  and  slate  into  the  main 
body  of  dark  gray  slate,  which  is  very  distinctly  and  finely 
banded,  and,  on  the  whole,  strongly  contrasted  with  the  Cam- 
brian slate  south  of  the  conglomerate.  This  slate,  which  is 
exposed  for  a  breadth  of  about  three  hundred  feet,  and  dips 
N.  85°— 90°,  unquestionably  belongs  to  the  same  series  as  the 
conglomerate. 

About  one  fourth  of  a  mile  east  of  the  section  just  described, 
the  intervening  ground  being  a  complete  blank,  is  the  well- 
known  section  on  Newjiort  Avenue,  the  Old  Colony  Railroad 
and  Hancock  Street.  The  Cambrian  slate,  south  of  the  con- 
glomerate, is  wholly  wanting  here,  as  well  as  the  southern 
part  of  the  conglomerate  itself.  The  exposure  of  the  con- 
glomerate beo-ins  about  150  feet  north  of  the  brook,  and 
extends  north  for  160  feet.  It  is  of  irregular  composition, 
mostly  rather  fine,  and  largely  of  a  slaty  character,  a  super- 
abundant arenaceous  and  slaty  paste  inclosing  small  and  often 
scattering  pebbles  of  granite  (fine  and  normal),  felsite,  mela- 
phyre  and,  probably,  Cambrian  slate  ;  and  it  finally  passes 
through  some  40  feet  of  mixed  conglomerate,  sandstone  and 
slate  into  the  true  slate,  identical  with  that  west  of  the  rail- 
road —  dark  gray  and  finely  laminated  or  banded.  It  is  exposed 
for  a  breadth  of  150  feet,  —  distinctly  for  75  feet  and  then  very 
obscui'ely,  on  the  railroad,  for  75  feet  more.  These  beds  are 
in  line  with  those  of  the  preceding  section,  but  all  dip  S.  80°- 
90°,  averaging  about  87°,  the  series  being  thus,  apparently, 
slightly  overturned.  In  part  the  conglomerate  resembles  that 
of  the  North  Common  Hill  section,  but  it  lacks  the  flinty  hard- 
ness —  the  silicified  and  metamorphic  character —  of  that  rock. 
As  before,  the  conglomerate  passes  gradually  northward  into 


485 

the  sandstone;  and  the  sandstone,  in  turn,  seems  to  blend 
perfectly  with  the  slate,  so  that  it  is  impossible  to  draw  a  line 
of  demarcation  between  them.  The  sandstone  is  thus  a  mere 
bed  of  passage  between  the  conglomerate  and  slate  ;  and  in 
the  fact  that  the  slate  north  of  the  conglomerate  is  thus 
unquestionably  a  part  and  almost  certainly  a  newer  part  of  the 
same  series  with  the  conglomerate,  we  have  its  strongest  and 
most  vital  or  significant  contrast  with  the  Cambrian  slate  south 
of  the  conglomerate  :  for  the  conglomerate  is  clearly  newer  than 
the  granite,  and  the  granite,  in  turn,  undoubtedly  belongs  to 
a  much  later  period  than  the  Cambrian  slates  which  it  so  freely 
intersects.  That  we  have  in  these  Furnace  Brook  sections  the 
beginning  only  of  a  broad  area  of  the  newer  slates,  which  has 
been  previously  assumed  to  underlie  the  drift  plain  of  North 
Quincy,  is  rendered  much  more  certain  by  the  facts  that  the 
deep  sewers  recently  constructed  in  this  part  of  Quincy 
encountered  slate  of  the  same  character  at  a  depth  of  25  feet 
under  Hancock  Street,  about  eight  hundred  feet  north  of 
the  surface  exposure  on  this  street,  and  a  thousand  feet  north 
of  the  brook,  and  again  nearly  a  thousand  feet  northeast 
of  this  point,  in  the  northern  part  of  Merrymount  Park,  at  a 
depth  of  about  ten  feet  below  high  tide.  On  the  railroad  the 
conglomerate  is  cut  near  its  northern  edge  by  a  trap  dike  two 
to  three  feet  wide,  and  what  appears  to  be  the  same  dike  cuts 
the  slate  on  Hancock  Street  near  the  conglomerate  with  a 
breadth  of  at  least  four  feet. 

The  Hough's  JSFeck  Area.  —  Eastward  from  Hancock 
Street  in  the  latitude  of  Furnace  Brook,  the  mantle  of  drift  is 
unbroken  for  about  one  and  a  half  miles.  At  this  distance, 
on  Sea  Street,  the  road  leading  to  Germantown  and  Houghs 
Neck,  at  the  point  just  beyond  Mt.  Wallaston  where  the  sand 
plain  is  interrupted  by  a  narrow  strait  of  salt  marsh,  is  a  low 
and  solitary  outcrop  of  slate,  the  only  rock  exposure  of  any 
kind,  in  an  area  of  more  than  four  square  miles.  It  is  a  thin, 
gray  slate,  with  '  perfect   lamination  cleavage,   and    occasional 


486 


coarse  gritty  bands  strongly  suggestive  of  the  arkose  of  Milton. 
The  strikers  N.  80°  W.,  and  the  dip  N.  80°- 85°.  The 
north-south  breadth  of  the  outcrop  is  about  275  feet,  mostly 
on  the  south  side  of  the  road.  On  the  southern  edge  of  the 
outcrop  a  small,  irregular  dike  cuts  the  slate  approximately 
parallel  with  the  strike  ;  and  there  are  indications  of  a  similar 
dike  on  the  northern  edge. 

Nearly  a  mile  farther  east,  on  the  south  side  of  Hough's 
Neck,  is  the  rocky  ridge,  half  a  mile  long,  encircled  by  the 
salt  marsh,  and  known  as  Rock  Island.  Directly  east  of  and 
in  line  with  Rock  Island,  from  which  it  is  separated  only  by 
a  narrow  strip  of  the  salt  marsh  and  a  small  creek,  is  a 
second  and  slightly  shorter  and  narrower  ridge,  also  isolated  by 
the  marsh,  which  terminates  eastward  in  Rock  Island  Head. 
And  about  eight  Imndred  feet  northeast  of  this  promontory  is 
Raccoon  Island,  a  rocky  islet,  which  is  joined  to  Rock  Island 
Head  at  extreme  low  tide.  These  rocky  reliefs  are  clearly, 
only  the  higher  and  unsubmerged  portions  of  a  continuous 
rocky  ridge  separating  two  buried  east-west  valleys ;  and 
Rock  Island  and  Rock  Island  Head,  at  least,  evidently  owe 
their  prominence  to  the  exceptionally  hard  and  resistant  char- 
acter of  the  rocks.  Rock  Island  and  Raccoon  Island  afford 
very  satisfactory  and  virtually  continuous  exposures  of  the 
rocks,  and  the  same  is  true  of  the  narrow  western  end  of  the 
Rock  Island  Head  ridge  for  about  one  third  of  its  length, 
the  eastern  two  thirds  being  a  smoothly  rounded  drift  hill  of 
drumloid  form. 

As  the  outcrop  map  (PI.  27)  clearly  shows,  the  dominant 
and  determining  oeological  feature  of  Rock  Island  is  the  heavv 
bed  of  melaphyre,  which  forms  the  axis  and  main  mass  of  the 
ridge  ;  and  melaphyre  is  the  only  rock  exposed  in  the  Rock 
Island  Head  ridge.  The  melaphyre  forms  in  either  ridge  a 
continuous  east- west  belt,  which  varies  in  exposed  breadth 
from  two  hundred  to  four  hundred  feet  in  the  western  ridge, 
and   from    two   hundred  to  two  hundred   and  fiftv  feet  in   the 


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•487 

eastern  ridge.  The  inelaphyres  of  the  two  ridges  are  practi- 
cally indistinguishable,  by  either  macroscopic  or  microscopic 
characters  ;  and  whether  referred  in  origin  to  one  continuous 
mass  or  to  separate  masses,  they  must  be  regarded  as  litho- 
logically  identical. 

This  rock  was  first  described  lithologically  by  Ur.  J.  E. 
Wolff  ;^  and  his  description  is  substantially  confirmed  by  Mr. 
White.      Dr.  Wolff  says  :  — 

"  In  the  hand  specimens  the  groundmass  varies  in  color 
from  green  through  gray  to  red,  — the  last  color  characteristic 
of  the  rock  that  is  least  decomposed.  It  sometimes  encloses 
large  green  or  white  feldspar  crystals,  often  indented  by  the 
groundmass,  or  the  feldspar  crystals  may  be  comparatively 
minute  ;  grains  and  crystals  of  epidote  are  occasionally  seen. 
The  rock  generally  contains  greenish  spots  of  epidote  and  of 
chloritic  material,  in  part  true  amygdules,  and  spots  of  reddish 
decomposition.  There  are  also  amygdules  of  calcite  and 
quartz. 

"As  seen  under  the  microscope,  the  rock  is  composed  of 
large  and  small  feldspar  crystals,  magnetite,  epidote,  calcite, 
and  a  mass  of  chlorite,  viridite,  and  opacite.  The  large  por- 
phyritic  crystals  are  twinned  plagioclase,  and  occasionally 
Carlsbad  twins  of  sandin.  The  minute  feldspars  of  the  ground- 
mass  flow  around  them,   encroach   upon  them,  and  sometimes 

break  them The  degree  of  decomposition  that  the  feldspars 

have  unders^one  varies  in  the  different  sections  :  in  the  freshest 
rock  they  contain  immense  quantities  of  minute  fluid  inclusions, 
characterized  by  moving  bubbles,  and  occasional  larger  ones, 
rounded  or  irregular  in  shape,  together  with  inclusions  of  the 
base.  The  latter  are  cylindrical,  or  irregularly  reticulated  in 
form,  often  zonally  arranged  in  the  interior  or  exterior  parts  of 
the  crystal ;  they  are  absolutely  identical  in  shape,  and  in  their 
relations  to  the  enclosing  crystal,  with  the  inclusions  of  glass  or 
base  of  the  fresh  basalts  ;    they  are  now  altered  to  magnetite, 

1  Bull.  mus.  comp.  zool.  vol.  7,  pp.  231-242. 


488 

viridite,    and    other    products The    only    other    original 

mineral,  unless  it  be  part  of  the  magnetite  and  apatite,  is 
olivine.  This  was  found  in  well-marked,  large,  and  unde- 
composed  crystals  only  near  the  contact  of  the  amygdaloid  with 

the    conglomerate Their    relations    to    the    groundmass 

prove  an  anterior  origin  ;  some  of  the  magnetite  and  opacite  in 
the  sections  have  probably  been  derived  from  the  alteration  of 
small  fragments  of  olivine.  Between  the  large  and  small 
feldspar  crystals  lies  a  mass  of  greenish  alteration  products,  — 
chlorite  (often  dichroic),  viridite,  magnetite,  opacite,  consider- 
able epidote,  quartz,  and  calcite.  When  some  of  the  large 
feldspar  crystals  diverge,  the  triangular  space  between  them  is 
filled  with  very  small  feldspar  crystals  lying  in  this  greenish 
mass,  showing,  as  has  been  often  remarked,  that  it  is  merely 
an  original,  glassy  base,  much  altered,  for  we  find  this  same 
relation  in  the  unaltered  basalts.  Calcite,  quartz,  epidote, 
hornblende,  biotite,  apatite,  etc.,  in  the  decomposed  base, 
seem  to  belong  to  the  more  advanced  state  of  decomposition. 
Magnetite  is  always  present.  A  large  part  of  the  magnetite 
arises  from  the  decomposition  of  the  base,  and  it  is  generally 
difficult  to  say  what  part  of  it  is  original. 

"While  in  some  sections  true  amygdules  are  wanting,  yet 
they  generally  occur,  characterized  by  their  sharp  boundary, 
and  the  arrangement  of  the  feldspars  of  the  groundmass 
parallel  to  their  outline.  They  are  filled  by  epidote,  chlorite, 
viridite,  calcite,  or  quartz  ;  the  epidote  generally  on  the  outside 
when  other  minerals  occur  with  it.  Besides  these  true  amyg- 
dules, areas  of  decomposition  occur  in  the  groundmass,  con- 
sisting either  of  opaque  ferritic  material,  constituting  the 
macroscopical  red  spots,  or  of  epidote,  chlorite,  viridite,  etc., 
enclosing  the  small  feldspars." 

Dr.  Wolff  summarizes  this  description  as  follows:  "It  is 
found  by  study  to  be  a  rock  which,  in  the  original  state,  was 
composed  of  the  feldspars,  olivine,  magnetite,  a  base  (glassy, 
microlithic,  etc.)  and  probal)ly  some  augite  (though  this  can- 


489 

not  be  identified  now),  all  in  varying  proportions,  and  that 
these  original  constituents  have  been  largely  replaced  by  sec- 
ondary products.  It  is  therefore  an  altered  basalt," — in 
other  words,   a  nielaphyre. 

The  mekphyre  of  Rock  Island  (PI.  27)  and  the  conglomer- 
ate bordering  it  on  the  south  rise  abruptly  from  the  marsh  on 
tlie  west.  The  conglomerate  dips  S.  70°,  and  varies  in  the 
breadth  of  exposure  from  a  hundred  and  twenty-five .  to  two 
hundred  feet ;  while  the  exposed  breadth  of  the  melaphyre, 
which  slopes  down  beneath  the  drift  on  the  north,  varies  'from 
two  hundred  and  fifty  to  nearly  three  hundred  feet.  These 
conditions  continue  for  about  six  hundred  feet,  and  then  the 
contact  of  the  melaphyre  and  conglomerate  is  abruptly  shifted 
to  the  south  about  a  hundred  feet,  apparently  by  a  transverse 
fault ;  and  the  exposed  breadth  of  the  melaphyre  is  somewhat 
increased.  The  road,  where  it  turns  to  the  southeastward, 
appears  to  mark  the  position  of  another  obliquely  transverse 
fault,  which  breaks  the  melaphyre-conglomerate  contact  again, 
shifting  it  to  the  southward  about  forty  feet.  This  fault  quite 
certainly  crosses  the  melaphyre,  for  the  northern  border  shows 
a  similar  displacement  in  line  with  this.  From  this  line  east- 
ward the  melaphyre  is  closely  bordered  by  conglomerate  on  the 
north  as  well  as  the  south,  and  its  full  breadth  is  seen  to  be 
about  three  hundred  and  fifty  feet.  The  northern  conglomer- 
ate dips  N.  85°— 90°,  and  is  exposed  for  a  maximum  breadth  of 
a  hundred  and  twenty-five  feet,  and  an  extreme  length  of  about 
five  hundred  feet,  the  contact  being  exposed  in  part  as  an 
abrupt  escarpment  ten  to  twenty  feet  high. 

Beyond'this,  or  about  midway  of  its  length,  the  Rock  Island 
ridge  subsides  to  an  undulating  and  ledgy  field,  in  which  the 
melaphyre  outcrops  frequently  and  apparently  with  diminished 
breadth  eastward.  About  twenty-two  hundred  feet  from  its 
western  end,  and  five  hundred  feet  from  its  extreme  eastern 
end,  the  island  is  divided  somewhat  obliquely  (see  map)  by  a 
very  narrow  strait  of  the  marsh,  which  almost  certainly  con- 


490 

ceals  a  fault ;  for  beyond  this  the  melaphyre  is  wholly  wanting, 
the  conglomerate,  in  almost  continuous  outcrops,  extending 
directly  across  its  path,  the  total  exposed  breadth  of  the  con- 
glomerate in  this  part  of  the  field  being  about  eight  hundred 
feet.  An  alternative  view  is  that  the  melaphyre  gradually 
narrows  eastward  and  comes  to  an  end  in  its  last  outcrop  on 
the  edge  of  this  strip  of  marsh.  The  distribution  of  the  out- 
crops is  quite  consistent  with  either  view.  It  is  improbable, 
however,  that  so  thick  a  body  of  melaphyre  (350  feet),  whether 
effusive  or  intrusive,  would  die  out  in  so  short  a  distance,  and 
the  first  explanation  appears,  therefore,  the  more  acceptable, 
especially  in  view  of  the  very  clear  evidence  of  transverse 
faulting  in  the  western  half  of  the  rido-e.  It  is  interesting  in 
this  connection  to  note  that  this  hypothetical  eastern  fault  is 
parallel  in  trend  with  the  others,  but  apparently  reversed  in 
throw,  the  melaphyre  probably  passing  to  the  south,  but  pos- 
sibly to  the  north,  of  the  island  of  conglomerate.  Crossing  the 
marsh  and  creek  to  the  Rock  Island  Head  ridge,  it  is  seen  that 
another  transverse  fault,  partially  compensating  with  reference 
to  the  first,  would  satisfactorily  explain  the  relations  of  this  sec- 
ond band  of  melaphyre  to  the  first  band.  In  the  Rock  Island 
Head  ridge  the  melaphyre  is  exposed  for  a  length  of  seven  hun- 
dred and  fifty  feet  and  a  breadth  of  two  hundred  to  two  hundred 
and  fifty  feet,  with  no  indications  of  the  conglomerate  on  either 
side.  South  of  the  melaphyre  in  Rock  Island,  and  including 
the  elongated,  isolated  ledge  in  the  marsh,  the  total  exposed 
breadth  of  the  conglomei^ate  is  about  five  hundred  and  fifty 
feet.  The  frequent  arenaceous  and  slaty  layers  show  the  strike 
to  be  throughout  about  N.  80°  W.,  and  the  dip  S.  70°. 

The  conglomerates  bordering  the  melaphyre  on  either  side 
are  of  very  normal  character  —  typical  pudding-stone.  The 
northern  conglomerate  is  rather  fine,  few  of  tlie  pebbles  exceed- 
ing an  inch  in  diameter,  and  very  uniform,  tlie  only  noticeable 
band  of  finer  sediment  being  a  layer  of  slate  three  to  five  feet 
thick,  with  streaks  of  sandstone,  which  marks  the  contact  vith 


491 

the  melaphyre  for  the  eastern  half  of  the  outcrop.  The  south- 
ern conglomerate  is  iii  part  distinctly  coarser,  containing  many 
pebbles  from  two  to  "three  inches  in  diameter;  and  within  two 
hundred  feet  or  so  of  the  melaphyre  it  embraces  much  arenace- 
ous material,  —  perhaps  fifty  feet  in  all  of  more  or  less  distinct 
and  thin-bedded  sandstone. 

We  come  now  to  the  interesting  question  as  to  the  relations 
of  the  melaphyre  to  the  conglomerates.  In  1880  ^  I  regarded 
the  melaphyre  (amgydaloid)  as  an  igneous  rock  belonging  to 
an  older  series  (Shawmut  group)  than  the  conglomerate,  and 
as  marking  an  anticlinal  axis  in  the  latter.  In  1882,  Dr. 
Wolff,  in  the  paper  cited  on  page  487,  held,  in  accordance  with 
the  title  of  that  paper  (The  Grreat  Dike  at  Hough's  Neck, 
Quincy,  Mass.),  that  the  melaphyre  or  amygdaloid  forms  a 
great  dike,  or  possibly  two  dikes,  cutting  the  conglomerate 
parallel  with  the  strike,  and  necessarily  newer  than  the  con- 
glomerate on  either  the  north  or  the  south.  I  long  ago 
became  satisfied  that  the  melaphyre  is  not  older  than  all  the 
conglomerate:  and  since  no  one  doubts  its  igneous  origin,  the 
real  question  now  is  as  to  whether  the  melaphyre  is  older  than 
a  part  of  the  conglomerate  or  younger  than  all  of  it ;  or,  in 
other  words,  —  Is  the  melaphyre  intrusive,  as  held  by  Wolff, 
or  effusive  (contemporaneous)  — a  dike,  or  a  surface  flow  of 
lava  ?  Dr.  Wolff,  although  apparently  not  regarding  the  evi- 
dence as  entirely  satisfactory,  but  as  demanding  a  more  detailed 
study,  summarizes  as  follows  the  facts  supporting  his  pro- 
visional conclusion  that  the  melaphyre  is  a  dike  :  ' '  At  the 
western  end,  on  the  southern  side,  the  junction  Avith  the  con- 
glomerate and  red  sandstone  is  very  irregular,  —  large  and 
small  tongues  of  the  dike  penetrate  into  the  conglomerate,  this 
rock  having  a  strike  N.  60°-  80°  W.  and  a  dip  70°  south. 
The  junction  between  the  two  rocks  is  sharp  and  well-marked  ; 
the  dike  seems  often  amygdaloidal  near  the  junction.  Sections 
of  the  contact  of  the  two  rocks  show  that  the  dike  is  composed 

1  Occas.  papers  Boston  soc.  nat.  hist.,  vol.  3,  pp.  176,  209. 


492 

of  a  mass  of  very  small  feldspars,  having  a  beautiful  fluidal 
arrangement,  while  they  are  often  bent  when  in  contact  with 
the  line  of  the  conglomerate.  On  the  northern  side  a  fine 
vertical  exposure  of  the  junction  is  obtained,  which  is  seen  to 
stand  almost  vertical ;  the  dike  cuttino^  the  slate  and  cono-lomer- 
ate  a  little  irregularly,  but  standing  nearly  parallel  to  the 
stratification." 

My  later  observations  indicate,  on  the  contrary,  that  the 
melaphyre  is  unquestionably  effusive.  The  facts  which  appear 
to  compel  this  conclusion   are,  briefly,  as  follows  :  — 

First,  the  structure  of  the  melaphyre  itself,  and  especially 
the  contrast  in  structure  presented  by  a  north-south  section  of 
the  mass,  suggests  a  lava  flow  rather  than  a  dike.  Along  the 
north  side,  or  in  the  northern  half,  it  is  a  homogeneous,  dense, 
and  in  general  non-amygdaloidal,  compact  to  finely  crystalline 
and  porphyrytic  rock  of  gray  to  greenish  gray  and  purplish 
gray  color.  Mr.  White,  who  fully  accepts  the  effusive  nature 
of  the  melaphyre,  notes  that,  "At  the  base  (north  part)  of 
the  flow  the  rock  almost  wholly  lacks  amygdules,  and  seems  to 
be  a  true  basalt,"  the  only  traces  of  amygaloidal  structure 
being  occasional  round  amygdules  up  to  1  cm.  in  diameter,  of 
a  brownish  black,  uncertain  decomposition  product.  South- 
ward and  especially  along  the  southern  margin,  as  noted  also 
by  Mr.  White,  the  peroxidation  of  the  iron  in  the  lava  is  more 
general,  and  dull  reddish  to  purplish  tints  prevail ;  and  the 
rock  becomes  upward,  and  especially  near  the  southern  or 
upper  margin  of  the  flow,  gradually  amygdaloidal  and  scoria- 
ceous,  the  superficial  portion  of  the  flow  being  in  part  pro- 
fusely amygdaloidal.  Mr.  White  points  out  that  the  mela- 
phyre at  the  top  of  the  flow,  along  the  irregular  contact  with 
the  conglomerate,  is  of  a  slaty  character  and  exhibits  a  micro- 
fluidal  structure  parallel  with  the  contact,  as  described  by  Dr. 
Wolff.  In  short,  we  have  here  a  perfectly  normal  and  almost 
ideal  section  of  an  ancient  lava  flow  —  a  single,  simple  flow, 
with  its  base  against  the  northern  conglomerate  and  its  sur- 


493 

face  covered  by  the  southern  conglomerate.  The  gradation  in 
texture  from  dense  and  crystalline  to  amygdaloidal  and  scoria- 
ceous,  is,  if  possible,  more  perfect,  or  more  satisfactorily  exhib- 
ited, in  the  eastern  than  in  the  western  ridge,  although  the 
entire  thickness  of  the  eastern  melaphyre  is  quite  certainly  not 
exposed. 

Second,  the  formal  relations  of  the  melaphyre  to  the  con- 
glomerates, although  perhaps  not  wholly  unambiguous,  are 
certainly  entirely  consistent  with  the  view  that  it  is  effusive. 
The  irregularities  of  the  northern  contact  rioted  by  Wolff  are  in 
part,  probably,  due  to  faulting ;  and  this  explanation  is 
especially  demanded  where,  on  the  west,  the  melaphyre,  for 
more  than  a  hundred  feet,  cuts  obliquely  and  by  a  straight 
line  across  the  conglomerate,  as  shown  on  the  map.  Where 
not  so  explainable,  the  irregularities  are  no  more  than  we 
should  naturally  expect  where  a  stream  of  lava  over  three  hun- 
dred feet  thick  flowed  over  the  unconsolidated  gravelly  sedi- 
ments of  the  Carboniferous  sea.  At  a  point  about  a  hundred 
feet  west  of  the  eastern  end  of  the  large  outcrop  of  the  northern 
conglomerate,  a  vertical  dike  about  a  foot  wide  cuts  the  con- 
glomerate at  right  angles  to  the  contact  but  fails  to  penetrate 
the  melaphyre  ;  its  apparent  lithologic  identity  with  the  mela- 
phyre suggests,  however,  that  it  is  probably  a  tongue  or 
apophysis  penetrating  the  conglomerate  from  the  base  of  the 
flow.  The  phenomena  of  the  upper  or  southern  contact  are 
naturally  of  greater  significance  and  interest ;  and  it  may  be 
noted  that  the  sharp  line  of  demarcation  between  the  melaphyi'e 
and  conglomerate,  and  the  conformity  of  the  fluidal  structure 
of  the  melaphyre  with  the  contact,  as  described  by  Dr.  Wolff, 
are  demanded  by  the  effusive  no  less  than  by  the  intrusive 
theory.  As  regards  the  irregularities  of  the 'contact,  the  sup- 
posed tongues  of  the  melaphyre  penetrating  the  conglomerate 
and  inclusions  of  conglomerate  in  the  melaphyre,  I  have  been 
able  to  find  absolutely  nothing  that  is  not  readily  and  satisfac- 
torily explained  on  the  supposition  that  the  conglomerate  was 


494 

deposited  over  the  naturally  uneven  and  scoriaceous  surface  of 
a  lava  flow,  and  especially  of  a  submarine  flow,  such  as  this 
most  probably  is.  AVhen  the  slaggy  surface  of  a  lava  stream 
is  filled  and  covered  by  sand  and  fine  gravel,  we  must  expect 
that  the  chance  and  imperfect  sections  which  erosion  affords 
Avill  often  be  puzzling  and  inconsistent. 

Third,  the  composition  of  the  conglomerate  tells  strongly 
against  the  intrusive  origin  of  the  melaphyre,  and  in  favor  of 
its  eff'usive  origin.  The  southern  or  upper  conglomerate, 
especially  very  near  the  melaphyre,  contains  numerous  large 
and  small  pebbles  of  precisely  the  same  kind  of  melaphyre. 
In  fact,  the  layer  of  conglomerate  immediately  covering  the 
melaphyre  is  at  some  points  largely  or  chiefly  composed  of  this 
melaphyre  debris.  On  the  other  hand,  the  melaphyre  pebbles 
appear  to  be  wholly  wanting  in  the  lower  or  northern  con- 
glomerate, and  in  the  southern  conglomerate,  as  a  rule,  beyond 
a  few  feet  from  the  contact. 

To  summarize,  the  chief  facts  proving  that  the  melaphyre  is 
effusive  and  not  intrusive,  —  a  record  of  volcanic  activity  in 
this  region  contemporaneous  with  the  formation  of  the  con- 
glomerate, and  not  of  plutonic  activity  at  some  subsequent 
period,  —  are  :  the  amygdaloidal  and  scoriaceous  character  of 
the  upper  part  of  the  flow  ;  the  highly  unsymmetrical  section 
of  the  melaphyre,  so  nor.mal  for  a  flow  and  so  abnormal  for  a 
dike  ;  the  highly  but  minutely  irregular  form  of  the  upper  con- 
tact, and  the  compact  and  fluidal  texture  of  the  melaphyre 
near  this  contact ;  and  the  abundant  occurrence  of  debris  of 
this  melaphyre  in  the  base  of  the  overlying  conglomerate  and 
its  absence  in  the  underlying  conglomerate. 

That  the  eruption  was  submarine  is  probable  from  the  sim- 
ilarity of  the  underlying  and  overlying  sediments,  the  complete- 
ness of  the  section,  and  the  absence  of  evidence  of  extensive 
erosion  of  the  melaphyre  before  it  was  covered.  That  the 
eastern  and  western  ridges  are  dislocated  portions  of  one  and 
the  same  flow  is  indicated  by  their  lithological  similarity,  and 


495 

by  their  great  thickness,  which  would  be  inconsistent  with  basic 
flows  of  limited  area.  That  the  structure,  as  now  revealed,  is 
not  anticlinal,  as  I  formerly  supposed,  is  proved  by  the  un- 
symmetrical  section  of  the  melaphyre  or  the  evidence  that  it  is 
efl^usive,  and  by  the  contrast  between  the  two  conglomerates, 
the  lower  conglomerate  containing  no  melaphyre  debris,  and 
but  few  arenaceous  or  slaty  layers,  while  both  are  conspicuous 
features  of  the  upper  conglomerate.  •  And,  finally,  that  the 
strata  are  not  inverted  is  clearly  indicated  by  the  fact  that  this 
great  bed  of  lava  is  still  right  side  up.  The  outcrops  are  con- 
sistent with  the  view  that  the  melaphyre  narrows  gradually 
both  east  and  west  from  the  middle  of  Rock  Island,  where  it 
attains  its  maximum  breadth  (thickness)  of  three  hundred  and 
fifty  feet ;  and  the  diverging  dips  of  the  upper  and  lower  con- 
glomerates suggest  also  that  it  may  increase  in  thickness  down- 
wards. At  all  events,  it  is  improbable  that  this  sheet  or  flow 
of  melaphyre  was  ever  indefinitely  or  very  greatly  extended  in 
the  direction  of  the  strike  beyond  its .  present  exposures  ;  and 
hence  there  is  no  occasion  for  surprise  at  its  non-appearance  in 
the  sections  on  the  Old  Colony  Railroad,  Furnace  Brook,  Ran- 
dolph Avenue,  etc.  The  interesting  complex  of  diabase  dikes 
traversing  the  melaphyre  and  to  some  extent  the  conglomerate 
of  Rock  Island  will  be  described  in  a  later  section. 

Passino-  now  to  Raccoon  Island,  which  is  about  a  thousand 
feet  in  extr,eme  length  by  three  hundred  feet  in  breadth,  we 
find  that  it  consists  wholly  of  slate  of  a  very  uniform  character. 
The  slate  is  dark  gray,  distinctly  and  finely  banded,  but  rather 
massive,  the  lamination  cleavage  not  being  very  distinct ;  and 
in  the  main  it  is  of  rather  coarse  texture,  some  layers  being 
almost  arenaceous.  Some  layers,  also,  are  minutely  contorted, 
although  the  adjoining  layers  and  the  slate  as  a  whole  are 
remarkably  straight  and  regular.  The  strike  is  E.-W.,  or 
N.  85^  W.  ;  and  the  dip  N.  85°- 90'' ;  corresponding  closely 
with  the  conglomerate  north  of  the  melaphyre  on  Rock  Island. 
The  slate  outcrops  practically  all  over  the  island,  the  exposed 


550  feet 

350 

i  i 

125 

i  i 

496 

breadth  (equivalent  to  the  thickness)  being  nearly  three  hun- 
dred feet. 

The  south  side  of  Raccoon  Island  is  about  in  line  with  the 
northern  edge  of  Rock  Island,  as  is  also  the  outcrop  of  slate 
already  described,  on  the  road  a  mile  west  of  Rock  Island, 
although  the  finer  and  softer  texture  of  the  last-mentioned  slate 
would  indicate  that  stratigraphically  it  belongs  further  from 
(above)  the  conglomerate  than  the  slate  of  Raccoon  Island. 
In  the  Rock  Island  monocline,  which  is,  of  course,  simply  the 
south  side  of  a  great  anticline,  we  have,  from  the  south, 
approximately, 

Upper  conglomerate  and  sandstone 

Melaphyre 

Lower  conglomerate 

1025     " 

The  melaphyre  seems  to  agree  well  in  character,  thickness, 
and  stratigraphic  position  (low  in  the  conglomerate  series) 
with  the  great  bed  of  melaphyre  in  Hingham,  and  a  provisional 
correlation  is  clearly  justified.  Below  the  melaphyre  of  Hing- 
ham the  conglomerate  is  of  uncertain  but  inconsiderable  thick- 
ness, while  above  it  the  conglomerate  series  (conglomerate, 
sandstone,  and  slate)  measures  in  several  good  sections  from 
eight  hundred  to  twelve  hundred  feet  before  we  come  to  the 
great  slate  series,  the  lower  part  of  which,  as  well  exposed  on 
Huit's  Cove,  is  related  in  its  coarse  and  gritty  texture  to  the 
slate  of  Raccoon  Ishxnd.  These  considerations  indicate  that 
the  anticlinal  axis  of  Hough's  Neck  is  not  far  to  the  noi'th  of 
the  melaphyre  ;  but  the  repetition  on  the  north  side  of  this 
axis  of  the  melaphyre  and  the  great  thickness  of  the  upper 
conglomerate  would  throw  the  slate  so  far  to  the  north  as  to 
necessitate  two  ti'ansverse  faults  (one  at  either  end  of  the 
melaphyre)  similar  to  those  dividing  the  melaphyre,  but  of  far 
greater  throw,  involving  a  horizontal  dis[)lacement  of  certainly 


497 

one  thousand  and  possibly  fifteen  hundred  feet.  This  consider- 
ation, and  the  entire  absence  nortli  of  the  Rock  Ishind  ridge  of 
any  indication,  topographic  or  otherwise,  of  either  the  mehi- 
phyre  or  conglomerate,  suggests  an  alternative  solution  of  the 
problem  in  the  form  of  a  longitudinal  fault  along  tiie  north 
side  of  Rock  Island,  with  a  down-throw  to  the  north  sufficient 
to  conceal  both  the  conglomerate  and  melaphyre.  It  might 
even  be  suggested  that  in  the  irregular  vertical  contact  of  the 
melaphyre  and  the  lower  conglomerate  we  have  some  indica- 
tion of  such  a  fault.  Between  these  two  views  it  is  difficult  to^ 
choose  ;  but  since  the  longitudinal  fault  appears,  on  the  whole,, 
to  agree  best  with  the  topography,  it  has  been  preferred  in  the 
construction  of  the  map.  In  looking  eastward  for  a  possible 
extension  of  this  broken  anticline,  it  may  be  suggested  that  we 
-have  some  indication  of  it  in  the  coarse  gritty  slate  on  the 
extreme  north  corner  of  Grape  Island  (see  Part  2,  p.  257),. 
and  the  strike,  N.  70°  E.,  is  distinctly  favorable  to  this  view. 
If,  as  we  may  reasonably  suppose,  the  anticline  pitches  east- 
ward, the  absence  of  the  conglomerate  and  melaphyre  on  Grape 
Island  is  readily  accounted  for.  Still  farther  to  the  northeast, 
as  previously  suggested  (Part  1,  page  104),  we  find  the  prob- 
able continuation  of  this  axis  in  Bumkin  Island,  Strawberry 
Hill,  Strawberry  Ledge,  and  Harding's  Ledge.  The  slate  on 
the  north  side  of  Grape  Island  dips  S.  70°,  evidently  passing 
beneath  the  fine,  soft,  dark  gray  and  nearly  vertical  slate  on 
the  south  side  of  Grape  Island  and  on  Slate  Island  and  the 
north  shore  of  Hingham.  Although  this  area  south  of  the 
Rock  Island  anticline  is  undoubtedly  characterized  by  great 
structural  complexity,  it  must  be  in  a  general  way  synclinal, 
and  hence  occupied  chiefly,  as  we  see  in  Slate  Island,  by  the 
soft,  black  slates.  To  the  south  side  of  this  trough  may  be 
referred  the  coarse,  gray,  massive  slates  outcropping  on  the 
beach  at  the  end  of  Sea  Street,  northwest  of  Great  Hill  in 
Weymouth,  with  E.-W.  strike  and  nearly  vertical  southerly 
dip.      In   the  field  immediately  south  of  the  beach,  the  slate, 

OCCA.S.  PAPERS  B.  S.  N.  H.  IV.  32. 


498 

now  black  and  fissile,  outcrops  again  with  the  same  strike  and 
dip  and  an  eixposed  breadth  of  about  twenty  feet.  These  out- 
crops have  more  the  character  of  the  newer  or  Carboniferous 
slates  than  of  the  known  Cambrian  slates  :  and  since  there  is  a 
possibility,  as  described  on  page  422,  that  the  arkose  sand- 
stone is  in  situ  on  Neck  Street  in  Weymouth,  it  appears  best 
to  carry  the  boundary  fault  of  the  Blue  Hills  Complex  as  far 
south  as  Washington  Street  in  Quincy  and  Old  Spain  in  Wey- 
mouth, and  to  regard  the  absolutely  blank  area  between  this 
line  and  the  Rock  Island  anticline  as  the  westward  continua- 
tion of  the  Carboniferous  slate  basin. 

Takino;  a  o-eneral  view  of  the  Carboniferous  border  of  the 
Blue  Hills  Complex,  it  is  evident  that  although  the  Rock 
Island  anticline  is,  apparently,  continuous  for  its  entire  length, 
west  of  the  Old  Colony  Railroad,  the  southern  limb  of  the 
anticline  has  been  more  or  less  completely  cut  away  by  the 
boundary  fault ;  while  east  of  the  railroad  the  fault,  as  sketched 
in  the  preceding  paragraph,  bears  to  the  southward  and  the 
synclinal  basin  of  the  newer  slate  is  gradually  developed 
between  it  and  the  anticline.  West  of  Hancock  Street,  Quincy, 
the  outcrops  are  all  referable  to  the  northern  limb  of  this 
rather  hypothetical  anticline,  the  beds  of  arkose,  sandstone, 
conglomerate  and  slate  lying  in  monoclinal  fashion  against  the 
northern  edge  of  the  Complex ;  but  on  Hough's  Neck  and 
Raccoon  Island  the  conglomerate-  and  melaphyre  must  be 
referred  wholly  to  the  southern  limb  and  the  slate  wholly  to 
the  northern  limb  ;  and  nowhere  do  we  obtain  a  clear  or  com- 
plete section  of  the  anticline,  which  is  evidently  much  broken 
bv  transverse  as  well  as  longitudinal  faults.  That  the  struc- 
ture along  this  line  is  essentially  anticlinal  is  a  reasonably  safe 
inference  from  the  general  fact  that  throughout  the  Boston  Basin 
the  conglomerate,  so  far  as  known,  underlies  the  newer  slate, 
and  hence  marks  as  a  rule  in  its  outcrops,  the  positions  of  anti- 
clinal axes.  Probably  the  best  general  summary  of  the  stratig- 
raphy   of   this   border    is    that    the   actual    but    much   broken 


499 

anticline  east  of  the  Old  Colony  Railroad  becomes,  west- 
ward, a  northward-dipping  monocline,  due,  perhaps,  at  least  in 
part,  to  the  drag  of  the  boundary  fault. 


COMPARISON  OF   THE  CARBONIFEROUS    STRATA  OF   THE  CON- 
TIGUOUS PORTIONS  OF  THE  BOSTON  AND  NORFOLK  BASINS. 

Incomplete  and  scattering  as  are  the  outcrops  of  post-Cam- 
brian sediments  along  the  southern  border  of  the  Blue  Hills, 
they  are  still  more  meager  and  fragmentary  along  the  northern 
border  ;  and  in  the  absence  of  approximately  continuous  sections 
a  close  comparison,  not  to  mention  a  close  correlation  is  out 
of  the  question,  unless  it  be  on  the  basis  of  a  broader  survey  of 
both  basins  than  we  are  prepared  for  now. 

The  salient  characters  of  the  sediments  in  the  eastern  part 
of  the  Norfolk  Basin  are:  (1)  the  prevailing  coarseness,  as 
expressed  in  the  predominance  of  conglomerates,  the  great 
development  of  arenaceous  beds,  and  the  comparative  absence 
of  slates  and  shales:  (2)  the  apparent  great  thickness  of  the 
section,  which  may,  however,  be  due  in  part  to  repetition  by 
folding  and  faulting  ;  (3)  the  dominant  red  color  of  the  lower 
half  of  the  series  ;  and  (4)  the  absence  of  contemporaneous 
lavas  and,  in  general,  of  dikes.  East  of  the  Blue  Hills  Com- 
plex, in  Hingham  and  Nantasket,  the  conglomerate  series,  to 
which  alone  the  Norfolk  Basin  sediments  can  be  regarded  as 
in  any  sense  equivalent,  embraces,  besides  the  interbedded 
volcanics,  many  alternations  of  conglomerate,  sandstone  and 
slate,  the  finer  sediments,  especially,  being  chiefly  of  a  reddish 
color,  although  the  color  is  in  general  darker  and  less  pro- 
nounced than  in  the  Norfolk  Basin.  In  the  Nantasket  area,  in 
Hingham,  and  in  the  Norfolk  Basin,  the  base  of  the  conglomer- 
ate series  and  the  granite  floor  on  which  it  was  deposited  are 
clearly  exposed  ;  and  omitting  the  volcanic  sheets,  of  which 
there  are  many  in  Nantasket,  one  in  Hingham,  and  none  in  the 


500 

Norfolk  Basin,  a  certain  general  correspondence  can  be  traced 
between  these  three  areas  ;  but  the  iSTorfolk  Basin  sediments 
are  unquestionably  much  thicker,  average  much  coarser,  are 
less  perfectly  assorted,  and  more  intensely  colored  than  those 
of  the  more  eastern  areas. 

Passing  now  to  the  north  side  of  the  Blue  Hills  Complex, 
the  contrast  with  both  the  eastern  and  southern  areas  is  much 
more  marked ;  although  the  melaphyre  of  Hough's  Neck  is 
an  important  connecting  link  between  this  area  and  Hing- 
hara.  The  base  of  the  conglomerate  series  is,  apparently,  not 
exposed  ;  red  sediments  are  much  less  in  evidence  ;  the  distinct 
alternations  of  conglomerates  with  sandstones  and  slates  are, 
apparently,  less  frequent ;  and  especially  the  arkose  sandstone 
series,  which  is  the  most  distinctive  feature  of  this  area,  cannot 
be  recognized  in  Hingham  and  Nantasket,  nor  as  a  sharply 
defined  series  in  the  Norfolk  Basin,  although  the  coarser  sedi- 
ments of  this  basin  are  throughout  more  or  less  of  arkose 
character. 

That  the  sediments  of  these  different  areas  are  essentially 
contemporaneous,  can  hardly  be  questioned  by  any  one  con- 
versant with  all  the  facts  bearing  upon  this  problem  ;  and  we 
must  conclude,  in  view  of  the  striking  lithologic  and  strati- 
graphic  contrasts  which  they  present,  that  in  Carboniferous  time 
the  conditions  of  deposition  in  these  closely  contiguous  areas 
were  strongly  differentiated.  But  this  sharp  differentiation  is 
inconceivable  if  we  must  consider  that  the  Blue  Hills  were  then 
practically  non-existent  and  the  sea  spread  freely  over  the  entire 
area  ;  furthermore,  the  formation  of  the  giant  conglomerate, 
hundreds  of  feet  thick,  along  the  southern  base  of  the  Blue  Hills 
demands  a  bold  shore  freely  exposed  to  a  vigorous  surf;  and 
finally,  the  great  thicknesses  of  these  coarse,  shallow  water  sedi- 
ments, especially  in  the  Norfolk  Basin,  compels  us  to  postulate  a 
contemporaneous,  progressive  and  localized  subsidence  ;  that  is, 
a  graben.  In  other  words,  the  demarcation  of  the  orographic 
block  known  as  the  Blue  Hills  Complex,  occurred  early  in  the 


501 

Carboniferous  age  ;  and  the  development  of  the  boundary  faults 
went  on  during  the  deposition  of  the  conglomerate  series.  The 
Complex  is  divided  by  the  great  transverse  fault  already  postu- 
lated in  the  vicinity  of  the  Old  Colony  Railroad  into  two 
principal  blocks  ;  and  while,  as  will  be  more  fully  shown  in 
the  next  section,  the  eastern,  block  probably  experienced  a 
massive  and  somewhat  equable  elevation,  the  western  block, 
embracing  besides  the  Blue  Hills  proper  the  area  now  included 
in  the  Norfolk  Basin,  suffered  a  north-south  tilting,  the  north- 
ern edge  rising  and  the  southern  edge  sinking.  The  depression 
of  the  southern  margin  of  this  block  gave  rise  to  a  deep  trough 
into  which  were  rapidly  washed  :  first,  the  coarse  detritus  worn 
by  the  waves  from  the  quartz  porphyry  and  Cambrian  cover 
of  the  Blue  Hills,  to  make  the  basal  conglomerate  ;  second,  the 
highly  ferruginous  residuary  chemical  detritus  of  a  wide  area  to 
form  the  complex  of  red  beds  ;  and,  third,  after  the  practical 
exhaustion  of  the  red  soil  through  the  rapid  erosion  incident 
to  the  strong  elevation  of  the  land  contiguous  to  the  graben, 
the  normal  gray  detritus  formed  at  a  time  when  the  denuda- 
tion of  the  land  went  on  too  rapidly  to  permit  the  formation 
of  the  red  residuary  earths.  We  are  thus  able  to  find  in  this 
progressively  deepening  trough,  which  was  filled  as  fast  as  it 
deepened,  a  natural  and  satisfactory  explanation  of  the  great 
thickness,  the  tripartite  structure,  and  all  the  other  original 
characters  of  the  Norfolk  Basin  sediments. 

During  the  development  and  filling  of  the  Norfolk  trough,  the 
northern  edge  of  the  Blue  Hills  Complex  was  rising  cliff-like 
above  the  broader  and,  apparently,  shallower  Boston  Basin, 
and  the  erosion  and  especially  the  granular  disintegration  of 
this  granite  escarpment  afforded  in  large  part  the  material  of  the 
bordering  Carboniferous  strata,  including  the  heavy  bed  of 
arkose  sandstone.  Being  on  the  down-throw  side  of  the  great 
displacement,  the  base  of  the  sedimentary  series,  as  previously 
noted,  is  not  exposed ;  but  the  available  facts  suggest  a  thick- 
ness materially  less  than  that  of  the  Norfolk  sediments.      East- 


502 

ward,  the  great  longitudinal  faults  appear  to  terminate  in  trans- 
verse displacements,  the  southern  boundary  fault  being  traceable 
as  far  as  Hingham  Harbor,  while  the  northern  boundary  fault 
quite  certainly  does  not  cross  Hingham  and  may  be  supposed 
to  terminate  with  the  Blue  Hills  Complex  in  the  valley  of  Wey- 
mouth Back  River.  Beyond  Hingham,  in' the  Nantasket  area, 
the  exposures  do  not  afford  a  complete  section  of  the  conglom- 
erate series  from  the  granite  up  to  the  overlying  slates  ;  but  the 
evidence  favors  a  moderate  thickness,  as  in  Hingham,  and  a 
corresponding  subsidence  for  both  areas,  with  no  indications  of 
a  deep  local  trough  or  graben.  But  we  do  find  in  these  eastern 
fields  the  locus  of  volcanic  activity  which  was  clearly  contem- 
poraneous with,  and  in  some  way,  no  doubt,  causally  connected 
with,  the  profound  displacements  to  the  westward. 

To  find  a  parallel  to  the  conditions  of  deposition  proposed 
here  we  need  go  no  further  than  the  Connecticut  Valley,  where, 
according  to  Davis, ^  the  Triassic  monocline  is  cut  off  on  the 
east  by  a  fault  with  a  down-throw  to  the  west  of,  probably, 
many  thousands  of  feet :  and  the  great  thickness  of  the  Trias- 
sic strata  (roughly  estimated  at  two  miles),  all  bearing  evi- 
dence of  shallow  water  deposition,  may  be  accounted  for  by  a 
progressive  development  of  this  displacement  during  the  ac- 
cumulation of  the  sediments,  the  wash  of  red  residuary  earths 
from  adjoining  land  areas,  into  this  trough  keeping  pace, 
approximately,  with  the  localized  subsidence.  According  to 
the  as  yet  unpublished  observations  of  the  writer,  similar 
conditions  obtained  in  the  Triassic  area  of  the  Minas  Basin  in 
Nova  Scotia.  Along  the  northern  or  PaiTsboro  shore  of  this 
basin  the  gently  dipping  Triassic  beds  meet  the  strongly  folded 
and  usually  vertical  Carboniferous  and  Devonian  strata  in  an 
extremely  obvious  fault  having  a  throw  approximating  the 
entire  thickness  of  the  Triassic  series.  That  this  fault  was  in 
process  of  development  during  the  deposition  of  the  Triassic 
sediments   is    practically    proved    by    the    fact    that   it  is  now 

'  W.  M.  Davis,  Eighteenth  aim.  rept.  U.  S.  geol.  surv. 


503 

marked  by  a  chain  of  elongated  or  lenticular  volcanic  necks 
which  supplded  the  thick  beds  of  contemporaneous  lava  or  trap 
constituting  such  an  important  and  conspicuous  feature  of  the 
Acadian  Triassic.  Two  of  these  necks,  clearly  lying  in  linear 
fashion  in  the  main  fault  fissure,  are  superbly  exposed  in  Was- 
son's  Bluff,  five  miles  east  of  Parrsboro  ;  and  the  volcanic 
aofo-lomerate  fillino-  these  ancient  vents  is  the  great  storehouse 
of  the  zeolites  and  other  minerals  for  which  this  shore  is  so 
famous.  If  these  necks  and  the  connecting  interbedded  sheets 
of  columnar,  amygdaloidal,  or  agglomeratic  lava  are  contem- 
poraneous, as  probably  few  geologists  familiar  with  the  facts 
would  deny,  then  the  fault  must  also  be,  in  part,  contempora- 
neous ;  and  we  need  not  suppose  that  the  Triassic  series 
ever  extended  with  its  full  thickness,  if  at  all,  over  the  older 
formations  on  the  up-throw  side  of  the  fault.  This  view,  it 
will  be  observed,  minimizes  post-Triassic  erosion,  as  well  as 
greatly  diminishing  the  difficulty  of  finding  an  adequate  supply 
of  red,  residuary  detritus  for  the  building  of  the  Triassic  strata. 
For  still  clearer  examples,  not  belonging  wholly  to  a  remote 
past,  but  in  part  still  in  process  of  development,  of  deposition 
strongly  localized  and  accelerated  by  faulting,  we  may  refer  to 
the  Great  Basin,  between  the  Wasatch  Mountains  on  the  east 
and  the  Sierras  on  the  west.  The  numerous  north-south  ranges 
of  mountains  by  which  the  Great  Basin  is  divided  are,  almost 
without  exception,  monoclinal  in  structure.  In  other  words, 
each  range  is  a  tilted  orographic  block,  presenting  a  long  gentle 
bedding  slope  on  one  side  and  a  short,  abrupt  fault  scarp  on  the 
other  ;  and  the  intervening  valleys  necessarily  share  the  struc- 
ture of  the  mountains.  The  level  desert  and  play  a  floors  of  the 
valleys  consist  of  silt  and  coarse  forms  of  detritus  washed  from 
the  bordering  mountains,  and  indicate,  in  connection  with  the 
inclinations  of  the  bordering  slopes,  that  the  valleys  have  been 
filled  to  depths,  in  some  cases,  of  several  thousand  feet.  Ac- 
cording to  Russell  ^  and  other  competent  authorities,  the  throw 

1 1.  C.  Russell,  Monogr.  11,  U.  S.  geol.  surv. 


504 

of  the  more  important  faults  of  the  Great  Basin  ranges  from 
5,000  to  10,000  feet  or  more,  and  nearly  every  fault  scarp  af- 
fords evidence  of  recent  slipping  :  showing  that  the  faults  are 
still  growing  and  probably  have  been  growing  during  the  entire 
period  represented  by  the  silting  up  of  the  valleys,  which  have 
never  had  the  topographic  depth  indicated  by  the  converging 
mountain  slopes.  If,  instead  of  being  an  excessively  arid  re- 
gion, where  erosion  is  almost  at  a  standstill,  the  Great  Basin 
enjoyed  the  hot  and  humid  climate  of  the  southern  Atlantic 
States  and  the  tropics,  with  a  comparatively  rapid  development 
of  red  residuary  soils,  deposition  would  have  kept  pace  with  the 
growth  of  these  contemporaneous  fault  valleys,  and  they  would 
be  now  more  completely  filled  Avith  a  complex  of  red  sediments 
analosrous  to  those  of  the  Triassic  trouo-hs  of  the  Atlantic  sea- 
board  and  of  the  Norfolk  Basin. 

Accepting  then,  as  a  working  hypothesis,  the  view  that  dur- 
ing the  formation  of  the  lower  part  of  the  conglomerate  series 
of  the  Boston  and  Norfolk  Basins,  deposition  was  more  or  less 
sharply  localized  in  strongly  contrasting  areas  ;  it  is,  neverthe- 
less, probable  that  the  deposition  of  the  upper  part  of  the  con- 
glomerate series  was  continuous,  under  somewhat  uniform  con- 
ditions,  across  the  Complex.  But,  however  this  may  be,  it 
can  hardly  be  doubted  that  the  overlying  slates  Avere  spread 
over  the  entire  area  north  if  not  south  of  the  southern  boundary 
fault.  It  will  be  noticed  that  both  of  the  boundary  faults 
down-throw  to  the  north,  giving  a  cumulative  displacement  and 
at  the  last  differentiating  the  Boston  Basin,  by  its  exceptional 
depth,  from  the  area  immediately  to  the  southward. 

One  important  contrast  of  tlie  Boston  and  Norfolk  Basins 
remains  to  be  stated  :  The  post-Cambrian  or  Carboniferous 
sediments  of  the  Boston  Basin,  notwithstanding  the  great  ex- 
tent to  which  they  have  been  folded,  faulted,  sheared,  cleaved 
and  invaded  by  igneous  masses,  are  rather  seldom,  in  the  full 
and  normal  sense  of  the  term,  appreciably  metamorphic.  Along 
the  borders  of  the  igneous  masses,  more  or  less  pronounced  local 


505 

or  contact  luetamorpliism  may  often  be  observed,  taking  the 
form  especially  of  baking  and  induration  ;  but  true  regional  or 
normal  metamorphism  with  the  development  of  an  indigenous 
micaceous  element,  as  in  the  Cambrian  slates  near  the  granite, 
is  practically  wanting.  In  the  Norfolk  Basin,  on  the  contrary, 
lustrous  surfaces  and  a  semi-  or  incipiently  micaceous  facies, 
suggestive  of  a  transition  to  inica  schist,  are  prevalent  charac- 
ters of  the  sandstones  and  to  some  extent  of  the  shales  and  the 
finer  part  or  paste  of  the  conglomerate.  This  contrast  may, 
perhaps,  be  attributed  in  part  to  the  greater  thickness  and  con- 
sequent more  severe  compression  .and  shearing  of  the  sediments 
in  the  Norfolk  Basin.  But  the  writer  is  disposed  to  find  the 
explanation  chiefly  in  the  more  generally  arkose  character  of 
the  Norfolk  Basin  sediments,  which,  in  turn,  is  a  necessary 
correlative,  of  their  more  rapid  deposition.  The  essence  of  the 
explanation  is  in  the  relatively  alkaline  composition  of  arkose, 
the  feldspathic  detritus  insuring  the  presence  in  the  sediments 
of  the  alkalies  —  potash  and  soda  —  so  essential  to  the  develop- 
rnent  of  mica  and  normal  metamorphism  :  and  we  are,  per- 
haps, justified  m  correlating  metamorphism  somewhat  generally 
with  arkose  composition  and  consequently,  as  a  rule,  with  rapid 
and  thick  deposition.  This  would  explain  the  absence  of  met- 
amorphism (or  indigenous  mica)  in  non-arkose  sediments,  like 
the  slates  and  sandstones  of  the  Boston  Basin,  even  when  they 
have  been  buried  to  a  great  depth,  as  in  the  case  of  the  Pots- 
dam sandstone  of  the  Appalachian  region.  In  confirmation  of, 
and  as  a  necessary  corollary  of,  this  view  we  have  the  well- 
established  fact  that  volcanic  tufi^s,  which  are  pre-eminently  of 
arkose  character,  are  so  peculiarly  susceptible  of  metamorphism 
that  in  the  older  formations,  as  a  rule,  their  original  structural 
features  have  been  completely  effaced  and  their  histories  are 
difficult  to  trace. 


506 


THE   BLUE    HILLS   COMPLEX   DURING   THE   APPALACHIAN 
EEVOLUTION. 

Although  the  Subcarboniferoiis  period  left  in  this  region  no 
identifiable  records  either  in  erosion  or  deposition,  the  closing 
episode  of  the  Carbonifei'ous  age  and  of  the  Paleozoic  era  —  the 
Appalachian  revolution  —  is  indelibly  recorded  in  strong  and 
salient  structure  lines :  and  it  must  ever  stand  as  the  chief  event 
in  the  geological  history  of  this  part  of  the  continent.  As  else- 
where, the  strata  yielded  chiefly  by  plication  ;  but  the  displace- 
ments are  also  numerous  and  extensive.  The  faulting  is  prob- 
ably due  to  the  fact  that  the  thickness  of  imperfectly  lithified 
sediments  (2000  to  3000  feet  of  Carboniferous  beds  only  in  the 
Boston  Basin)  was  insufficient  to  induce  softening  of  the  crys- 
talline floor  by  rise  of  the  isogeotherms.  The  fissures  were 
sufficiently  profound  to  tap  reservoirs  of  basic  magma,  and  the 
numerous  dikes  and  intrusive  sheets  of  diabase  which  intersect 
the  rocks  of  the  Boston  Basin,  and  to  a  very  limited  extent  those 
of  the  Norfolk  and  Narragansett  Basins,^  were  formed. 

It  is  impossible  to  present  here  in  full  even  the  principal  re- 
sults of  this  grand  crustal  adjustment ;  but  we  must  confine  our 
attention  to  the  Boston  and  Norfolk  Basins  and  especially  to 
their  dividing  wall,  the  Blue  Hills  Complex.  The  Boston  Basin 
has  been  described  as  a  composite  trough.  A  north-south  sec- 
tion shows  belts  of  conglomerate  and  melaphyre  alternating 
with  belts  of  the  newer  slate.  The  conglomerate  marks  anti- 
clinal and  the  slate  synclinal  axes  ;  and  in  the  most  general 
view  we  recognize  one  dominant,  central,  anticlinal  arch  three 
miles  broad  with  several  subordinate  folds  on  either  side  of  it 
before  we  reach  the  sharply  defined  northern  and  southern 
boundaries,  which  are  undoubtedly  displacements  of  consider- 

1  Amer.  journ.  sci.,  vol.  20,  1880,  p.  417;  and  Proc.  Boston  soc.  nat.  hist.,  vol.  23,  pp. 
32,5-3.30. 


507 

able  magnitude,  the  bordering  crystalline  rocks  being  in  each 
case  upon  the  up-throw  side.  These  northern  and  southern 
boundary  faults  of  the  Boston  Basin  must  be  regarded  as  of 
primary  importance  in  the  geological  structure  of  the  region  ; 
for,  virtually,  they  present  on  the  up-throw  side  two  solid  walls 
of  crystalline  rock  between  which  the  great  central  area  of  sedi- 
mentary and  volcanic  rocks  has  first  settled  down  and  then  suf- 
fered compression  as  in  a  vise,  by  the  approach  of  those  walls, 
producing  the  folds  already  noticed  and  most  of  the  minor  faults 
of  the  region.  The  Norfolk  Basin,  at  least  in  its  northeastern 
extremity,  with  which  alone  we  have  to  do,  is  a  simple  trough 
of  monoclinal  structure.  All  along  the  south  side  of  the  Blue 
Hills  the  coarse  basal  conglomerate  of  the  Carboniferous  series 
lies,  as  already  noted,  directly  upon,  and  dips  steeply  away  from, 
the  quartz  porphyry  covering  of  the  Blue  Hills  Complex,  the  con- 
glomerate being  chiefly  composed  of  the  debris  of  the  quartz 
porphyry.  Going  directly  across  the  trough,  we  find  that  the 
prevailing  dip  is  southerly  ;  and,  although  the  southern  border 
is  not  clearly  exposed,  it  is  probable  that  we  pass  abruptly 
across  another  great  displacement  to  the  normal  granite  of  the 
region.  The  up-throw  is  again  on  the  south  and  has  been 
sufficient  to  carry  the  entire  Carboniferous  series  above  the 
present  plane  of  erosion. 

In  the  preceding  pages  these  principal  displacements  have 
been  referred  largely  but  not  necessarily  wholly  to  the  time 
when  the  conglomerate  series  was  being  deposited.  The  later 
Carboniferous  sediments  were  probably  deposited  during  a  rela- 
tively quiescent  period  when  this  region  was  experiencing  a 
massive  rather  than  a  differential  subsidence.  This  period  was 
closed  by  the  Appalachian  revolution,  during  which  the  growth 
of  the  primary  faults  was  renewed,  and,  in  obedience  to  a  power- 
ful compressive  force  acting  in  a  north-south  direction,  the  net- 
work of  minor  faults  and  the  complex  folds  of  the  strata  were 
developed.  With  the  primary  faulting  may  be  associated  or 
correlated  the  effusive  volcanic  activity  which  resulted  in  the 


508 

formation  of  the  contemporaneous  sheets  of  basic  lava  (mela- 
phyre  and  porphyrite)  in  the  lower  part  of  the  conglomerate 
series  ;  and  with  the  subsequent  period  of  faulting  and  folding 
known  as  the  Appalachian  revolution,  may  be  correlated  the 
formation  of  the  numerous  dikes  intersecting  alike  the  earlier 
and  the  later  Carboniferous  strata.  The  effusive  portions  of  the 
later  or  dike  eruptions,  if  there  were  any,  were  not  covered  and 
preserved  by  sedimentary  deposits,  and  hence  were  long  since 
removed  by  erosion. 

Its  northern  and  southern  boundary  faults,  begun  in  early 
Carboniferous  time  and  completed  during  the  Appalachian  revo- 
lution, and  both  throwing  in  the  same  direction  (northward), 
make  of  the  Blue  Hills  Complex  an  orographic  block  about  ten 
miles  lono^  and  increasing  in  breadth  westward  from  a  little  more 
than  one  mile  in  Weymouth  to  more  than  four  miles  in  the  Blue 
Hills  region,  embracing  in  its  western  half  not  only  the  Blue 
Hills,  but  also  the  northeastern  extremity  of  the  Norfolk  Basin. 
This  orographic  block  must  be  conceived  as  terminated  abruptly 
east  and  west  by  transverse  faults  in  the  valleys  of  Weymouth 
Back  River  and  the  Neponset  River  ;  and  it  is  quite  certainly 
divided  in  the  longitude  of  the  Old  Colony  Railroad  by  another 
transverse  fault,  with  the  up-throw  to  the  east  (p.  411),  into 
two  A^ery  unequal  blocks  which  appear  to  have  moved  some- 
what independently.  That  the  smaller  eastern  block  has 
moved  en  masse  is  indicated  especially  b}^  the  fact  that  the  fine 
granite  cover  of  the  Complex  is  preserved  over  nearly  all  parts 
of  this  block,  alike  on  the  north  and  the  south.  The  large  Avest- 
ern  block,  on  the  other  hand,  has  evidently  been  tilted,  the 
northern  edge  elevated  and  the  southern  edge  depressed,  and  in 
this  depression  lies  the  narrow  eastern  extremity  of  the  Norfolk 
Basin,  Avhile  the  northern  scarp  overlooks  the  Boston  Basin. 

Concerning  the  magnitude  of  the  displacements  bounding  the 
principal  orographic  block  of  the  Blue  Hills,  outlined  above,  it 
is  impossible,  at  present,  to  make  definite  statements.  Al- 
though this  block  is,  in  itself,  monoclinal,  it  is  seen  in  a  broader 


509 

view  to  be  essentially  anticlinal  in  its  relations  to  the  bordering 
Carboniferous  basins  ;  for  the  strata  dip  away  from  it  on  both 
sides  at  high  angles.  The  gigantic  and  unsymmetrical  arch  of 
the  later  Carboniferous  strata  finally  broke  along  its  northern 
crest,  when  displacement  by  flexure  and  stretcliing  through  the 
renewed  growth  of  the  ancient  boundary  fault  had  reached  its 
utmost  limit ;  and  the  granitic  axis,  bearing  its  cover  of  quartz 
porphyry  and  a  considerable  thickness  of  the  newer  Carbonifer- 
ous strata,  rose  high  above  the  crumpled  beds  forming  the  floor 
of  the  Boston  Basin.  From  this  northern  fault-scarp  the  con- 
tact zone  of  quartz  porphyry  and  overlying  strata  probably 
sloped  southward  very  gently  at  first,  and  then  more  steeply  as 
they  bent  down  to  form  the  trough  of  the  Norfolk  Basin,  finally 
abutting  against  the  fault-scarp  forming  its  southern  wall. 
The  northern  slip,  forming  the  southern  wall  of  the  Boston 
Basin,  conceals  almost  the  entire  thickness  of  the  conglomerate 
series  ;  and  the  base  of  the  conglomerate,  considering  the  high 
inclination  of  the  beds,  is  probably  not  less  than  two  thousand 
feet  below  the  present  surface.  To  this  must  be  added  the 
thickness  of  the  quartz  porphyry  and  an  equal  or  greater  thick- 
ness of  the  underlying  granite,  giving  twenty-five  hundred  to 
three  thousand  feet  as  a  minimum  or  reasonably  safe  estimate 
of  the  total  throw.  The  throw  of  the  southern  fault,  forming 
the  southern  wall  of  the  Norfolk  Basin,  must  be,  in  view  of  the 
great  thickness  of  the  Carboniferous  beds  in  this  trough,  at 
least  double  this  amount,  or  say  five  thousand  feet  as  a  mini- 
mum. 

Eastward  from  the  fault  on  the  line  of  the  Old  Colony  Rail- 
road in  the  Quincy  Adams  valley,  terminating  the  quartz  por- 
phyiy  and  felsite  area,  and  dividing  the  Complex,  we  do  not 
find  another  transverse  slip  of  similar  magnitude  until  we 
come  to  Weymouth  Back  River.  East  of  this  north-south 
valley,  in  Hingham,  we  have,  besides  the  granitic  series,  only 
an  exceptionally  complete  development  of  the  conglomerate 
series  and  the  newer  slates,  or  the  strata  now  believed  to  be  of 


510 

Carboniferous  age.  Evidence  that  either  the  Cambrian  or 
Carboniferous  beds  cross  this  line,  south  of  the  northern  bound- 
ary fault  of  the  Complex,  is  entirely  wanting ;  and  since  the 
Carboniferous  series  on  the  Hingham  side  has  a  measured 
thickness  of  about  two  thousand  feet,  a  slip  of  at  least  that 
magnitude  is  a  necessity  here,  the  down-throw  being,  in  this 
case,  to  the  east.  The  slates  outcropping  near  Great  Hill, 
on  the  north  shore  of  Weymouth,  have  been  referred,  with 
the  slate  of  Raccoon  Island  and  the  conglomerate  and  mela- 
phyre  of  Hough's  Neck,  to  the  Carboniferous  series ;  and 
somewhere  beneath  the  broad  expanse  of  drift  and  water  be- 
tween these  outcrops  and  the  Lower  Cambrian  beds  of  Mill 
Cove  and  eastern  Quincy  must  extend  a  continuation  of  the 
o-reat  displacement  forming  the  southern  wall  of  the  Boston 
Basin  ;  but  east  of  the  Old  Colony  Railroad  the  exact  course  of 
the  fault  must  remain  a  matter  of  conjecture.  The  fault  mark- 
ing the  southern  boundary  of  the  Complex  is,  as  previously 
described,  very  clearly  and  instructively  exposed  in  the  railroad 
cuttings  in  Weymouth,  the  fissile  and  lustrous  Cambrian  slate 
resting  against  a  nearly  vertical,  slickensided  wall  of  granite. 

Among  the  minor  displacements  of  the  Blue  Hills  Complex 
described  in  the  preceding  pages  and  probably  dating  from  the 
Appalachian  revolution  may  be  mentioned  :  first,  the  longitudinal 
faults  bounding  the  Hayward  Creek  and  Ruggles  Creek  belts 
of  Cambrian  slate  ;  and,  second,  the  transverse  faults  breaking 
the  southern  boundary  fault  in  the  valleys  of  Mill  Cove,  Wey- 
mouth Fore  River  and  Smelt  Brook,  and  the  more  hypothetical 
fault  in  the  vicinity  of  Braintree  Great  Pond  and  Purgatory 
Road,  which  we  suppose  to  determine  the  eastern  end  of  the 
Norfolk  Basin  and  the  common  boundary  of  the  quartz  porphyry 
and  effusive  aporhyolite  in  the  southeastern  part  of  the  Blue 
Hills.  The  fact  that  the  east-west  boundary  faults  of  the  Blue 
Hills  Complex  so  closely  follow  belts  of  Cambrian  slate  is  prob- 
ably due  to  the  circumstance  that  these  narrow  zones  of  slate, 
extending  to  a  great  depth  in  the  granitic  floor  over  which  the 


511 

Carboniferous  strata  were  spread,  must  have  formed  planes  of 
weakness  in  the  Complex,  and  hence  determined  the  position 
and  direction  of  great  displacements.  The  northern  belt  prob- 
ably extends  out  under  the  Carboniferous  beds,  and  is  not  so 
narrow  as  it  appears  on  the  map. 


DIABASE   DIKES   NEWER   THAN   THE    GRANITIC   SERIES. 

Undoubtedly,  the  closing  event  in  the  development  of  the 
lithified  formations  or  hard  rocks  of  the  Blue  Hills  Complex 
was  the  making  of  the  extensive  series  of  the  post-granitic  basic 
or  diabase  dikes.  These  dikes  are  not  only  contrasted  chrono- 
logically with  the  earlier  or  pre-granitic  dikes,  which  they  other- 
wise closely  resemble,  by  being  post-granitic  ;  but  they  are  also 
essentially  post-Carboniferous,  clearly  dating,  in  part  at  least, 
from  the  close  of  the  Appalachian  revolution,  when  the  Carbon- 
iferous strata,  having  become,  through  severe  plication,  close- 
folded  and  rigid,  suffered  farther  deformation  by  faulting.  That 
the  distinctly  basic  or  diabase  dikes  of  this  area  belong,  chrono- 
logically, to  two  series  is  indicated  by  their  relations  to  the 
granitic  rocks.  The  dikes  of  the  fii^st  or  older  series,  which 
have  been  cleai'ly  recognized  only  in  the  valley  of  Pine  Tree 
Brook,  as  already  described,  cut  the  Cambrian  sti-ata  and  are 
cut  and  enclosed  by  the  granitic  rocks  ;  while  the  dikes  of  the 
second  or  later  series  cut  alike  the  Cambrian  beds,  the  entire 
granitic  series  and  the  bordering  Carboniferous  strata.  It  is 
only  where  cutting  the  Cambrian  sediments  alone  that  the  dikes 
ai-e  ambiguous;  and  of  such  dikes  there  are,  fortunately,  but 
few. 

One  of  the  most  striking  features  of  the  pre-granitic  dikes,  as 
developed  in  the  Pine  Tree  Brook  area,  is  their  close  parallelism 
with  the  bedding  of  the  Cambrian  slate,  a  parallelism  which  is  per- 
sistently maintained  through  an  important  change  of  strike  (PI. 
26) .     Although  these  slates  probably  once  possessed  a  good  lami- 


512 

nation  cleavage,  they  have  become,  through  the  metamorphic 
influence  of  the  enclosing  granite  and  porphyry,  decidedly  mass- 
ive and  non-fissile  in  structure.  This  is  another  clear  indica- 
tion that  these  dikes  were  formed  before  the  invasion  of  the 
slates  by  the  plutonic  rocks,  although  they  are,  quite  certainly, 
subsequent  to  the  folding  of  the  slate.  The  post-granitic  dikes, 
on  the  other  hand,  show  no  special  disposition  to  conform  witli 
the  bedding  planes  of  either  the  Cambrian  or  Carboniferous 
strata  ;  but  they  tend  rather  to  agree  in  direction,  and  to  some 
extent  in  position  or  distribution,  with  the  fault  lines  of  the  dis- 
trict, which  are  also  the  directions  of  the  principal  joint  systems. 
In  the  further  consideration  of  the  distribution  of  the  post- 
granitic  dikes,  it  is  interesting  to  note  that  they  are  chiefly  con- 
fined to  the  Boston  Basin  side  of  the  Complex,  or,  in  other 
words,  to  the  region  north  and  east  of  the  main  range  of  the 
Blue  Hills.  Over  the  main  or  quartz  porphyry  range  of  the 
Blue  Hills  and  the  Norfolk  Basin  dikes  are  almost  entirely  want- 
ing, and  but  very  few"  have  been  noted  in  the  granitic  area  south 
of  the  Norfolk  Basin.  It  has  been  suggested  that  this  disparity, 
which  was  pointed  out  by  Professor  Barton  and  the  writer  nearly 
twenty  years  ago,^  may  be  attributed  to  the  infrequency  of  the 
outcrops.  But  this  explanation  is  certainly  not  applicable  to 
the  Blue  Hills,  where  the  outcrops  are  broad  and  continuous, 
covering,  on  some  of  the  hills,  a  large  part  of  the  surface  ;  nor 
do  the  Carboniferous  beds  outcrop  any  less  freely  along  the 
southern  than  the  northern  side  of  the  Complex,  although  the 
dikes  are  a  conspicuous  feature  of  the  latter  area  and  entirely 
unknown  in  the  former.  In  fact,  the  only  dike  which  Professor 
Barton  and  I  discovered  in  a  rather  thorough  examination  of 
the  outcrops  of  the  Norfolk  Basin  for  its  entire  length  of  over 
thirty  miles,  is  the  small  example  previously  cited  in  the  long- 
cutting  ill  the  red  beds  on  the  Boston  and  Providence  Railroad, 
north  of  Canton  Junction.      The  generalization  appears,  there- 

1  Amcr.  joivni.  sci.,  vol.  120(1880),  p.  417;  see  also  I'roc.  Boston  soc.  nat.  liist,,  vol.  23, 
p.  330. 


513 

fore,  to  be  well  sustained,  that  in  later  as  in  earlier  Carbonifer- 
ous times  the  Boston  and  Norfolk  Basins  were  contrasted  by  the 
fact  that  igneous  activity  was  practically  confined  to  the  former. 

The  diabase  dikes  of  the  Nantasket  and  Cohasset  area  are 
readily  referred  by  their  directions  to  three  distinct  systems  — 
two  approximately  east-west  (N.  75°  E.  and  S.  75°  E.)  and 
the  third  north-south.  These  systems  are  proved  by  intersec- 
tions to  be  also  chronologically  distinct,  the  north-of-east  system 
being  the  oldest  and  the  north-south  system  the  newest.  The 
dikes  of  the  east-west  systems  have  suffered  the  deep-seated 
alteration  known  as  chloritization  and  are  composed  of  typical 
greenstone  ;  columnar  jointing  is  seldom  well  developed  in 
them  ;  and  they  are  frequently  porphyritic.  The  north-south 
dikes,  on  the  other  hand,  have  not  as  a  rule  experienced  the  green- 
stone alteration  or  chloritization,  are  usually  distinctly  cross- 
jointed,  readily  decompose  to  a  brown  earth,  and  are  never 
distinctly  or  coarsely  porphyritic.  The  east-west  systems  are 
recognized  in  Hingham,  although  exposed  intersections  are  want- 
ing to  establish  their  relative  ages  ;  and  dikes  clearly  referable 
to  the  north-south  system  are  wanting.  West  of  Weymouth 
Back  Kiver,  in  the  area  of  the  Blue  Hills  Complex,  both  east- 
west  and  north-south  dikes  are  well  represented  :  but,  as  in 
Hingham,  intersections  are  nowhere  clearly  exposed.  Both 
series  of  east-west  dikes  are  represented,  although  the  younger 
or  south -of-east  system  appears  to  prevail ;  all  of  these  and  some 
of  the  north-south  dikes  are  greenstone  ;  and  there  is  a  consid- 
erable proportion  of  abnormal  trends,  especially  in  the  northwest- 
southeast  quarters.  The  Nantasket  classification  is  clearly 
applicable  to  the  majority  if  not  to  nearly  all  of  the  dikes  of 
this  more  western  area ;  and  it  is  especially  interesting  to  note 
that,  with  practically  no  exceptions,  the  cross-jointed,  black, 
rusty-weathering  dikes,  decomposing  readily  to  a  brown  earth, 
have  north-south  trends. 

In  his  microscopic  examination  of  the  dikes  of  the  Blue  Hills 
Complex  Mr.  White  has  failed,  as  did  Professor  Merrill  for  the 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.         33. 


5M 

Nantasket  area,  to  discover  any  distinctive  characters  for  the 
different  systems.  This  is  the  more  surprising  since,  macro- 
scopically,  the  east-west  systems,  which  are  clearly  seen  at  Nan- 
tasket to  be  of  approximately  the  same  age  and  possibly  chrono- 
logically distinct  only  in  individual  instances,  are  certainly 
strongly  contrasted  with  the  decidedly  younger  north-south 
dikes.  In  view  of  all  the  facts,  it  appears  to  be  a  reasonably 
safe  generalization  that  the  east-west  dikes,  which  often  show, 
besides  the  general  chloritization,  unmistakable  shearing  and 
crushing  to  which  the  absence  of  the  columnar  structure  or 
cross-jointing  may  in  part  be  due,  were  formed  in  the  main 
during  the  general  folding  or  north-south  compression  of  the 
Carboniferous  strata,  in  the  temporary  east- west  or  longitudinal 
fissures  naturally  incident  to  the.  plication  or  bending  of  the  for- 
mations, with  or  without  faulting  ;  while  the  north-south  dikes, 
which  are  practically  free  from  dynamic  metamorphism,  may  be 
referred  to  a  somewhat  later  stage  of  the  Appalachian  revolu- 
tion, when,  the  formations  having  become  well  stiffened,  north- 
south  or  transverse  fissures  naturally  resulted  from  the  contin- 
ued north-south  compression  and  consequent  east-west  tension 
or  stretching.  The  north-south  fissuring,  in  harmony  with  this 
explanation  of  its  origin,  was  in  general  unaccompanied  by 
either  contemporaneous  or  subsequent  thrust  faulting  or  shear- 
ing. 

A  few  of  the  east-west  dikes  hade  to  the  south,  and  others 
are  vertical,  but  the  great  majority  have  a  decided  hade  to  the 
north.  This  generalization  holds  not  only  for  the  Blue  Hills 
Complex,  but  for  the  entire  southern  margin  of  the  Boston 
Basin,  as  is  so  clearly  shown  by  the  tabulated  list  of  dikes  in 
Pai-t  1.  Master  joints,  shear  planes  and  fault  planes  of  south- 
erly hade  are  not  at  all  uncommon  ;  but  under  the  stresses  of 
the  dike-forming  period  they  have  seldom  opened  to  afford  a 
pathway  for  the  basic  magma.  The  dominant  northerly  hade 
of  the  dikes  appears  the  more  surprising  since  that  is  also  the 
attitude  of  the  slatv  cleava2:e  throus-hout  both  the  Boston  and 


515 

Norfolk  Basins,  indicating  compression  normal  to  the  dikes. 
We  must  conclude,  thei-efore,  that  the  formation  of  the  east- 
west  dikes,  as  well  as  of  the  north-south  dikes,  post-dates  the 
main  period  of  plication  and  cleavage  development  in  this  region, 
accompanying  the  subsequent  more  massive  deformation  or 
period  of  faulting,  when,  apparently,  the  fissures  of  northerly 
hade  widened  downward,  and  the  fissures  of  southerly  hade,  as 
a  rule,  remained  closed  or  widened  upward.  It  may  also  be 
noted  in  this  connection  that  the  north-south  dikes  throughout 
the  entire  region,  with  few  exceptions,  either  are  vertical  or  have 
a  slight  hade  to  the  west.  That  the  hade  of  either  system  of 
dikes  has  been  sensibly  influenced  or  determined  by  massive 
movements  or  tilting  of  the  crust  subsequent  to  their  formation 
is  perhaps  improbable  ;  although  it  is  certain  that  the  east-west 
dikes  at  least,  as  previously  noted,  and  to  a  greater  degree  than 
the  north-south  dikes,  suffered  dislocation  and  shearing  before 
the  final  close  of  the  Appalachian  revolution. 

Among  the  post-granitic  dikes  of  the  Blue  Hills  Complex  the 
two  which  accompany,  the  one  the  northern  and  the  other  the 
southern,  boundary  faults,  stand  out  with  special  prominence 
by  virtue  of  both  size  and  structural  importance.  During  the 
prosecution  of  this  work  no  systematic  or  thorough  search  for 
dikes  has  been  made  ;  and  it  is  certain  that  the  following  de- 
scriptions, copied  from  ray  note-books,  are  not  exhaustive, 
although  it  is  probable  that  every  important  or  distinct  type  of 
dike  occurring  in  this  area,  is  represented.  On  account  of  their 
readier  decomposition,  the  nortli-south  dikes  usually  outcrop 
less  prominently  than  the  east-west  dikes,  and  are  thus  more 
likely  to  escape  observation.  The  dikes  may  be  most  conven- 
iently described  in  topographic  order. 

Dikes  in  the  Region  South  of  the  Blue  Hills  Complex. — 
In  the  railroad  cutting  northeast  of  King  Oak  Hill  we  have  ex- 
posed,  south  of  the  lower  Cambrian  slate  and  the  boundary 
fault,  several  hundred  feet  of  coarse  reddish  granite.  It  is  much 
crushed  and  slickensided  and  profoundly  altered  ;   and  it  is  trav- 


516 

ersed  by  several  east-v/est  dikes  of  greenstone  diabase  one  to 
several  feet  in  width,  all  as  badly  smashed  up  and  altered  as  the 
granite. 

In  the  cutting  at  East  Weymouth  Station  six  greenstone 
dikes  are  clearly  exposed  in  the  granite.  They  are  approxi- 
mately east-west  in  trend,  vertical,  or  with  a  slight  hade  to  the 
south,  and  vary  in  width  from  eighteen  inches  to  five  feet,  ex- 
cept the  most  northerly  dike,  which  is  only  four  inches.  They 
seem  to  agree  closely  in  trend ;  and  parallel  with  them  are  two 
strongly  marked  shear-planes  or  faults. 

On  the  southwest  corner  of  Middle  and  Broad  Streets,  in 
Weymouth,  an  ideally  regular  dike  is  very  handsomely  exposed. 
It  is  greenish  black,  cross-jointed,  three  feet  wide,  trends  N. 
75°  E.  and  hades  S.  10°  approximately.  Southeast  of  this 
locality,  on  the  east  side  of  Whitman  Pond,  north  of  Washing- 
ton Street,  is  a  fine  series  of  dikes.  Only  one  has  been  observed 
in  the  granite  south  of  the  triangular  ai^ea  of  diorite  ;  one  in  the 
diorite  ;  and  half  a  dozen  or  more  in  the  granite  north  of  the 
diorite  ;  occurring,  evidently,  at  more  frequent  intervals  north- 
ward. They  are  composed  of  fine-grained  greenstone  diabase, 
vary  in  width  from  six  inches  to  fifteen  feet,  and  trend  approxi- 
mately east-west,  belonging  mainly  at  least  to  the  north-of-east 
system.  Mr.  White  describes  a  I'epresentative  dike  of  this  series 
as  rich  in  pyrite  and  showing  considerable  effects  of  ci'ushing. 
East  of  this  group  of  dikes,  on  the  west  side  of  Pleasant  Street, 
between  East  Weymouth  and  Lovell  Corners,  the  normal  bio- 
tite  granite  has  been  blasted,  and  several  dikes,  two  inches  to 
two  feet  wide,  are  exposed,  trending  N.  80°  W.  Farther 
south  there  are,  evidently,  but  few  dikes  ;  and  in  the  seam-fiice 
o-ranite  district  of  Weymouth  and  Hingham  I  have  observed  but 
one.  This  is  in  Gilbreth's  quarry,  six  hundred  feet  east  of 
Pleasant  Street,  in  Weymouth.  It  is  only  from  six  to  twelve 
inches  wide  and  trends  N.  70°  W. 

West  of  Whitman  Pond  dikes  in  the  granitic  rocks  are  cer- 
tainly relatively  fewer.      In  fact,  east  of  the  Old  Colony  Rail- 


517 

road  only  two  have  been  observed,  besides  those  accompanying 
the  boundary  fault.  One  of  these  is  northwest  of  the  pond, 
midway  between  Washington  and  Essex  Streets.  It  is  four 
feet  wide  and  trends  N.  70°  E.  The  other  outcrops  on  the 
summit  of  Liberty  Hill  in  Braintree,  with  a  breadth  of  nine 
feet,  and  also  trending  N.  70°  E.  Both  of  these  dikes  are  ver- 
tical, and  consist  of  the  normal  greenstone  diabase  ;  and  the 
latter  is  profusely  porphyritic. 

Between  the  Old  Colony  Railroad  and  the  Boston  and  Provi- 
dence Railroad,  a  distance  of  eight  miles,  the  only  dikes  ob- 
served are  on  the  highway  between  South  Braintree  and  Ran- 
dolph. On  the  direct  road  to  Randolph,  about  half  a  mile 
south  of  the  road  leading  west  to  Great  Pond,  is  an  east-west 
greenstone  dike  three  feet  wide  ;  and  nearly  half  a  mile  south 
of  this  the  granite  is  cut  by  three  vertical  north-south  dikes 
from  two  to  six  inches  wide.  In  the  granite  cutting  on  the 
Boston  and  Providence  Railroad,  immediately  north  of  Canton 
Junction,  several  rather  irregular  dikes  of  moderate  width  are 
exposed. 

To  this  area  belongs  also  the  great  southern  boundary  dike, 
which  has  been  described  with  sufficient  fullness  on  page  405. 
This  is  probably  the  largest  dike  in  the  entire  area  of  the  Blue 
Hills  Complex.  It  has  been  traced,  with  a  maximum  breadth 
of  three  hundred  feet,  for  about  one  and  a  half  miles  in  a  gen- 
eral east-northeast  direction  from  East  Braintree  nearly  to  King 
Oak  Hill  in  Weymouth.  It  is  a  true  boundary  dike  for  not 
more  than  half  this  distance.  Where  the  boundary  fault  is 
thrown  to  the  northward  by  the  Fore  River  fault,  the  dike 
enters  the  granite.  It  is  clearly  later  than  the  main  part  at 
least  of  the  displacements,  and  yet  evidently  genetically  related 
to  them. 

Mr.  White  describes  the  groundmass  of  this  dike  as  composed 
of  dirty  brown  alteration  products  in  which  are  imbedded  lath- 
shaped  feldspars.  Bending  of  the  crystals  and  flow-structure 
are  observable.     Triangular  magnetite  crystals  are  present,  and 


518 

a  little  ilmenite  somewhat  altered  to  leucoxene.  In  what  is 
virtually  a  part  of  this  dike,  on  Mt.  Pleasant,  Mr,  White  found 
a  handsome  rose-colored  augite  that  appears  like  garnet  except 
for  its  low  refraction.  He  also  says  the  rock  is  much  chloritized, 
the  chlorite  extending  all  through  the  feldspar,  while  many 
minute  veins  of  quartz  ramify  through  the  rock. 

Dikes  in  Worth  Weymoiith.  —  On  account  of  the  nearly 
continuous  mantle  of  modified  drift,  the  dike  phenomena  of  this 
area  are  exceedingly  scanty.  Near  the  head  of  Mill  Cove,  and 
four  or  five  hundred  feet  north  of  the  railroad,  is  a  prominent 
outcrop  of  trap  bounded  on  all  sides  by  salt  water  and  marsh, 
and  connected  with  White's  Neck  by  a  tide-w^ater  dam.  The 
rock  is  a  finely  and  somewhat  obscurely  crystalline  greenstone 
in  which,  according  to  Mr.  White,  the  original  microscopic 
characters  have  been  almost  entirely  eifaced.  The  north-south 
breadth  of  the  outcrop  is  nearly  two  hundred  feet.  A  little 
more  than  half  a  mile  to  the  eastward,  in  the  laro-e  s^ravel  exca- 
vation  between  East  and  Gi'een  Streets,  is  another  large  and 
ill-defined  exposure  of  trap.  To  refer  these  two  outcrops  to 
one  great  east- west  dike  parallel  with,  or  possibly  a  dislocated 
portion  of,  the  boundary  dike,  is  a  tempting  generalization. 
The  only  difficulty  is  that  they  are  not  lithologically  very  simi- 
lar, the  more  eastern  outcrop  being  an  entirely  normal,  dis- 
tinctly crystalline  greenstone  diabase.  The  only  other  dike 
exposed  in  this  area  is  that  cutting  the  red  Cambrian  slate  and 
limestone  on  the  north  side  of  Mill  Cove  (see  PI.  20).  It  is 
a  well-defined,  regular  dike  of  greenstone  thirty  inches  wide,  of 
rather  abnormal  attitude,  trending  N.  40°  E.  and  hading  N.  W. 
40°.      Mr.  White  describes  it  as  highly  altered. 

Dikes  in  the  Area  between  JVeymoiUh  Fore  River  and 
the  Old  Colony  Railroad. — In  the  Monatiquot  Valley  belt 
of  Cambrian  strata  a  few  small  longitudinal  or  east-west  dikes 
have  been  observed,  some  of  which  have  been  noted  incidentally 
in  the  descriptions  of  the  Cambrian  outcrops.  In  the  railroad 
cut  east  of  Weymouth  Station,  about  six  feet  from  the  granite, 


519 

is  a  gray,  compact  dike  twelve  to  eighteen  inches  wide  and 
parallel  with  the  bedding  of  the  slate.  In  the  cutting  on 
Quincy  Avenue  south  of  the  railroad,  two  vertical  east-west 
dikes  of  similar  gray  comj^act  trap  traverse  the  slate  about  fifty 
feet  north  of  the  great  boundary  fault.  They  are  only  about 
one  foot  apart,  and  the  north  dike  averages  one  foot  wide  and 
the  south  dike  half  a  foot.  At  the  Jenkins  Rubber  Mills,  in 
the  green  slates  along  the  middle  of  the  valley,  is  a  nearly  ver- 
tical but  conformable  dike  about  one  foot  wide  trending  N. 
82°  E.  And  in  the  same  slates,  on  the  north  side  of  the  river, 
east  of  Commercial  Street,  we  have  exposed  at  intervals  for 
about  six  hundred  feet  what  appears  to  be  a  continuation  of 
this  dike  with  a  width  of  one  foot  or  less  and  perfectly  conform- 
able with  the  slates  :  but  it  is  here  highly  chloritized  and  largely 
replaced  by  vein  quartz.  North  of  this,  in  the  red  and  green 
slate  about  the  East  Braintree  Station,  two  east-west  dikes 
outcrop  obscurely,  trending  with  the  slate  N.  77°  E.  The 
southern  dike  is  of  normal  character  and  three  feet  wide,  but 
the  northern  dike,  under  the  railroad,  shows  much  alteration 
and  is  distinctly  amygdaloidal.  The  possibility  must  be  recog- 
nized that  alfof  these  narrow,  highly  altered,  conformable  dikes 
in  the  slates  may  be  pre-granitic. 

In  the  Wyman  Hill  area  of  fine  granite  separating  the 
Cambrian  strata  of  the  Monatiquot  Valley  and  Hayward  Creek, 
no  dikes  have  been  observed.  On  the  north  side  of  the  Para- 
doxides  Quarry,  at  the  mouth  of  Hayward  Creek,  a  vertical 
north-south  black  dike  about  six  feet  wide  cuts  the  massive 
flinty  slate  ;  and  in  the  next  quarry  immediately  to  the  west 
of  this  is  an  irregular  dike  ten  to  twenty  feet  wide  trending 
about  southeast.  Mr.  White  calls  this  a  decomposed  diabase 
in  which  the  augite  is  mostly  changed  to  chlorite,  giving  the 
rock  a  green  color.  Pyrite  is  present  in  microscopical  crystals, 
with  much  ilmenite  slightly  altered  to  leucoxene.  Following 
up  the  valley  of  Hayward  Creek  to  Echo  Quarry  at  the  base  of 
Payne's  Hill,  we  find  at  the   east   end  of  the   quarry  a   small 


N. 


Fig.  46.  —  A  north-south 
dike  of  diabase  in  the  nor- 
mal granite  northeast  of 
Payne's  Hill,  Quincy. 

Scale  ;  1  inch  =  16  feet. 


520 

but  veiy  typical  example  of  the 
north-south  dikes.  It  is  vex'tical, 
cross-jointed,  brown-weathering, 
from  three  to  twelve  inches  wide 
and  rather  crooked.  It  is  faulted 
about  ten  feet  by  the  very  perfect 
slickensided  joint  plane  described 
on  page  340.  In  the  principal 
quarry  next  northeast  of  Echo 
Quarry,  in  the  direction  of  Quin- 
cy Avenue,  are  two  larger  but 
equally  typical  dikes  of  the 
black,  brown-weathering  dia- 
base. They  have  the  normal 
north-south  trend,  are  only 
twenty  to  thirt}^  feet  apart,  and 
are  beautifully  cross-jointed. 
The  east  dike  is  best  exposed, 
is  two  feet  in  normal  width, 
vertical  and  curiously  irregular 
as  shown  in  the  accompanying 
sketch  (Fig.  46),  which  shows 
it  to  be  virtually  two  dikes. 
The  second  or  west  dike  hades 
east  some  thirty  degrees  and 
seems  to  be  narrower  than  the 
other.  They  are  probably  united 
below  ;  and  the  west  dike  may 
be  simply  the  oiFset  southern 
extension  of  the  western  mem- 
ber of  the  east  dike. 

The  large  dike  of  finely  crys- 
talline greenstone  diabase  sepa- 
rating; the  slate  and  fine  gran- 
ite  alona'   the    straio-ht   northern 


521 

fault  margin  of  the  Eldridge  Hill  Cambrian  area  has  been 
described  on  page  443.  Mr.  White  describes  it  as  very  highly 
decomposed  and  chloritized.  Mr.  Watson  has  traced  this  dike 
westward  through  the  normal  gi-anite  nearly  to  Quincy  Avenue. 
It  appears  to  be  shifted  to  the  northward  somewhat,  as  if  by  an 
oblique  foult,  on  the  line  of  Quincy  Avenue.  Beyond  the 
avenue,  Mr.  Watson  has  followed  it  in  frequent  outcrops, 
showing  a  breadth  of  thirty  feet,  to  Phipps  Street,  where  it 
seems  to  end  at  a  point  just  south  of  the  double  angle  in  the 
street.  But  about  six  hundred  feet  farther  up  the  hill,  on 
Phipps  Street,  Mr.  Watson  has  found  what  appears  to  be  a 
displaced  continuation  of  the  same  dike,  agreeing  closely  in 
width,  trend  (E.-W.)  and  lithological  character.  This  dis- 
placed member  of  the  dike,  if  such  it  be,  is  exposed  on  the 
west  side  of  the  street  for  about  a  hundred  feet,  and  for  twenty 
or  thirty  feet  on  the  east  side;  but  beyond  that,  eastward,  on 
the  same  direct  line  are  granite  ledges,  with  no  trace  of  the 
dike  in  either  outcrops  or  bowlders.  It  is  probable  that  we 
have  here  an  original  jog  or  offset  in  the  dike  fissure,  rather 
than  a  true  displacement  or  fault  subsequent  to  the  formation 
of  the  dike. 

Where  the  fine  granite  on  the  south  side  of  the  Puo-o-les 
Creek  valley  has  been  quarried  on  the  line  of  South  Street,  it 
is  traversed  by  another  large  east- west  dike,  very  similar  in 
breadth  and  character  to  the  Eldridge  Hill  dike.  It  is  exposed 
for  several  hundred  feet ;  but  Mr.  Watson  and  I  have  been 
unable  to  find  another  indubitable  outcrop  on  this  line,  although 
three  or  four  hundred  feet  farther  west  there  is  a  small  expo- 
sure of  trap  which  is  probably  a  bowlder  but  possibly  a  ledge. 
The  only  other  dike  observed  in  the  Ruggles  Creek  Valley  is 
at  the  extreme  west  end  of  the  slate  belt.  It  belongs  to  the 
north-south  series,  is  about  eight  feet  wide,  and  cuts  directly 
across  the  slate  a  hundred  to  a  hundred  and  fifty  feet  east  of 
New  Road  ;  but  it  cannot  be  traced  either  way  into  the  granite, 
seeming^  to  end  as^ainst  the  faults  boundinsf  the  slate. 


522 

No  dikes  have  been  noted  in  the  granite  north  of  Ruggles 
Creek  :  but  north  of  the  granite,  in  the  Union  Street  Cambrian 
outcrops  is  another  important  east-west  dike.  It  is  about 
ten  feet  wide,  and,  crossing  Union  Street  at  the  corner  of 
Edwards  Street,  can  be  traced  eastward,  parallel  with  the  strike 
of  the  slate,  for  nearly  four  hundred  feet.  This  dike  and  the 
greenstone  dike  near  the  Paradoxides  Quarry  might  be  regarded 
as  pre-granitic,  but  it  appears  best  to  class  them  with  the  very 
similar  Eldridge  Hill  and  Ruggles  Creek  dikes,  which  are  very 
clearly  post-granitic. 

Dikes  in  the  Pine  Hill  and  JSTorth  Common  Hill  A.rea. — 
Three  dikes  only  have  been  observed  in  the  Pine  Hill  area,  or 
in  the  angle  between  the  railroad  on  the  northeast  and  Willard 
and  West  Streets  on  the  northwest.  Advancing  northward, 
the  first  dike  is  a  very  important  member  of  the  east- west  series, 
cutting  the  normal  gray  granite  along  the  line  midway  between 
the  quarries  of  I'ed  granite  on  the  southern  margin  of  this  tract 
and  the  quarry  of  gray  granite  on  the  northeastern  margin, 
opposite  the  end  of  Liberty  Street.  Where  most  clearly 
exposed,  the  dike,  which  is  about  thirty-five  feet  wide  and 
hades  N.  30°  to  40°,  forms  the  bottom  of  a  well-defined  chasm, 
fifteen  to  twenty  feet  deep  and  trending  N.  75°-  80°  W.  It  is  a 
dark  green,  crystalline  diabase,  showing  a  very  normal  grada- 
tion in  texture  from  the  middle  toward  the  walls,  and  quite 
distinctly  cross-jointed.  According  to  Mr.  White  the  ophitic 
texture  is  strikingly  developed  ;  the  plagioclases  are  in  lath- 
shaped  forms,  sometimes  fibrous,  and  of  a  greenish  color;  the 
interspaces  are  filled  by  masses  of  a  handsome  pink  augite  :  and 
iron  ore  is  abundant,  chiefly  in  grains  of  irregular  outline,  hence 
presumably  ilmenite.  The  chasm  is  a  distinct  topographic 
feature  for  some  three  hundred  feet ;  and  beyond  this,  east- 
ward, the  dike  can  be  traced  by  the  drift  for  about  a  quarter  of 
a  mile,  or  nearly  to  the  margin  of  the  swamp  which  here  bor- 
ders the  Pine  Hill  tract  on  the  east.  At  the  west  end  of  the 
chasm  the  trend  of  the  dike  changes  abruptly  to  N.   25°  W. 


523 

and  continues  with  the  same  breadth  and  hade  as  before  for 
fully  three  hundred  feet  farther,  where  it  is  lost  beneath  a  well- 
defined  and  swampy  transverse  depression,  which  probably 
marks  a  fault,  for  no  trace  of  the  dike  could  be  found  in  the 
rocky  northwestern  wall  of  the  depression.  Going  northerly 
from  this  point  we  come,  just  before  reaching  the  quarry  oppo- 
site the  end  of  Liberty  Street,  to  the  second  dike,  which,  having 
about  the  same  trend  (N.  25°  W.)  as  the  first  dike  where  it 
disappears  beneath  the  swamp,  may  be  regarded  as  possibly 
the  displaced  continuation  of  it.  It  is  a  true  greenstone 
diabase,  very  compact  on  the  borders,  and  sends  branches  into 
the  bordering  granite  and  porphyry.  It  is  forty  to  fifty  feet 
wide,  and  is  exposed  along  the  northeastern  margin  of  the  Pine 
Hill  tract,  parallel  with  the  railroad,  for  nearly  two  thousand 
feet.  Farther  to  the  northwest  it  is  clearly  cut  off  by  the 
normal  granite,  being  first  shifted  to  the  west  several  hundred 
feet.  The  third  dike,  having  approximately  the  same  aberrant 
trend,  cuts  the  basic  porphyry  near  the  eastern  end  of  the 
street  running  into  the  Pine  Hill  tract  from  the  junction  of 
Willard  and  West  Streets.  It  is  of  the  same  character  as  the 
preceding  dikes,  but  not  as  wide.  Obviously  some  cause  has 
operated  to  give  the  east-west  dikes  distinctly  abnormal  trends 
in  the  main  part  of  the  Pine  Hill  tract ;  and  this  cause  is,  no 
doubt,  closely  related  to  or  dependent  upon  the  northwesterly 
trend  of  the  bands  of  basic  porphyry  and  fine  granite  which  we 
suppose  to  indicate  the  former  extension  of  the  Pine  Hill 
slate  belt. 

Northeast  of  the  Pine  Hill  tract,  between  the  railroad  and 
Center  Street,  is  the  hill  on  which  Wilson's  Quarry  is  situated. 
A  north-south  dike  one  foot  wide  crosses  the  east  wall  of  the 
quarry,  hading  W.  5°.  It  is  black,  weathering  brown,  com- 
pact in  texture  and  cross-jointed.  About  ten  feet  east  of  this 
dike  is  its  exact  duplicate  in  every  respect,  but  not  so  well  ex- 
posed. 

Crossing  the  valley  northward,  we  find  in  the  numerous  quar- 


524 

I'ies  of  North  Common  Hill  several  interesting  dikes.  Com- 
mencing with  the  Hardwick  Quarry,  on  the  north  side  of 
Quarr}'  Street,  we  find  that  a  north-south  dike  forms  its  eastern 
wall  and,  crossing  the  ridge,  forms  also  the  eastern  Avail  of  the 
next  quarry  to  the  north.  It  is  normally  nearly  three  feet 
wide,  trends  almost  due  north-south,  and  hades  W.  about  5°. 
After  the  manner  of  its  class,  it  is  distinctly  cross-jointed  ;  but 
it  belongs  lithologically  with  the  east- west  dikes,  its  dark  green 
color  indicating  a  good  degree  of  chloritization.  The  trap  is 
very  compact  and  slaty,  and  minutely  pseudo-amygdaloidal  with 
calcite,  which,  weathering  out,  leaves  the  rock  cellular,  the 
cavities  being  distinctly  angular.  The  calcite  is  evidently  sec- 
ondary and,  probably,  a  necessary  concomitant  of  the  chloriti- 
zation of  the  original  augite.  The  dike,  although  very  regular 
in  form,  is  really  a  series  of  beautiful  lenses,  tapering  to  a 
thin  edge  in  both  quarries.  It  is  exactly  parallel  to  a  system 
of  north-south  joints  in  the  granite,  and  incloses  long,  thin 
sheets  of  granite  one  fourth  of  an  inch  to  three  inches  thick 
and  parallel  to  the  walls  of  the  dike.  These  inclusions  are  ex- 
tremely dike-like,  suggesting  at  first  dikelets  of  granite  cutting 
Cambrian  slate  ;  but  a  close  examination  shows  that  this  cannot 
be  the  case. 

Immediately  beyond  the  Hardwick  Quarry,  on  the  south 
side  of  Quarry  Street,  is  a  large  quarry  at  right  angles  to 
the  street.  Across  the  street  end  of  this  quarry  is  another 
north-south  dike.  It  is  really  a  double  or  composite  dike, 
consisting  of  two  dikes,  each  about  a  foot  wide  and  separated 
by  about  a  foot  of  granite ;  and,  as  before,  it  hades  west 
about  5°.  It  is  the  same  compact,  slaty,  greenish  chloritic 
trap  ;  and  yet  both  members  of  the  dike  are  beautifully  cross- 
jointed.  On  the  north  wall  of  this  quarry  is  a  dike  of 
slaty,  greenish  trap,  four  feet  wide,  which  trends  N.  80°  W. 
and  hades  S.  about  10°.  It  is  much  broken,  but  sliows 
some  columnar  jointing.  In  the  Lombard  Quarry  immediately 
northwest  of  this,  the  same  dike  reappears,  forming  the  south- 


525 

ern  wall.  It  is  admirably  exposed,  is  four  feet  wide,  cross- 
jointed,  trends  exactly  E.-W.  and  hades  S.  10°.  The  north 
wall  of  this  quarry  is  formed  of  a  dike  of  the  same  chai'acter  and 
trend,  but  hading  N.  5°.  Passing  several  quarries  which  afford 
superb  examples  of  parallel  and  master  joints,  we  come  to  the 
only  other  dike  observed  in  the  quarries  on  this  hill.  It  is  the 
small  north-south  dike  crossing  the  middle  of  the  Craig  and 
Richards  Quarry,  on  the  northeast  side  of  Quarry  Street.  It 
is  six  to  possibly  eighteen  inches  wide,  irregular,  and  divided 
on  the  north  wall  of  the  quarry  into  two  overlapping  parts.  It 
trends  N.  10°  W.  and  hades  W.  5°.  The  east  and  west  ends 
of  the  quarry  are  broad,  flat  and  very  rusty  joint  faces  exactly 
parallel  with  this  dike.  This  dike  is  especially  remarkable  for 
its  deep  and  complete  kaolinization,  to  which  my  attention  was 
first  called  by  Professor  George  H.  Barton.  In  fact,  it  is,  in 
this  respect,  rather  unique  among  the  dikes  of  the  Boston  Basin. 
On  account  of  its  small  size  it  is  naturally  very  fine-grained 
and  compact;  and  to  the  greatest  depth  of  the  quarry,  nearly 
fifty  feet,  it  is  more  or  less  completely  decomposed  to  a 
greenish  and  brownish  earth.  The  chemical  decay  is,  how- 
ever, somewhat  localized,  being  most  marked  in  the  central  and 
southern  part  of  the  quarry,  where  a  coarsely  crystalline  vein 
of  quartz  occupies  the  same  fissure.  The  vein,  which  is  not 
clearly  exposed,  appears  to  present  a  series  of  pockets,  yielding 
well-formed  crystals  of  quartz  from  one  to  several  inches  in 
diameter,  and  affording  meteoric  waters  easy  access  to  relatively 
deep  portions  of  the  dike.  Among  the  residuary  products  of 
the  decay  of  the  dike,  decidedly  the  most  striking  and  interest- 
ing is  the  snow-white  and  plastic  kaolin  which  appears  to  belong 
normally  between  the  quartz  vein  and  the  green  residuaiy  earth 
or  clay  representing  the  main  part  of  the  dike.  Where  most 
perfectly  developed  and  when  freshly  dug,  the  kaolin  is  not  only 
perfectly  white  but  as  soft  and  plastic  as  butter;  with,  appar- 
ently, a  rather  sharp  line  of  demai'cation  between  it  and  the 
green  clay  or  earth.      Its  distribution  seems  to  be  very  localized 


526 

or  bunchy  :  and  it  has  a  thickness  in  some  cases  of  several 
inches.  That  the  kaolin  really  is  a  product  of  the  greenish  black 
trap  appears  certain.  The  quartz  vein  shows  no  sign  of  a  peg- 
matitic  character,  that  is,  of  carrying  feldspar  which  might  have 
yielded  the  kaolin  ;  and  although  the  enclosing  granite  is  some- 
what decomposed  in  places,  as  would  be  expected,  there  is  no 
indication  of  a  passage  into  the  kaolin  ;  and,  furthermore,  the 
kaolin  is  absolutely  free  from  grains  of  quartz  or  any  gritty 
material.  The  following  p.nalyses  were  made  in  1894  by  Miss 
H.  L.  Gates,  then  a  student  in  the  Massachusetts  Institute  of 
Technology. 

A  soft,  plastic,  freshly  dug  sample  of  the  white  kaolin  lost 
26.38  per  cent,  of  water  by  drying  in  the  hot  closet  for  one 
week,  and  then  yielded  :  — 

Silica  (SiO.)  45.44  per  cent. 

Aluminum  sesquioxide  (Al^Og)  37.85         " 

Water  (H,0)  ^  13.93 


97.22 

A  proper  distribution  of  the  loss  error  would  make  this  a  very 
close  approximation  to  the  normal  composition  of  kaolin  :  and 
it  is  evident  that  the  process  of  kaolinization  has  here  accom- 
plished its  perfect  work.  The  kaolin  becomes  quite  hard  and 
firm  by  ordinary  drying  ;  and  such  an  aii'-dried  sample  was  also 
analyzed,  yielding  essentially  the  same  results  as  the  plastic 
sample.  The  associated  greenish  clay,  free  from  grit  and  cut- 
ting with  a  knife  like  cheese,  gave  on  analysis  :  — 

Silica  (SiO.2)  51.44  per  cent. 

Aluminum  sesquioxide  (xVl.Os)  35.21         " 

Iron  oxide  (estimated  as  F.^Og)  1.14         " 

AVater  (HoO)  -      6.62 


94.41 
The  silica  and  water  both  suggest  incomplete  kaolinization 


527 

and  the  probable  presence  of  residual  portions  of  protoxide  bases 
(alkalies  or  alkaline  earths)  which  were  not  determined.  The 
coloring  agent  is  undoubtedly  iron  in  ferrous  form. 

The  unusual  features  of  this  occurrence  of  residuary  earth 
are:  (1)  the  extent  and  completeness  of  the  decomposition; 
(2)  the  prevailing  green  or  ferrous  oxide  color  of  the  residue, 
instead  of  the  brown  tints  commonly  observed  in  the  decomposed 
rocks  and  residuary  earths  of  this  region  and  due  to  the  peroxi- 
dation of  the  iron  :  and  (3)  the  complete  bleaching  of  a  part  of 
the  residuary  kaolin  contiguous  to  the  quartz  vein.  The  condi- 
tion of  the  outcrop  of  this  dike  before  the  opening  of  the  quarry 
is  not  apparent ;  but  the  rather  unique  features  of  the  decom- 
posed part  of  the  dike  seem  to  find  a  ready  explanation  in  the 
not  improbable  assumption  that  the  surface  was  locally  somewhat 
of  a  swampy  character.  This  condition,  in  connection  with  the 
open  vein  of  quartz,  insures  (1)  an  abundance  of  carbonic  acid 
and  the  various  humus  acids  for  the  kaolinization  of  the  trap, 
(2)  the  organic  matter  requisite  to  prevent  the  peroxidation 
of  the  iron,  and  thus  to  permit  its  solution  and  removal  as  a  car- 
bonate or  organic  salt. 

Dilces  in  the  West  Qaincy  Quarry  District. — In  the 
numerous  quarries  and  ledges  of  this  area  four  dikes  of  the 
east-west  series  have  been  observed,  but  none  that  can  be 
referred  to  the  north-south  system.  The  most  southerly  dike 
is  also  the  most  important  in  length  of  outcrop,  being  traceable 
for  nearly  a  mile.  It  first  appears  along  the  north  side  of  the 
branch  of  the  Quarry  Railroad  which  runs  oflf  to  the  eastward 
toward  the  Bunker  Hill  Quarry.  The  outcrop  continues  for 
several  hundred  feet,  trending  N.  75°-80°  W.,  but  does  not 
show  the  hade  or  full  width  of  the  dike.  Proceedino;  westward 
on  the  line  of  the  dike,  we  find,  north  of  the.  main  line  of  the 
Quarry  Railroad,  and  some  five  or  six  hundred  feet  beyond 
the  junction,  considerable  diabase  in  the  drift,  of  similar  char- 
acter to  the  dike,  and  again  nearly  a  quarter  of  a  mile 
farther    west    and    a    few   rods    northwest    of  the    Jones    and 


528 

Desmond  Quarry.  The  dike  next  appears  beyond  Blueberry 
Swamp,  in  the  northern  pai't  of  the  Fitzgerald  and  Lyons 
Quarry,  with  a  thickness  of  eight  to  ten  feet  and  hading  N.  20°. 
Its  next  and  last  appearance  is  on  the  south  side  of  the  Fuller 
and  Clarke  Quarries,  with  the  same  hade  ;  but  the  full  width 
is  not  exposed.  That  all  of  these  exposures  are  referable  to 
one  dike  is  further  indicated  by  their  lithologic  uniformity. 
The  rock  is,  throughout,  the  normal  dark  green  diabase  with 
very  compact  or  slaty  margins. 

The  second  dike  of  this  series  is  the  small  example  forming 
the  southern  walls  of  the  Berry  Quarries,  some  eight  hundred 
feet  southeast  of  the  Granite  Railway  Quarry.  It  is  only  from 
six  to  twelve  inches  thick,  trends  N.  85°  W.,  and  hades  X. 
15°.  The  third  dike,  which  is  also  the  largest  and  most  con- 
spicuous, is  that  forming  the  southern  wall  of  the  Granite 
Railway  Quarry.  It  trends  X.  70°  W.,  hades  JST.  30°  to  35°, 
and  is  about  fifteen  feet  thick.  Lithologically  it  is  a  very  normal 
east-west  dike.  Converging  downwards  toward  this  dike,  with 
a  hade  S.  25°,  is  the  strongly  marked  shear  and  probable  fault 
plane  described  on  page  339.  This  is  simply  one  striking 
example  among  many  master  joints  of  southerly  hade  which 
serve  by  contrast  to  emphasize  the  prevailing  northerly  hade  of 
the  dikes  for  the  entire  southern  mai'gin  of  the  Boston^  Basin. 
The  fourth  and  last  dike  of  this  series  crosses  the  extreme  north 
end  of  the  quarry  on  the  west  side  of  Willard  Street,  north  of 
West  Quincy  Station.  It  is  about  four  feet  wide,  is  very 
regular,  trends  almost  due  east-west,  and  hades  N.  about  10°. 
It  is  nearly  black  in  color,  but  not  deeply  weathered. 

Dikes  in  the  Ma{7i  Range  of  the  Slue  Hills. — This  sec- 
tion may  be  very  brief;  for  in  the  Blue  Hills  proper,  as  pre- 
viously stated,  there  are  practically  no  dikes.  In  fact  it  is 
probably  the  most  nearly  dikeless  area  in  the  Boston  Basin  ; 
and  in  the  entire  area  south  of  the  Pine  Tree  Brook  belt  of  slate 
and  west  of  Pine  Hill  and  Willard  Street  I  have  noted  but  one 
example.      This  is  a  large  and   imperfectly  exposed   body   of 


529 

diabase  in  the  extreme  northeast  corner  of  the  area,  contiguous 
to  the  West  Quincy  quarry  district,  to  which  it  might  properly 
be  referred.  The  chief  outcrop  forms  the  south  side  and  sum- 
mit of  the  small  hill  known  as  Babel  Rock,  or  bearing  a 
granite  bowlder  distinguished  by  that  name,  south  of  Old 
Furnace  Brook  and  west  of  the  junction  of  Willard  and  West 
Streets.  The  breadth  of  the  outcrop  would  indicate  an  east- 
west  dike  some  two  hundred  feet  or  more  wide  ;  and  that  it 
really  belongs  to  the  east-west  series  is  clear  from  its  lithologi- 
cal  character  —  fine-grained  greenstone  —  and  from  other  out- 
crops to  the  westward.  The  first  one  of  these  is  about  five 
hundred  feet  distant,  on  the  north  side  of  the  road  leading  to  the 
Rattlesnake  Hill  quarries.  A  little  more  than  a  quarter  of  a 
mile  beyond  this,  in  a  west-northwest  direction,  on  the  south 
border  of  Furnace  Brook  swamp  and  on  an  island  in  the 
swamp  crossed  by  the  path  to  Sawcut  Notch,  is  more  trap,  so 
distributed  as  to  bear  out  the  theory  of  a  large  east- west  dike. 
Farther  west  and  east  the  outcrops  are  all  granite,  and  the  dike 
cannot  be  traced. 

Dikes  along  the  Northern  Border  of  the  Blue  Hills  Com- 
jplex.  —  The  most  westerly  dike  of  which  we  have  any  knowledge 
along  this  line  is  the  great  boundary  dike  of  the  Randolph  Avenue 
area  described  on  pages  438  and  478.  It  outcrops  on  Randolph 
Avenue  wuth  a  breadth  of  a  hundred  and  twenty-five  feet ;  and 
it  can  be  traced  westward  a  thousand  feet  to  Reedsdale  Road, 
and  eastward  a  little  more  than  two  thousand  feet  to  Pleasant 
Street,  where  its  breadth  is  reduced  to  about  forty  feet.  It 
has  a  strong  northerly  hade,  and  its  normal  trend  is  north 
of  east ;  but  east  of  Gun  Hill  Road  it  apparently  shifts  to  a 
south-of-east  trend,  the  only  alternatives  being  to  refer  the 
Pleasant  Street  outcrop  to  a  branch  or  to  an  independent  and 
narrower  dike.  The  view  that  both  of  the  east-west  systems 
are  represented  here  by  independent  dikes  is,  perhaps,  the 
best,  everything  considered.  It  derives  strong  support  from 
the  fact  that  in  the  granite  south  of  the  junction  of  Pleasant 

OCCAS.  PAPERS  B.  S.  J^.  H.  IV.  34. 


530 

Street  and  Gun  Hill  Road  is  another  dike  with  approximately 
the  same  south-of-east  trend.  It  is  a  typical  greenstone,  forty- 
five  to  fifty  feet  wide,  which  is  being  worked  at  the  pres- 
ent time  for  road  metal.  Its  trend  intersects  Pleasant  Street 
at  a  point  six  hundred  feet  west  of  Gun  Hill  Road.  Farther 
east  these  south-of-east  dikes,  as  well  as  the  north-of-east  Ran- 
dolph Avenue  dike,  supposing  that  to  be  independent,  are  lost 
beneath  the  drift ;  but  that  the  former  are  in  some  way  repre- 
sented in  the  south-of-east  dikes  of  the  West  Quincy  quarry 
district  seems  most  probable.  Among  the  ledges  of  Cambrian 
slate  and  granite  in  the  valley  of  Furnace  Brook  south  of  Edge 
Hill  Road  there  are  indications  of  a  large  east- west  dike.  One 
of  the  ledges  in  the  complex  of  granite  and  Cambrian  slate  south 
of  Furnace  Brook  and  west  of  the  Granite  Branch  Railroad, 
shows  an  east-west  dike  eis^hteen  inches  wide  cutting-  the  slate. 
This  appears  to  exhaust  the  exposed  dike  phenomena  along  this 
border. 

Dikes  in  the  Rock  Island  Anticline.  —  This  broken  fold 
may  be  considered  to  begin  on  the  west  in  the  outcrops  of  con- 
glomerate and  bordering  slate  in  the  valley  of  Furnace  Brook 
west  and  east  of  the  Old  Colony  Railroad  ;  and  in  its  eastern 
extension  it  embraces  the  ledges  of  Houo-h's  Neck  and  Raccoon 
Island.  Between  the  railroad  and  Hancock  Street  an  east-west 
dike,  widening  eastward  from  two  to  four  feet,  runs  through 
the  conglomerate  and  slate,  as  previously  noted.  The  ledge  of 
slate  nearly  three  hundred  feet  wide  on  the  road  to  Hough's 
Neck  is  bordered  on  the  north  and  south  by  obscurely  exposed 
east-west  dikes,  as  described  on  page  486. 

So  far  as  known,  no  part  of  either  the  Blue  Hills  Complex 
or  the  contiguous  Carboniferous  areas  is  so  much  cut  up  by 
dikes  or  presents  such  an  interesting  group  of  fissures  in  a  lim- 
ited area  as  the  western  half  of  Rock  Island  (PI.  27)  ;  and  it 
is  a  significant  fact  that,  as  the  map  so  plainly  shows,  the  dikes, 
or  at  least  their  outcrops,  are  almost  entirely  confined  to  the 
great  bed  of  melaphyre.     Still  more  curious  are  the  facts  that 


531 

in  their  relations  to  the  melaphyre  and  the  geological  structure 
generally  they  are  neither  distinctly  longitudinal  nor  transverse  ; 
and  that,  while  showing  little  close  agreement  in  trend,  they 
are  all  confined  to  the  northwest  and  southeast  quarters  of  the 
compass.  The  hade  of  the  dikes  is,  with  one  or  two  vertical 
exceptions,  distinctly  to  the  north  and  east.  Lithologically 
they  ai-e  nearly  all  normal  greenstone ;  the  one  important 
exception  being  the  great  dike  (No.  12).  This  weathers 
brown  and  has  the  general  facies  of  the  north-south  dikes.  It 
is  sixty  and  possibly  seventy  feet  wide,  of  uncertain  hade,  and 
cuts  both  the  melaphyre  and  conglomerate,  as  well  as  several 
of  the  greenstone  dikes.  So  far  as  known,  this  is  the  largest 
dike  of  ~the  north-south  system  on  the  south  side  of  the  Boston 
Basin.  The  small  dike  (No.  11)  agreeing  most  nearly  in 
trend  with  the  great  dike  should  probably  be  referred  to  the 
same  system.  All  the  remaining  dikes  should,  apparently,  be 
referred  to  the  east-west  systems,  although  several  of  them  are 
quite  aberrant  in  trend.  The  east-west  dikes  are  more  or  less 
interrupted  by  transverse  faults,  as  the  map  shows  ;  but  no 
clear  intersection  of  dikes  of  this  system  has  been  observed. 
The  apparent  intersection  of  dikes  3  and  4  is  quite  possibly  a 
branching.  This  explanation  is  suggested  by  the  facts  that 
both  dikes  are  porphyritic  and  that  they  are  the  only  por- 
phyritic  dikes  in  the  entire  area. 

The  Rock  Island  dikes  have  been  studied  microscopically  by 
Dr.  J.  E.  Wolff'  in  connection  with  the  melaphyre.  He  de- 
scribes the  great  north-south  dike  (12)  as  a  coarse-grained, 
dark  green  rock  containing  crystals  of  feldspar,  pyrite,  magne- 
tite and  hornblende,  in  a  dark  green  groundmass.  The  section 
shows  rather  large  feldspar  crystals  ;  fibi-ous,  greenish,  dichroic 
hornblende ;  crystals  of  magnetite  and  pyrite ;  decomposed 
crystals  of  olivine  ;  epidote  ;  and  viridite,  quartz,  apatite,  etc. 
The  feldspars  are  to  a  great  extent  kaolinized.  The  hornblende 
occurs  in  irregular  masses  and  contains  a  great  deal  of  epidote 

1  Bull.  Mus.  comp.  zool.,  vol.  7,  pp.  231-242. 


532 

in  rounded  grains.  Some  of  the  feldspar  crystals  lie  imbedded 
in  the  hornblende,  or  cross  it,  just  as  they  do  in  the  case  of 
the  augite  of  the  less  decomposed  diabases,  so  that  this  and  the 
whole  character  of  the  hornblende  indicate  that  it  is  (in  part  at 
least)  a  product  of  the  decomposition  of  the  original  augite. 
The  olivine  occurs  generally  in  shattered  crystals,  with  the 
usual  blackened  borders.  The  magnetite  is  found  in  extremely 
irregular  forms,  while  the  pyrite  grains  often  contain  magnetite, 
and  therefore  arise  probably  from  its  decomposition. 

The  large,  coarsely  porphyritic  dike  (No.  4)  is  described  as 
consisting  of  a  grayish  green  groundmass  holding  crystals  of 
greenish  feldspar  and  grains  of  pyrite.  The  section  shows  the 
rock  to  be  much  altered.  The  feldspars  retain  their  outlines, 
but  are  filled  with  chloritic  material,  kaolin,  epidote  and  calcite. 
Magnetite  and  menaccanite  are  very  plentiful  in  crystalline  and 
irregular  forms.  Pyrite  occurs  in  occasional  grains  and  square 
crystals,  generally  close  to  or  mingled  with  the  iron  oxides,  and 
is  therefore  probably  an  alteration  product.  The  remaining 
portion  of  the  rock  is  a  confused  mixture  of  chlorite,  epidote, 
quartz,  viridite,  hornblende,  calcite,  and  colorless  needles,  in 
part  probably  apatite,  — all  products  of  alteration.  Dr.  Wolff 
adds,  "  This  rock  is  the  most  coarsely  crystalline  and  the  most 
decomposed  of  any  examined." 

A  non-porphyritic  east-west  dike  is  described  as  a  greenish 
gray,  felty-looking  rock,  containing  minute  grains  of  pyrite  and 
small  feldspar  crystals,  and  traversed  by  veinlets  of  epidote. 
The  section  shows  white,  opaque,  feldspar  crystals  and  masses  of 
opacite,  magnetite,  and  pyrite,  in  a  green,  chloritic  groundmass. 
The  feldspars  have  generally  the  long  ledge  form  of  the  basaltic 
triclinic  feldspars,  but  occasionally  the  form  of  Carlsbad  twins 
of  sanidin.  They  are  entirely  altered  to  a  fibrous  and  scaly 
aggregate.  Colorless  needles  of  apatite  occur  occasionally  in 
the  feldspars,  and  also  aggregates  of  quarts.  Between  the 
feldspars  lies  a  mass  of  green  fibrous  products, — chlorite,  viri- 
dite, etc.,  considerable  epidote,  magnetite,  quartz,  etc.,  rarely 


hematite  and  biotite.      The  feldspars  occasionally  have  a  fluidal 
arrangement. 

Dikes  of  Hock  Island^  IIougKs  Neck. 


Width 

No. 

Trend. 

Hade. 

in  feet. 

Remai'lis. 

1 

N.39"  W. 

Vertical. 

0.75 

Wholly  in  conglomerate. 

2 

N.  65°  W.  to 
K  86°  W. 

N.  E.  0°-5°. 

2-3.5 

Eatilted  and  cut  by  No.  12. 

3 

N.  85'-  W. 

N.  20°. 

3 

Sparingly  porphyritic;  cut  by 
No.  4  or  possibly  a  branch. 

4 

N.  56°  W. 

N.  E. 15°-20° 

15-20 

Coarsely  porphyritic  on  south 
side. 

5 

N.  62°  W. 

N.  E.  5°. 

7 

Faulted  to  left,  60  feet. 

6 

N.  52°  W.  to 
N.  61°  W. 

N.  E.  22°. 

5 

Faulted  and  cut  by  No.  12. 

7 

N.  50°  W. 

N.  E.  27°. 

1-1.3 

Only  one  outcrop. 

8 

N.  32°  W. 

N.  E.  15°-20°. 

16 

Probably  faulted. 

9 

N.  32°  W. 

N.  E.  10°. 

3.5 

Probably  faulted  twice. 

10 

N.  45°  W. 

? 

'? 

Irregular  and  not  well  exposed. 

11 

N.  23°  W. 

N.  E.  10°-15°. 

2-3 

Exposed  for  50  feet  ;  dies  out 
southward. 

12 

N.  20°  AV. 

? 

60-70 

Brown-weathering  ;  cuts  Nos. 
2  and  6.         > 

No  dikes  have  been  observed  in  either  the  melaphyre  or  con- 
glomerate in  the  eastern  half  of  Rock  Island,  nor  in  the  Rock 
Island  Head  ridge  of  melaphyre.  There  are,  however,  two 
east- west  dikes  on  Raccoon  Island.  They  are  exactly  parallel 
with  the  bedding  of  the  slate  (trend  IST.  85°  W.  and  hade  N". 
0°-5°)  and  run  directly  through  the  middle  of  the  island,  under 
the  house,  from  the  extreme  easterly  point  to  the  west  shore. 
They  are  composed  of  normal,  fine  greenish  trap  ;  and  are  fif- 
teen feet  apart  east  of  the  house  and  about  thirty  feet  west  of 
the  house.  The  south  dike  averages  about  one  foot  wide,  and 
the  north  dike  varies  from  two  feet  to  nearly  five  feet,  averaging 
about  three  feet. 


534 


POST-CARBONIFEROUS   EROSION   OF  THE  BLUE 
HILLS    COMPLEX. 

"  The  hills  are  shadows,  and  they  flow 

From  form  to  form,  and  nothing  stands ; 
They  melt  like  mist,  the  solid  lands, 
Like  clouds  they  shape  themselves  and  go." 

Structurally,  the  Blue  Hills  were  now  complete  in  the  two 
principal  fault-blocks,  extending  from  Weymouth  Back  River 
to  the  Neponset  River,  out  of  which  they  were  to  be  carved. 
Each  of  these  blocks  embraced  in  vertical  section  from  the  base 
upward  :  (1)  the  batholite  of  normal  granite  with  its  covering  of 
fine  granite  and  quartz  porphyry;  (2)  an  unknown  and  vari- 
able thickness  of  the  strongly  folded  Cambrian  slates  ;  (3)  over 
a  part  of  the  area  at  least  a  remnant  of  the  effusive  felsites  ; 
and  (4)  several  thousand  feet  of  Carboniferous  strata.  The 
eastern  block,  having  been  lifted  bodily,  without  important  tilt- 
ing, presented  a  plateau  character  ;  but  the  strongly  tilted  west- 
ern block  must  have  formed  from  the  beginning,  a  sharp- 
crested  monoclinal  mountain  range.  We  thus  account  for 
the  existing  topographic  contrast  of  the  two  blocks,  and  learn 
that,  prominent  as  the  Blue  Hills  are  in  the  modern  landscape, 
they  possess  scarcely  a  tithe  of  their  original  mass  and  height. 

So  far  as  we  know,  this  area  has  been  above  sea-level  con- 
tinuously since  the  Appalachian  revolution,  with  the  exception 
of  a  brief  partial  submergence  during  the  close  of  the  glacial 
epoch.  There  are  no  indications  that  the  sea  invaded  eastern 
Massachusetts  in  Triassic  times,  when  the  Connecticut  Val- 
ley was  a  Bay  of  Fundy  ;  nor,  apparently,  did  the  elevation 
and  faulting  of  the  Triassic  beds  add  to  the  complexity  of  our 
local  geology.  The  Appalachian  revolution,  as  previously 
noted,  gave  eastern  Massachusetts  mountain  relief,  which  ex- 


535 

plains  the  non-occurrence  of  later  sediments  ;  and  it  also  devel- 
oped a  high  degree  of  rigidity  in  this  part  of  the  earth's  crust, 
which  has,  at  least  in  large  measure,  precluded  subsequent 
flexures  and  fractures.  The  grand  result  accomplished  by  post- 
Carboniferous  erosion  has  been  the  substantial  efFacement  of 
the  Appalachian  reliefs  and  the  sweeping  away  of  the  entire 
Carboniferous  series  from'  many  hundreds  of  square  miles  of 
the  granitic  and  Cambrian  floor  on  which  it  was  deposited, 
these  strata  remaining  now  only  in  the  deepest  troughs  into 
which  they  were  down-folded  and  faulted  during  the  Appalach- 
ian readjustment.  That  the  subjacent  floor  has  itself  been 
deeply  planed  down  is  obvious  from  the  fact  that  even  the  axes 
of  the  folds  which  must  once  have  existed  outside  the  present 
basins  can,  as  a  rule,  no  longer  be  traced.  We  have  one  im- 
portant exception,  however,  in  the  Blue  Hills  Complex,  the 
denuded  but  still  salient  axis  of  the  faulted  anticline  three  thou- 
sand feet  or  more  in  height  and  four  to  five  miles  broad  which 
once  divided  the  Boston  and  Norfolk  Basins.  The  details  of 
the  erosion  of  this  axis  can  be  followed  most  closely  and  are 
most  intei-esting  for  the  tilted  western  half.  The  crest  of  the 
range  was  here  the  brink  of  the  great  northern  fault-scarp  ;  and 
it  must  have  so  continued  until  erosion  had  cut  down  to  and 
through  the  contact  zone  of  quartz  porphyry. 

In  the  existing  relations  of  relief  to  oreoloajical  structure  in 
the  western  fault  block  two  facts  are  especially  prominent. 
First,  the  main  range,  including  all  the  summits  from  Great 
Blue  Hill  to  Rattlesnake  Hill,  is  in  the  southern  half  of  the 
block.  Second,  the  main  range  embraces,  superficially,  but 
little  granite,  consisting  almost  wholly  of  quartz  porphyry,  and 
follows  quite  closely  the  northern  edge  of  the  porphyry.  Since 
the  time  when  erosion  first  uncovered  the  quartz  porphyry,  its 
elevated  northern  edge  has,  apparently,  formed  the  crest  of  the 
Blue  Hill  Range.  In  other  words,  during  the  gradual  wearing 
away  of  the  quartz  porphyry  this  crest-line  has  migrated  south- 
ward, and  its  migration  will  probably  continue  until,  through 


536 

the  more  rapid  descent  of  the  sheet  where  it  bends  to  pass  be- 
neath the  Norfolk  Basin,  it  is  finally  reduced  to  the  base-level 
of  the  region.  This  must  mean  that  the  quartz  porphyry  is  a 
harder  and  more  resistant  rock  than  the  granite  ;  for,  other- 
wise, as  soon  as  the  granite  was  exposed  by  the  wearing  away 
of  the  northern  edo;e  of  the  felsite  the  rano-e  would  either  cease 
to  present  a  single  well-defined  and  dominant  crest,  or  the  crest 
would  be  transferred  to  the  granite.  This  differential  erosion, 
so  clearly  indicated  by  the  fact  that  at  all  points  along  the  range 
the  quartz  porphyry  rises  above  the  granite,  the  difference  of 
level  varying  from  less  than  one  hundred  feet  at  the  eastern 
end  of  the  range  to  more  than  four  hundred  feet  at  the  western 
end,  is  confirmed  by  observations  upon  the  present  rates  of  de- 
cay and  waste  of  these  rocks.  Tlie  coarse  granular  texture  of 
the  granite  is  its  weak  point.  Differential  expansion  and  con- 
traction of  its  component  minerals  gradually  loosen  the  grains 
and  permit  the  ingress  of  water  and  other  agents  of  decay  ;  and 
both  the  chemical  decomposition  and  disintegration  by  frost 
action  become  more  and  more  marked  with  increased  absorp- 
tivity. Even  where  the  surface  of  the  granite  is  comparatively 
hard  and  fresh,  the  angles  of  the  ledge  are  usually  well  rounded 
off,  and  in  the  soil  at  the  base  of  the  ledge  or  in  depressions  on 
its  surface  the  detached  grains  of  quartz  and  feldspar  have  ac- 
cumulated in  the  form  of  a  coarse  sand.  This  detritus  is  really 
much  more  abundant  than  it  usually  appears,  since  it  is  easily 
overlooked  after  becoming  incorporated  with  the  vegetable  mold. 
The  quartz  porphyry,  on  the  other  hand,  does  not  suffer  gran- 
ular disintegration  to  an  appreciable  extent ;  and  its  outlines 
are  much  more  angular.  It  is  a  more  brittle  rock  than  the 
granite  and  therefore  more  closely  jointed  ;  and  the  action  of 
frost  in  incipient  joint-cracks  tends  to  reduce  the  surface  of  the 
ledge  to  a  mass  of  angular  fragments.  A  quite  remarkable 
talus  of  this  kind  can  be  seen  on  the  precipitous  western  slope 
of  Hancock  Hill.  It  does  not  appear,  however,  that  these  an- 
gular blocks  of  quartz  porphyry  are  readily  reduced  to  finer 


detritus  wliich  can  be  transported  by  rain  wash.  It  [)rcsents, 
in  consequence,  broader  continuous  exposures,  and  the  traces 
of  grlaciation  have  been  less  erenerallv  effaced  than  on  the  ijran- 
ite.  The  relations  of  these  rocks  to  the  agents  of  erosion  are 
thus  seen  to  be  in  harmony  with  and  to  explain  their  topographic 
relations.  The  more  rapid  erosion  of  the  Pine  Tree  Brook 
band  of  Cambrian  slate  than  of  either  the  quartz  porphyry  or 
the  granite  graduallj^  developed  the  broad  depression  or  valley 
separating  the  elevated  quartz  porphyry  range  on  the  south  and 
the  low  granite  range  on  the  north  ;  and  we  have  in  the  limited 
length  of  this  belt  of  slate  an  explanation  of  the  fact  that  the 
duality  of  the  Blue  Hill  Range  cannot  be  traced  either  way  to 
the  end  of  the  Complex,  but  only  so  far  as  the  slate  belt  extends. 
In  its  geological  relations  Great  Blue  Hill  is  comparable  with 
Mt.  St.  Elias,  which  Russell  has  shown  to  be  the  high  corner 
of  a  tilted  orographic  block. 


EELATIONS   OF   THE   BLUE    HILLS   COMPLEX   TO   THE   PENE- 
PLAINS  OF   EASTEEN  MASSACHUSETTS. 

If  the  entire  region  were  base-leveled  or  worn  down  to  an 
ideal  peneplain,  as  it  may  have  been  more  than  once  since  the 
Appalachian  revolution,  the  Blue  Hills  Complex  would  still 
have,  by  virtue  of  its  boundary  faults,  great  sti-uctural  relief. 
This  we  have  duly  considered  ;  and  we  come  now  to  regard  the 
Blue  Hills  as  an  erosion  monument,  as  measuring  in  their  actual 
topographic  relief  the  erosion  of  the  country  which  they  domi- 
nate. It  is  not  proposed  to  make  this  phase  of  the  subject  a 
prominent  feature  of  the  present  study,  and  a  few  summary 
statements  must  suffice. 

The  most  perfect  peneplain  must  slope  seaward.  The  mass- 
ive elevation  which  often  follows  the  development  of  the  pene- 
plain may  be  and  usually  is  attended  by  crustal  deformation  or 


538 

tilting,  thus  accentuating  the  seaward  slope  ;  and  where  during 
subsequent  base-leveling  the  inter-stream  surfaces  are  reduced 
to  mere  peaks  and  crests  the  unequal  lowering  of  these  give& 
rise  to  still  other  differences  of  level  beyond  what  is  implied  in 
the  name  peneplain  (approximately  a  plain). 

The  greatly  dissected,  but  still  readily  recognized  and  approxi- 
mately continuous.  Tertiary  or  coastal  peneplain  of  eastern 
Massachusetts  has  a  normal  height  near  the  coast  of  from  one 
hundred  to  two  hundred  feet.  But  as  we  recede  from  the  shore 
the  elevation  of  this  coastal  peneplain  gradually  rises  to  three 
hundred  feet  and  more.  Rising  from  the  coastal  peneplain  are 
widely  isolated  remnants  (monadnocks),  including  the  Blue 
Hills "^(635  feet),  Moose  Hill  (536  feet),  Nobscot  Hill  (606 
feet).  Prospect  Hill  (460  feet),  etc.,  of  an  older  land  surface. 
Farther  inland  this  older  and  higher  Cretaceous  or  submoun- 
tainous  peneplain  is  represented  by  many  summits,  which  are 
to  a  large  extent  connected  in  more  or  less  continuous  ridges 
and  plateaus,  and  the  coastal  peneplain  is  represented  only  in 
the  lowlands  or  valleys. 

Northwestward  from  Boston  Harbor,  the  first  approximately 
continuous  development  of  the  submountainous  peneplain  i& 
found  in  the  eastern  rim  or  ridge  of  the  Nashua  Valley,  with  a 
normal  heio;ht  of  from  five  hundred  to  seven  hundred  feet. 
East  of  this  rido;e  elevations  exceedino-  five  hundred  feet  are  few 
and  widely  separated,  and  continuous  areas  above  the  upper 
limit  of  the  coastal  peneplain,  although  numerous,  are  individ- 
ually limited,  except  on  the  Marlboro  ridge  or  height  of  land 
between  the  Assabet  and  Sudbury  Rivei's. 

Westward,  along  the  southern  rim  of  the  Nashua  Valley,  the 
submountainous  peneplain  gains  gradually  in  height  and  con- 
tinuity as  we  approach  the  great  water-parting  of  eastern 
Massachusetts,  the  divide  between  the  Atlantic  and  the  Con- 
necticut, with  a  normal  elevation  of  a  thousand  to  twelve  hun- 
dred feet.  From  this  height  of  land  rise  Watatic  (1840  feet), 
Wachusett  (2000  feet)  and  Little  Wachusett  (1560  feet),  as- 


539 

the  last  vestiges,  in  all  this  region,  of  a  still  oldei'  and  higher 
land  surface,  not  necessarily  a  peneplain,  which  is  represented 
by  the  mountain  peaks  of  New  England. 

The  Nashua  Valley  has,  then,  been  carved  out  of  the  sub- 
mountainous  peneplain  during  the  development  of  the  coastal 
peneplain.  The  coastal  peneplain  is  represented  by  its  floor, 
the  submountainous  peneplain  by  its  eastern,  southern  and  west- 
ern rims,  and  the  still  more  ancient  surface  from  which  the  sub- 
mountainous  peneplain  was  carved,  by  the  sentinel  peaks  or 
monadnocks  risinsr  from  its  hio;h  western  rim.  The  trenching 
or  dissection  of  the  coastal  peneplain,  many  of  the  valleys  near 
the  coast  extending  far  below  sea-level,  probably  dates  chiefly 
from  early  Pleistocene  times,  that  is,  from  the  period  of  eleva- 
tion that  finally  ushered  in  the  great  ice  age.  This  last  distinct 
or  strongly  accentuated  stage  in  the  topographic  development 
of  eastern  Massachusetts,  omitting  the  effects  of  the  Pleistocene 
glaciation,  is  also  represented  in  the  Nashua  Valley  ;  for  the 
recent  explorations  of  the  Metropolitan  Water  Board  in  the 
vicinity  of  Clinton  have  revealed  the  existence  beneath  the 
drift-covered  bottom  of  the  valley  of  a  deep  and  narrow  rock 
a;orge, 

A  peneplain  must  not  only  slope  seaward  ;  but  after  a  mass- 
ive elevation  has  inaugurated  a  new  topographic  cycle  the  old 
surface  disappears  most  rapidly  in  its  seaward  portion.  Hence 
the  submountainous  peneplain  is  no  longer  traceable  east  of  the 
Nashua  and  Bkckstone  Valleys  in  a  series  of  nearly  continuous 
crest-lines,  but  only  in  widely  isolated  summits,  which  rise 
abruptly,  by  crowded  contours,  from  the  broadly  and  gently 
undulated  surface  which  marks  an  advanced  stage  in  the  devel- 
opment of  the  coastal  peneplain,  to  heights  ranging  from  three 
hundi'ed  and  fifty  to  seven  hundred  feet. 

The  exceptional  prominence  of  the  western  end  of  the  Blue 
Hill  Range  indicates  that,  probably  on  account  of  the  intense 
hardness  of  the  quartz  porjjhyry,  it  was  not  com[)letely  base- 
leveled  on  the  submountainous  peneplain. 


540 

The  second  base-level  or  coastal  peneplain  which,  west  of 
Worcester,  we  have  only  in  a  very  immature  stage,  as  narrow 
valleys,  is  broadly  developed  in  the  coastal  area,  its  normal  ele- 
vations rano'iDor  from  a  hundred  feet  and  less  near  the  shore  to 
five  hundred  feet  and  more  forty  miles  inland.  The  consider- 
able slope  of  the  first  and  second  ( submountainous  and  coastal ) 
peneplains  is  an  indication,  as  Davis  has  pointed  out,  of  crustal 
deformation,  the  elevation  to  start  a  new  cycle  being  differential 
and  not  strictly  massive.  Nowhei^e,  perhaps,  is  the  coastal 
peneplain  more  perfectly  developed  than  along  the  southern 
border  of  the  Boston  Basin,  from  Cohasset  to  the  Neponset 
Kiver,  the  main  or  quartz  porphyry  range  of  the  Blue  Hills 
Complex  alone  rising  above  it.  It  is  not  the  smoothness  of  the 
inter-stream  surfaces,  for  that  may  be  wholly  wanting,  but  the 
uniformity  of  the  rock  elevations  that  proves  the  peneplain  ; 
and  this  we  recognize  in  the  level  horizon  lines  all  over  the 
coastal  area,  where  the  drift  hills  or  drumlins  are  not  too  thickly 
planted. 

The  development  of  the  coastal  peneplain  was  interrupted  by 
the  marked  elevation  of  the  land  which,  according  to  Upham's 
theory  (accepted  by  the  writer),  ushered  in  the  glacial  epoch. 
The  elevation  was  sufficient  to  permit  the  formation  of  the  deep- 
est fiords  of  our  coast,  including  the  remarkable  submarine 
channel  of  the  Hudson,  nearly  three  thousand  feet  deep  ;  and 
sufficiently  prolonged  to  enable  the  streams  to  cut  back  their 
channels  into  the  land  and  develop  the  complex  and  mature 
drainage  systems  which  characterize  our  topography  today. 
From  the  close  of  the  first  cycle  ( the  development  of  the  sub- 
mountainous  peneplain )  dates  the  crest  of  the  main  range  of 
the  Blue  Hills.  From  the  close  of  the  second  cycle  (the  de- 
velopment of  the  coastal  peneplain)  date  the  relief  and  the 
notches  of  the  main  range  and  the  numerous  summits  of  the 
northern  range  and  of  the  eastern  half  of  the  Complex.  From 
the  massive  eleva,tion  that  inaugurated  the  glacial  epoch,  date 
the  valleys  of  Weymouth  Back  liiver,  Weymouth  Fore  River, 


541 

Monatiquot  River,  Pine  Tree  Brook,  etc.,  and  the  general  sub- 
division of  the  peneplain  and  isolation  of  the  rocky  hills  so  well 
seen  where  the  accumulation  of  drift  has  not  been  excessive,  as 
in  the  northern  range  from  West  Quincy  westward.  It  can- 
not be  affirmed  that  the  glaciation  profoundly  modified  the  rock 
contours  of  the  Blue  Hills  Complex  ;  but  the  chief  topographic 
function  of  the  ice-sheet  in  this  area  consisted  in  obstructing;  the 
drainage  and  generally  masking  the  outlines  of  the  rock  surface. 
The  post-glacial  depression  finally  left  the  land,  after  various 
oscillations,  one  hundred  to  two  hundred  feet  above  its  level 
during  the  development  of  the  second  or  coastal  peneplain. 
The  lower  portions  of  the  deepest  glacial  valleys,  such  as  the 
two  Weymouth  Rivers,  were  submerged  and  a  new  base-level 
established  ;  and  we  may  consider  that  a  new  cycle  of  topo- 
graphic development  is  now  well  inaugurated,  at  least  in  imper- 
fectly consolidated  formations  such  as  the  drift  hills  of  Boston 
Harbor.  The  bottoms  of  valleys  now  near  the  sea-level  con- 
stitute the  be2:innino;.of  a  fourth  base  level,  holdino-  the  relation 
of  a  terrace  to  the  rock  gorges  formed  during  the  glacial  uplift, 
which  it  is  not  improbable  will  be  matured,  if  the  existing  equi- 
librium proves  sufficiently  stable,  before  the  monument  of  the 
first  cycle  which  we  have  in  the  main  crest-line  of  the  Blue  Hills 
has  been  wholly  effiiced.  This  section  may,  then,  be  summa- 
rized as  follows  :  — 

Southern  New  England  exhibits  besides  the  bottoms  of  the 
buried  glacial  gorges,  and  the  ancient  land  represented  by  Mts. 
Wachusett,  Monadnock,  etc.,  three  base-levels  in  different 
stages  of  development  and  effacement,  the  development  of  the 
younger  plains  and  the  effacement  of  the  older  plains  being  in 
general  inversely  proportional  to  the  distance  from  the  sea. 
The  first  base-level  or  peneplain  is  still  well  developed  in  west- 
ern and  central  Massachusetts,  but  is  represented  only  by  iso- 
lated hills,  including  the  Blue  Hills,  in  the  coastal  area.  The 
development  of  the  second  peneplain,  which  arrested  the  devel- 
opment of  the  first,  was  well  advanced  over  eastern  and  slightly 


542 

80  over  central  and  western  Massachusetts  when  it  was,  in  its 
turn,  interrupted  by  the  great  oscillations  of  the  glacial  epoch 
with  the  final  establishment  of  the  existing  base-level  and  the 
advent  of  the  third  cycle,  or  the  fourth  cycle,  if  we  take  account 
of  the  buried  valleys  of  the  Charles,  Neponset  and  other  rivers 
and  the  floor  of  Boston  Harbor. 


THE   SURFACE   GEOLOGY,  OR  THE  BLUE  HILLS 
COMPLEX  IN  PLEISTOCENE   TIMES. 

Although  the  Blue  Hills  Complex  presents,  superficially, 
some  general  contrasts  with  the  areas  described  in  Parts  1  and 
2,  such  as  its  greater  extent,  more  inland  character  and  more 
diversified  and  rugged  topography,  all  of  which  must  tend  to 
influence  and  differentiate  its  surface  geology,  this  chapter  may 
still  be  cast  in  the  same  general  mold  as  before,  except  that  it  is 
necessary  now  to  take  more  particular  account  than  in  Part  2  of 
that  most  important  factor  in  the  later  glacial  history  of  this  part 
of  the  Boston  Basin  —  Lake  Bouve.  This  is  the  name  which,  in 
honor  of  Mr.  T.  T.  Bouve,  former  President  of  the  Boston  Society 
of  Natural  Histoiy,  friend  of  science  and  humanity,  and  a  life- 
long, enthusiastic  and  successful  student  of  the  geology  of  the 
South  Shore,  I  have  had  the  satisfaction,  in  association  with  Mr. 
A.  W.  Grabau,  of  giving  to  the  considerable  body  of  water 
which  was  impounded  along  the  margin  of  the  great  ice-slieet  dur- 
ing its  recession  northward  from  the  southern  water-parting  of 
the  Boston  Basin.  Its  intricate  and  chano-ino-  outline,  its  liead- 
lands,  islands,  successive  outlets  and  levels,  its  great  ice  barrier 
and  the  icebergs  which  must  have  adorned  its  surfiice,  and 
above  all  the  tributary  torrents  of  glacial  water  bursting  forth 
from  the  ice,  surcharged  with  glacial  detritus,  and  the  wonderful 
grace  and  beauty  of  the  resulting  winding  eskers  and  lobate  and 
dimpled  delta  plains,  certainly  make  the  history  of  Lake  Bouve 


543 

one  of  the  most  interesting  and  romantic  chapters  in  the  geology 
of  this  region.  The  features  and  history  of  Lake  Bouve  have 
been  worked  out  in  detail  by  Mr.  Grabau  ;  and  it  has  afforded 
me  much  pleasure  to  be  able  to  intrust  this  task  to  such  compe- 
tent hands. 


GLACIAL   GEOLOGY  — OCCUPATION   PHENOMENA. 

It  is  proposed  to  describe  in  this  section  those  phases  of  the 
surface  geology  due  to  the  actual  and  complete  occupation  of 
this  area  by  the  great  ice-sheet ;  or,  in  other  words,  those  features 
of  the  bed-rock  surface  and  of  the  drift  which  were  developed 
while  the  ice  still  rested  heavily  upon  the  land  and  was  capable 
of  abrading  and  disrupting  the  hard  rocks  and  transporting  the 
resulting  debris.  More  specifically,  these  phenomena  include  : 
glacial  striae;  glacial  pot-holes;  the  till  or  unmodified  (un- 
washed) drift  and  the  special  accumulations  of  the  till. known 
as  drumlins  ;  and  erratics  or  glacial  bowlders. 

Grlacial  Striae.  — As  for  the  more  eastern  areas,  these  rec- 
ords of  the  movement  of  the  ice-sheet  fall  naturally  into  two 
groups  ;  first,  those  agreeing  closely  with  the  normal  direction 
of  the  ice  movement  for  this  region  (S.  20°-25°  E.)  ;  and,  sec- 
ond, those  deviating  from  the  normal  in  an  easterly  direction. 
The  first  group  is  especially  characteristic  of  the  more  elevated 
tracts  on  the  rim  of  or  wholly  south  of  the  Boston  Basin,  or 
more  generally  still,  of  the  areas,  be  they  high  or  low,  where 
the  movement  of  the  ice  was  uninfluenced  by  the  easterly  trend 
of  the  Boston  Basin.  The  second  group,  on  the  other  hand,  is 
almost  wholly  confined  to  the  basin,  at  least  in  its  topographic 
sense  ;  and  they  record  the  influence  of  the  southern  wall  of  the 
basin  in  deflecting  the  ice  currents.  In  the  following  table,  the 
normal  striae  are  given  first,  followed  by  the  deflected  striae  ; 
and  the  latter,  it  will  be  observed,  occur  chiefly  on  the  lowlands, 
or,  more  exactly,  just  where  they  should,  with  reference  to  the 


.      544 

higher  land,  to  accord  with  the  theory  of  glacial  deflection.  It 
is  clearly  not  necessary  to  suppose  that  the  normal  and  deflected 
striae  are  strictly  contemporaneous  ;  but  it  is  quite  as  probable 
that  the  deflected  striae  date  in  part  from  a  time  after  the  nor- 
mal movement  of  the  ice  had  ceased  :  on  the  same  general  prin- 
ciple that  the  striae  on  stoss  slopes  must  be,  as  a  rule,  later  than 
those  on  lee  slopes,  the  ice,  in  its  later  stages,  having  refused 
to  follow  closely  and  forcibly  the  lee  contours.  Close  compari- 
sons are,  of  course,  difficult  and  unprofitable  on  account  of  the 
comparatively  wide  range  of  directions  often  to  be  observed  on 
a  single  outcrop  ;  and  an  attempt  has  been  made  to  obtain  the 
average  trend  in  each  case. 

Directions  of   Glacial    Striae. 

North  side  of  Great  Blue  Hill,  on  quartz  porphyry     .         .  S.  23°  E. 

Summit  of  Buck  Hill,  on  quartz  porphyry  .         .         .  S.  25°  E. 

Pine  Tree  Brook,  on  Cambrian  slate,  ....  S.  20"- 23°  E. 

Jones  and  Desmond's  granite  quarry,  West  Quincy  .  S.  27°  E. 

Dean  and  Horrigan's  granite  quarry,  West  Quincy         .  S.  25°  E. 

Babel  Rock,  west  of  Willard  St.,  on  trap        .        .        .  S.  20°  E. 

Pine  Hill,  on  Cambrian  slate S.  17°-23°  E. 

Pine  Hill  on  Cambrian  slate,  another  ledge      .         .         .  S.  25°-28°  E. 

Mill  Cove,  on  Cambrian  slate  S.  23°-28°  E. 

North  end  of  Weymouth  Great  Pond,  on  granite  .  S.  22°  E. 

Main  St.,  south  of  Washington  St.,  Weymouth,  on  granite  S.  25°  E. 

Corner  Winter  and  Main  Sts.,  Weymouth,  on  granite     .  S.  25°  E. 

Washington  St.,  south  of  Whitman  Pond,  on  granite     .  S.  23°-25°  E. 

Washington  St.,  west  of  Whitman  Pond,  on  granite        .  S.  30°  E. 

Between  Commercial  and  Middle  Sts.,  Weymouth,  on  granite  S.  30°  E. 

Pleasant  St.,  east  of  Whitman  Pond,  on  granite       .         .  S.  33°  E. 

Northwest  slope  of  Broken  Hills,  on  quartz  porphyry     .  S.  30°  E. 

Northeast  base  of  Wampatuck  Hill,  on  aporhyolite         .  S.  35°  E. 

North  side  of  North  Common  Hill,  on  Cambrian  slate   .  S.  30°  E. 

East  side  of  North  Common  Hill,  on  Cambrian  slate      .  S.  38°  E. 

North  side  of  Furnace  Brook,  Quincy,  on  slate       .        .  S.  33°  E. 

Hancock  St.,  north  of  Eurnace  Brook,  on  slate  and  trap  S.  37°  E. 

Washington  St.,  Quincy,  on  Cambrian  slate     .         .        .  S.  35°  E. 

East  Braintree,  on  the  boundary  dike       .        .        .        .  S.  35°  E. 

Raccoon  Island,  on  slate S.  50°  E. 

Grape  Island,  on  slate S.  60°  E. 


545 

Undoubtedly  the  most  notable  exposure  of  a  glaciated  surface 
in  this  area  is  the  high  sloping  ledge  of  granite  with  inclusions- 
of  basic  porphyry  on  the  northeast  side  of  the  Pine  Hill  tract,, 
on  the  southwest  side  of  the  railroad  between  West  Quincy  and 
Braintree,  opposite  the  end  of  Liberty  Street.  It  forms  a 
steeply  sloping  and  undulating  wall  thirty  to  forty  feet  high  and 
several  hundred  feet  long,  which  has  been  uncovered  during 
the  grading  of  the  railroad,  and  which  was  so  well  polished  by 
the  movement  of  the  ice  that  it  fairly  shines  in  the  sunlight. 

Glacial  Potholes. — -Although  situations  extremely  favor- 
able to  the  formation  of  potholes  by  subglacial  streams,  or 
streams  falling  through  crevasses  in  the  ice-sheet  (moulins)  to 
become  subglacial,  are  very  numerous,  only  one  good  example 
has  been  observed.  This  was  brought  to  my  attention  by  Mr. 
John  J.  Loud  of  Weymouth,  while  it  was  temporaril}^  exposed 
during  the  excavation  for  the  foundation  of  the  new  Congrega- 
tional Church  on  Commercial  Street  near  Quincy  Avenue,  in 
East  Braintree.  It  was,  unfortunately,  destroyed  by  the  blast- 
ing of  the  ledge,  and  its  site  is  now  covered  by  the  southeast 
corner  of  the  church.  It  was  formed  in  the  large  inclusion  of 
slate  in  the  great  boundary  dike  (p.  405),  and  close  to  the  con- 
tact with  the  trap,  which  rose  eight  to  ten  feet  above  the  pot- 
hole, on  the  north,  making  the  situation  almost  an  ideal  one. 
The  hole  or  basin  was  3  feet  deep,  12  feet  long  (east-west), 
and  from  4  to  Q^  feet  wide.  It  resembled  a  huge  bathtub,  ex- 
cept that  it  was  entirely  open  at  the  west  end,  the  smooth, 
water-worn  surface  of  the  pothole  rounding  out  in  tliat  direction 
and  blending  with  the  general  surface  of  the  ledge,  which  here 
showed  several  incipient  potholes.  It  is  quite  evident  that  the 
ice  was  broken  by  its  passage  over  the  crest  of  this  east-west 
rock  ridge,  and  that  the  pothole  marks  the  point  where  a  super- 
glacial  stream  fell  through  the  resulting  crevasse  and,  impinging 
as  a  glacial  moulin  upon  the  sloping  surface  of  the  ledge,  exca- 
vated the  pothole  and  flowed  away  to  the  west  and  south  in  its 
subglacial  course. 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  35. 


546 

There  are  many  points  in  this  region,  and  these  are  not 
wholly  confined  to  low  levels  or  areas  of  modified  drift,  where 
the  recently  uncovered  ledges  present  in  part  at  least  a  distinctly 
water-worn  aspect.  This  phenomenon  is  so  wide-spread  that  it 
is  needless  to  discuss  its  distribution  or  cite  many  examples.  It 
first  distinctly  attracted  my  attention  while  I  was  studying  the 
slate  of  Raccoon  Island  ;  and  the  following  description  is  taken 
directly  from  my  field  notes.  "  Wherever  the  slate  has  been 
exposed  in  recent  times  by  the  washing  away  of  the  drift,  it 
presents  a  smoothly  rounded  or  flowing  and  unmistakably  water- 
worn  surface,  with  many  incipient  potholes.  My  first  thought 
was  that  it  must  be  due  to  the  action  of  the  sea  in  recent  times  ; 
but  I  soon  found  that  it  extends  considerably  above  the  high 
tide  level  and  probably  all  over  the  island.  I  also  observed 
that  these  same  surfaces  are  distinctly  glaciated.  The  striae 
(S.  50°  E.)  are  not  very  strongly  marked,  but  can  be  seen  on 
all  stoss  and  the  gentler  lee  slopes.  The  whole  eflfect  is  clearly 
due  to  the  combined  or  simultaneous  action  of  the  moving  ice 
and  subglacial  waters  at  the  close  of  the  ice  age,  when  the  rap- 
idly wasting  ice-sheet  was  moving  feebly  and  with  but  little 
abrasive  power."  It  was  not  so  much  a  cooperation  as  a  con- 
flict of  ice  and  water,  each  agent  tending  to  efface  the  distinctive 
characters  engraved  upon  the  slate  by  the  other,  the  water,  in 
this  instance,  being  sufficiently  in  the  ascendant,  so  that  lee 
slopes  are  nowhere  craggy  or  broken. 

Till  and  Drutnlins. — In  the  more  eastern  areas  —  Nantas- 
ket,  Cohasset  and  Hingham  —  the  till  or  bowlder  clay  not  cov- 
ered by  modified  drift  occurs  chiefly  in  the  form  of  drumlins  ; 
and  this  relation  holds  also  for  the  lower  eastern  and  northern 
portions  of  the  area  now  under  consideration,  or,  more  generally, 
within  the  range  of  the  lower  levels  of  Lake  Bouve.  But  it  is 
safe  to  assume  that  the  broad  delta  plains  of  Lake  Bouve  have 
throughout  a  more  or  less  continuous  foundation  of  bowlder 
clay,  although  few  sections  are  deep  enough  to  expose  it ;  and 
the  nearly  continuous  mantle  of  bowlder  clay,  existing  independ- 


547 

ently  of  the  clrumlins,  over  the  higher  land  to  the  south  and 
west,  and  notably  in  the  Blue  Hills,  means  simply  the  absence 
of  the  lake  deposits.  The  general  map  (PL  13)  is  a  graphic 
presentation  of  all  the  essential  facts  concerning  the  distribution, 
forms,  sizes,  elevations  and  trends  of  the  drumlins  in  the  area 
of  the  Blue  Hills  Complex.  It  shows  at  a  glance  that  in  their 
distribution  the  drumlins  favor  the  lowlands  along  the  north  or 
Boston  Basin  side  of  the  Complex,  and  the  east  rather  than  the 
west,  having  their  largest  and  most  perfect  development  in  the 
district  contiguous  to  Boston  Harbor.  These  data  are  I'epeated 
on  the  map  of  Lake  Bouve  (PI.  25),  which  also  shows  the  rela- 
tions of  the  drumlins  to  the  sand  plains  or  deltas  and  the  succes- 
sive shore  lines  of  the  lake. 

Closer  attention  to  the  facts  shows  that  the  drumlin  form  of 
the  till  fades  out  somewhat  gradually  as  we  recede  from  the 
great  valley  of  the  Boston  Basin.  On  many  of  the  more  promi- 
nent rock  hills  the  stoss  slopes,  and  to  some  extent  the  lee  slopes, 
bear  distinctly  drumloid  accumulations  of  till,  suggesting  that 
the  normal  or  fully  developed  drumlins  probably  have,  as  a  rule 
at  least,  elevated  rock  foundations  or  cores,  which  served  as 
gathering-points  for  the  till.  As  examples  of  these  embryo 
drumlins  we  have  the  northwest  or  stoss  slopes  of  Payne's  Hill 
in  Quincy,  Liberty  Hill  in  East  Braintree,  Monatiquot  Heights 
in  North  Braintree,  ChickataAvbut  Hill,  Hawk  Hill,  Buck  Hill, 
and  to  a  less  extent  other  members  of  the  main  range  of  the 
Blue  Hills,  and  Ponkapog  Hill  in  Canton. 

Where  the  rock  contours  are  not  strongly  accentuated  or  dif- 
ferentiated, the  drumlins,  especially  if  closely  grouped,  become 
subdued  in  relief,  tending  to  flatten  out  into  gently  undulating 
plains  of  till.  This  is  well  illustrated  by  the  drumlins  of  North 
Braintree,  east  of  the  railroad,  and  especially  by  the  group 
crossed  by  Middle  Street,  east  and  south  of  the  Monatiquot 
River.  In  other  cases  there  is  a  manifest  tendency  to  form  low 
ridges  of  till,  like  long-drawn-out  drumlins,  trending  in  the  di- 
I'ection  of  glacial  movement ;  and  we  have  an  exceptionally  good 


548 

example  of  this  type  in  the  low,  but  distinct  and  continuous 
rido'e  extendino-  alono-  the  west  side  of  the  railroad,  and  on  which 
Washington  Street  runs,  for  two  miles  from  the  bridge  over  the 
railroad  in  Xorth  Braintree  to  South  Braintree.  The  district 
south  of  the  Blue  Hills,  between  Great  Pond  and  Ponkapog 
Pond,  is  a  good  example  of  a  broadly  undulating  to  flat  till  sur- 
face, devoid  of  drumloid  character,  and  probably  due  to  a 
moderate  thickness  of  till  rather  evenly  distributed.  From  the 
map  it  can  readily  be  learned  that,  as  usual,  the  drumlins  trend 
rather  more  to  the  easterly,  as  a  rule,  than  the  glacial  striae, 
showing  that  the  formation  of  the  drumlins  represents  a  later 
period  in  the  history  of  the  ice-sheet,  when  it  was  experiencing 
basal  melting  and  was  depositing  the  ground  moraine  or  till, 
and  when  its  thinness  made  it  more  susceptible  to  topographic 
influences. 

A  more  particular  discussion  of  the  drumlins  of  this  area  may 
be  conveniently  reserved  for  Part  4,  Avhere  we  shall  have  a  broad- 
er and  safer  basis  for  generalization.  In  closing  this  section  it 
may  be  noted  that,  almost  without  exception,  the  hills  compos- 
ing the  main  range  of  the  Blue  Hills,  and  moi-e  especially  those 
of  the  higher  western  half  of  the  range,  are  more  or  less  drum- 
loid in  form,  or,  more  exactly,  they  exhibit  typical  roche  mou- 
tonee  contours.  The  northwestern  or  stoss  slopes  are  long  and 
comparatively  gentle,  this  eflect  being  heightened  by  an  increas- 
ino-  accumulation  of  till  downward  :  while  the  southeastern  or 
lee  slopes  are  usually  less  encumbered  by  drift  and  are  dis- 
tinctly more  abrupt,  or  even  broken  and  craggy.  These  features 
are  beautifully  shown  on  the  topographic  map  of  the  Blue  Hills 
Reservation  (PL  14).  The  drumlins  fairly  included  within 
this  area  afford  no  natural  or  artificial  sections  deep  enough  to 
satisfactorily  expose  the  unoxidized  or  blue  till,  which  in  other 
parts  of  the  Boston  Basin  has  yielded  a  goodly  variety  of  fossil 
shells.  Such  scanty  paleontological  data  as  have  come  to 
light  in  the  drumlins  of  this  area  will  therefore  be  reserved  for 
consideration  with  tlie  more  abundant  data  of  Part  4. 


549 

Bowlders  and  DiKviqited  Ledges.  — In  a  glaciated  area  so 
diversified  topographically  and  geologically  as  this,  and  embrac- 
ing several  formations,  like  the  conglomerate  of  the  Boston 
Basin  and  the  normal  granites,  exceptionally  well  adapted  to 
the  formation  of  large  bowlders,  erratics  are  necessarily  a  promi- 
nent feature  of  the  sm^face  geology.  A  few  of  the  bowlders 
scattered  over  the  Blue  Hills  Complex  are  interesting  on  ac- 
count of  their  size,  and  this  is  especially  true  of  House  Rock, 
in  Weymouth  ;  others  are  interesting  for  the  distance  or  direc- 
tion of  their  transportation  ;  and  many  others  simply  for  their 
pi-ofusion  on  limited  areas.  As  in  Hingham  and  Cohasset,  the 
great  majority  of  the  bowlders  ai^e  of  extremely  local  origin, 
being  within,  say  half  a  mile,  of  the  parent  ledges,  and  only  a 
small  proportion  of  the  whole  can  be  proved  to  have  travelled 
as  much  as  five  miles. 

Along  the  north  side  of  the  Blue  Hills,  in  Milton,  and  espe- 
cially near  Canton  Avenue,  west  of  Randolph  Avenue,  there  is 
a  great  profusion  of  conglomerate  bowlders,  some  of  which  are 
of  considerable  size  ;  but  in  walking  over  the  main  range  of  the 
Blue  Hills  I  have  been  impressed  by  the  infrequency  and  the 
small  size  of  the  conglomerate  erratics,  although  they  are  fairly 
common  over  the  lower  northern  hills  of  normal  granite.  This 
shows  most  conclusively  that  in  its  later  movement,  at  least, 
the  ice  transported  but  little  coarse  drift  directly  across  the  Blue 
Hills;  although,  as  we  have  seen,  the  glacial  scorings  on  the 
main  range  hold  rather  closely  to  the  normal  trend.  The  ex- 
ception that  proves  the  rule  is  a  block  of  coarse  conglomerate 
twenty  feet  square  and  twelve  feet  thick  on  the  northeast  side 
of  Chickatawbut  Hill.  The  largest  other  conglomerate  erratic 
which  I  have  observed  along  this  line  lies  a  little  beyond  the 
eastern  end  of  the  range,  on  the  northwest  side  of  West  Street, 
about  midway  between  Willard  Street  and  the  railroad,  and 
about  110  feet  above  the  sea.  It  consists  of  normal  pudding- 
stone  alternating  with  layers  of  sandstone  ;  and  its  horizontal 
dimensions  are  approximately  fifteen  by  twenty  feet.      It  really 


550 

lies  in  the  transverse  West  Quincy  and  Quincy  Adams  valley, 
and,  with  many  smaller  masses  of  conglomerate  in  this  broad 
depression,  it  testifies  to  the  influence  of  the  Blue  Hills  upon 
the  later  glacial  movement.  Of  conglomerate  bowlders  which 
appear  to  have  passed  through  this  depression  there  are  some 
notable  examples.  One  of  these,  some  twenty  feet  in  length, 
lies  at  the  northeastern  base  of  Monatiquot  Heights,  and  a  still 
larger  one  rests  upon  the  highest  part  of  the  hill,  directly  north 
of  the  stand-pipe  ;  while  another,  not  so  large  as  these,  lies 
upon  the  northwest  slope  of  the  drumlin  south  of  Payne's  Hill 
and  crossed  by  Elm  and  Commercial  Streets,  Perhaps  the 
most  striking  evidence  that  the  main  range  of  the  Blue  Hills 
was  an  important  factor  in  differentiating  the  movement  of  the 
ice-sheet  is  afforded  by  the  bowlders  of  red  felsite  derived,  pre- 
sumably, from  the  ledges  of  this  unique  and  attractive  rock  in 
Hyde  Park  and  the  Mattapan  district  of  Milton.  The  most 
easterly  outcrops  of  the  red  felsite  are  in  the  vicinity  of  Blue 
Hill  Avenue.  It  is  possible,  of  course,  although  highly  im- 
probable, that  it  also  occurs  in  place  beneath  the  broad  drift 
plain  of  East  Milton  and  North  Quincy  ;  but  in  my  opinion  we 
are  justified,  provisionally  at  least,  in  referring  the  erratics  of 
red  felsite  to  the  known  outcrops  of  the  Neponset  Valley. 

Now  this  felsite  is  a  hard,  massive  rock,  making,  as  a  rule, 
rather  small  bowlders,  which  may  be  described  as  long-lived 
and  capable  of  withstanding  the  wear  and  tear  of  glacial  trans- 
portation. The  noi'mal  movement  of  the  ice  would  have  car- 
ried these  bowlders,  together  with  the  conglomerate  bowlders, 
directly  aci'oss  the  main  range  of  the  Blue  Hills.  A  single 
example  has  been  observed  at  the  northern  base  of  Great  Blue 
Hill,  just  above  the  junction  of  Canton  and  Blue  Hill  Avenues 
with  Brush  Hill  Road,  and  several  others  were  noted  near  the 
eastern  end  of  the  Pine  Tree  Brook  belt  of  slate.  But  on  the 
main  range  of  the  Blue  Hills  the  red  felsite,  even  more  than  the 
conglomerate,  erratics  are  conspicuous  by  their  absence  ;  while 
around   the   eastern   end   of  the   hills   and   throuo-h   the    West 


551 

Quincy  and  Braintree  valley  tbey  occur  with  a  fair  degree  of 
frequency.  No  notes  have  been  made  on  the  smaller  examples, 
but  I  have  been  particularly  impressed  by  two  unusually  large 
bowlders  of  the  red  felsite  occurring  on  the  low  drumlins  of 
North  Braintree,  in  the  area  east  of  Middle  Street,  north  of 
Union  Street  and  south  of  Monatiquot  River.  Lithologically, 
they  are  identical  with  the  banded  and  brecciated  red  felsite  of 
the  ledges  in  Hyde  Park  and  Milton.  One  of  them  is  on  the 
northeast  slope  of  the  most  easterly  drumlin  of  this  group, 
about  seven  hundred  and  fifty  feet  south  of  the  railroad  and 
six  hundred  feet  west  of  the  mill-pond.  It  measures  eight  by 
ten  feet  horizontally,  and  is  evidently  deeply  buried.  The  other 
one  is  about  half  a  mile  south  of  this,  very  near  Union  Street 
and  nearly  a  thousand  feet  east  of  Middle  Street.  It  measures 
ten  by  eighteen  feet  and  is  seven  feet  high.  These  bowlders 
are  fully  six  miles  in  an  east-southeast  direction  (S.  65°-68°  E.) 
from  the  nearest  known  outcrop  of  this  rock,  on  Blue  Hill  Ave- 
nue ;  and  in  this  course,  which  crosses  Rattlesnake  and  Pine 
Hills,  no  red  felsite  bowlders  have  been  observed,  except  in  the 
low  land  north  of  the  Blue  Hills  and  in  the  upper  valley  of 
Pine  Tree  Brook.  There  can  be  little  doubt,  therefore,  that 
the  course  actually  travelled  by  these  two  bowlders  during 
their  glacial  transportation  was  first  nearly  east  for  three  miles 
to  East  Milton  and  the  West  Quincy  valley,  and  thence  about 
southeast  through  this  transverse  valley  three  miles  or  more  to 
the  points  where  they  now  rest.  These  facts  of  glacial  trans-  \ 
portation  confirm  the  indications  of  tlie  glacial  striae  and  the 
drumlins,  and  make  it  reasonably  certain  that  the  main  range 
of  the  Blue  Hills  was  a  controlling'  factor  in  determinino-  the 
local  movement  of  the  ice-sheet,  at  least  in  its  lower  levels. 
Above  the  level  of  the  crest  of  the  range  (three  hundred  to  six 
hundred  feet)  the  ice  probably  held  closely  to  its  normal  trend, 
due  to  the  general  slope  of  southeastern  New  England  ;  but 
below  the  sky-line  of  the  hills,  on  the  north  or  Boston  Basin 
side,  the  prevailing  movement,  at  least  during  the  closing  stages 


552 

of  glaciation,  was  easterly  to  the  first  important  break  in  the 
range,  where  the  contiguous  portion  of  the  basal  ice  regained 
to  some  extent  the  normal  trend,  although  the  main  body 
swept  on  to  the  eastward,  impelled  by  the  deepening  basin  of 
Boston  Harbor. 

In  his  study  of  the  distribution  of  the  bowlders  of  red  sand- 
stone derived  from  the  eastern  end  of  the  Norfolk  Basin,  Mr. 
M.  L.  Fuller^  has  observed  for  the  eastern  or  left-hand  boundary 
of  the  train  a  decided  eastward  trend,  the  easting  increasing 
from  about  S.  40°  E.  near  the  source  to  nearly  south  80°  E.  at 
a  distance  of  six  miles,  thus  describing  a  curve  concave  to  the 
north,  with  an  average  easting  of  S.  60°  E.  This  agrees  very 
closely  with  the  results  of  my  study  of  the  distribution  of  the 
red  felsite  bowlders  in  Hingham,^  the  average  trend  of  the  north- 
ern border  of  the  fan  being  N.  67°  E.  for  the  first  four  miles 
and  still  more  easterly  for  greater  distances. 

These  examples  of  the  distribution  and  dispersion  of  erratics 
evidently  demand  a  very  different  explanation  from  that  pro- 
posed above  for  the  red  felsite  and  conglomerate  of  Milton,  and 
an  explanation  in  which  topography  is  not  a  prominent  factor. 
It  is  obvious  that  the  erratics  near  the  parent  ledges,  say  within 
five  miles,  must  record,  in  general,  either  the  movement  of  the 
basal  portion  only  of  the  ice-sheet,  in  case  the  detritus  was  sub- 
glacial  or  confined  to  the  lower  part  of  the  ice,  or  the  later 
movement  chiefly  of  the  upper  part  of  the  ice-sheet,  in  case  the 
detritus  had  o-ained  a  hio;her  level  in  the  ice  and  was  truly  engla- 
cial.  That  the  motion  of  the  basal  and  upper  portions  of  a 
glacier,  as  of  a  river,  may  be  differential  in  both  velocity  and 
direction,  is  very  generally  conceded  ;  and  this  principle  is  be- 
lieved to  account  satisfactorily  for  the  occurrence  of  the  bowlders 
of  red  felsite  in  Braintree,  the  coarser  part  at  least  of  the  detritus 
of  both  the  felsite  and  the  conglomerate  of  the  Neponset  Valley 
having  generally  failed,  before  reaching  the  latitude  of  the  Blue 

I  Proc.  Boston  soc.  nat.  hist.,  vol.  28,  pp.  251-26-1. 
■■iPart  2,  p.  272. 


553 

Hills,  to  gain  a  level  in  tlie  ice  liigli  enough  to  free  it  from  the 
localized  basal  currents  and  enable  it  to  share  the  normal  on- 
ward movement  of  the  ice. 

South  and  east  of  the  Blue  Hills,  the  reliefs  of  the  low-lying 
peneplain  were  manifestly  insufficient  to  exert  any  important 
control  over  the  movement  of  even  the  basal  kiyers  of  the  ice  ; 
and  some  other  cause  of  the  decided  eastward  movement  of  the 
erratics  must  be  invoked.  The  general  lack  of  agreement  be- 
tween  the  indications  of  glacial  movement  afforded  (1)  by  the 
glacial  striae  (S.  20°— 30°  E.),  and  (2)  by  the  drumlins  and 
erratics,  is  readily  explained  by  the  principle  that  the  striae 
date  mainly  from  the  period  of  maximum  glaciation,  before  the 
beginning  of  glacial  melting  and  the  deposition  of  the  ground 
moraine,  while  the  building  of  the  drumlins  and  the  final  dis- 
position of  the  erratics  occurred  mainly  during  the  waning  and 
final  disappearance  of  the  ice  and  after  it  had  ceased  to  effec- 
tively abrade  the  bed-rock.  Xow  it  is  a  reasonable,  if  not  a 
necessary,  assumption  that  during  this  waning  stage,  when  the 
ice-sheet  was  greatly  reduced  in  thickness  and  its  margin  was 
near  instead  of  remote,  and,  lubricated  by  subglacial  waters,  it 
slipped  over  the  ground  moraine  instead  of  dragging  it  along, 
it  was  somewhat  diverted  from  its  former  course  by  the  basins 
of  Boston  Harbor  and  Massachusetts  Bay.  This  change  in 
direction,  which,  whatever  its  cause,  is  as  certain  as  any  fact  in 
the  geology  of  this  region,  and  cannot  be  reasonably  referred 
to  the  details  of  the  topography,  goes  far  also  to  explain  the 
fanning  out  or  radial  dispersion  of  the  erratics.  During  the 
period  of  maximum  glaciation  the  bowlders  were  dragged  by 
the  ice  along  lines  corresponding  to  the  glacial  striae,  while  dur- 
ing the  period  of  waning  and  more  easterly  movement,  when 
striation  had  practically  ceased,  they  were  dropped  by  basal 
melting  and  the  ice  flowed  over  them.  It  appears  entirely  un- 
necessary, therefoi^e,  in  this  region  at  least,  to  rely  for  the 
explanation  of  radial  dispersion  upon  the  principle  proposed  by 
Chamberlain  that  the  movement  of  the  ice-sheet  tended  every- 


554 

where  to  be  normal  to  the  adjacent  portion  of  its  lobate  margin. 
In  fact,  it  is  probable  that  during  the  waning  of  the  ice-sheet, 
which  is  the  only  period  we  need  take  account  of,  the  ice  upon 
this  plain  country  had  gradually  ceased  to  move  befoi'e  ablation 
had  brought  its  margin  near  enough  to  the  ledges  in  question 
to  exert  an  appreciable  influence  upon  the  direction  of  flow. 
On  the  contrary,  it  is  probable  that,  in  accordance  with  the 
principle,  also  proposed  by  Chamberlain,  that  the  ice  tends  to 
become  stagnant  first  upon  the  uplands,  where  it  has  the  least 
thickness  and  power,  the  ice  diverted  toward  Boston  Harbor  by 
the  Blue  Hills,  continued  to  flow  eastward  through  that  trough 
after  it  had  become  stagnant  or  had  completely  disappeared  upon 
the  peneplain  to  the  south. 

The  most  favorable  opportunity  for  observing  far-travelled 
erratics  and  their  courses  in  crossing  the  Boston  Basin,  is 
afforded  by  the  marine  sections  of  the  drumlins  of  North  Wey- 
mouth, Hough's  Neck,  and  the  neighboring  islands.  These 
are  on  a  broad  area  of  slate  ;  and,  in  accordance  with  the  prin- 
ciple that  the  drift  is  mainly  local,  we  find  that  the  slate  detri- 
tus very  largely  predominates.  There  are,  however,  many  large 
bowlders  of  conglomerate  from  Hough's  Neck,  Moon  Island 
and  Squantum,  one  of  these  on  the  extreme  west  end  of  Grape 
Island  measuring  twelve  by  twenty  feet,  and  six  to  nine  feet 
thick.  But  of  more  special  interest  are  the  occasional  bowl- 
ders of  granite  and  felsite,  which  it  is  certain  have  crossed  the 
entire  breadth  of  the  Boston  Basin  —  twelve  to  sixteen  miles 
—  from  the  ledges  of  Medford,  Maiden,  etc.  The  most  west- 
erly outcrops  of  felsite  on  the  north  side  of  the  basin  are  in 
Medfoi^d  ;  but  the  lithologic  character  of  the  felsite  erratics 
shows  that  very  few  have  come  from  further  west  than  Maiden 
and  Melrose;  and  it  is,  therefore,  a  safe  conclusion  that  these 
rather  far-travelled  masses  have,  during  their  transit  across  the 
basin,  held  very  closely  to  the  normal  south- southeast  course 
of  the  ice-sheet.  A  large  majority,  if  not  all,  of  tliese  masses 
were  certainly  englacial  during  the  main  part  of  their  transit ; 


.      555 

and  referring  once  more  to  the  differential  motion  of  the  upj^er 
and  basal  portions  of  the  ice  due  to  the  topographic  influence, 
the  generalization  is  suggested  that  for  the  drift  as  a  whole,  with, 
of  course,  many  exceptions,  the  transportation  of  the  local  and 
relatively  or  actually  basal  drift  was  more  aberrant  in  trend  than 
that  of  the  far-travelled  englacial  drift,  so  that  the  latter  may 
sometimes  be  the  more  readily  and  accurately  referred  to  its 
source.  In  other  words,  all  drift  is  at  first  subglacial  or  basal, 
and  its  movement  is  controlled  to'  some  extent  by  the  rock 
contours  ;  but  before  it  has  travelled  far  it  becomes  englacial 
and  shares  the  more  rapid  and  rectilinear  normal  movement  of 
the  ice. 

I  have  elsewhere  ^  shown  that  the  modified  drift  is  probably 
derived  chiefly  from  englacial  drift  during  the  superficial  and 
basal  melting  or  ablation  of  the  ice  ;  and  hence,  in  accoi'dance 
with  what  precedes,  it  should  be  in  general  of  less  local  char- 
acter. To  test  this  influence,  I  examined,  with  the  aid  of 
several  students  in  the  Massachusetts  Institute  of  Technology, 
the  composition  of  the  esker  forming  Hunt's  Point  on  the 
northwest  shore  of  Weymouth,  in  the  part  which  has  been 
extensively  excavated.  North  of  this  point,  in  the  line  of  gla- 
cial movement,  are  three  broad  belts  of  rocks  :  First,  slates  and 
conglomerates  of  the  Boston  Basin  (Carboniferous),  about 
thirteen  miles  ;  second,  hornblendic  granites,  diorite  and  fel- 
site,  with  some  Cambrian  slate  and  quartzite,  eight  to  ten 
miles ;  third,  mica  schists,  muscovite  granites  and  gneiss, 
pegmatite,  etc.,  extending  into  New  Hampshire.  We  found 
on  lookino;  over  some  tons  of  material  that  of  all  v/hich  was 
coarse  enough  for  easy  identification  about  50  per  cent,  is  from 
the  first  belt,  40  per  cent,  from  the  second,  and  10  per  cent, 
from  the  third.  Hence  it  is  probably  safe  to  assume  that  more 
than  half  the  coarse  material  (cobbles  and  bowlders)  of  the 
modified  drift  is  five  to  ten  miles  from  its  source,  and  a  good 
fraction  as  much  as  twenty  miles. 

1  Amer.  geol.,  vol.  17,  pp.  203-234;  Tech.  quart.,  vol.  9,  pp.  116-144. 


556 

"Of  conspicuous  bowlder  trains  or  patches  which  cannot  be 
regarded  as  mere  talus  accumulations,  the  Blue  Hills  afford  a 
number  of  good  examples,  which  are  quite  clearly  shown  on 
the  topographic  map  (PI.  14),  occurring,  as  a  rule,  on  the 
lee  or  southeast  sides  of  prominent  rock  elevations.  In  other 
cases  they  are  clearly  residuary  portions  of  the  till  w^here  it  has 
been  exposed  to  the  wash  of  subglacial  or  other  torrents.  A 
very  typical  example  of  this  kind  occurs  on  the  north  side  of 
West  Street,  in  Braintree,  southeast  of  the  Quincy  Reservoir; 
and  there  are  many  others,  as,  for  instance,  between  the  drum- 
lins  on  the  north  side  of  Elm  Street  near  Adams  Street  in 
Braintree. 

Of  bowlders  of  local  origin  arid  especially  notable  only  for 
size  or  situation,  there  are  many  besides  those  which  have  been 
mentioned.  Along  the  south  side  of  the  Blue  Hills,  and  par- 
ticularly of  Great  Blue,  are  many  large  and  often  quite  angular 
bowlders  of  quartz  porphyry  indentical,  lithologically,  with  the 
neighboring  ledges.  Some  of  these  are  fifteen  to  twenty  feet 
in  diameter.  On  the  southern  slope  of  Rattlesnake  Hill  is  a 
block  of  the  black  quartz  porphyry  (p.  363),  approximating  a 
twenty-foot  cube  ;  but,  although  detached,  it  rests  upon  a  ledge 
of  the  same  character  and  is  virtually  in  situ.  In  the  little 
swamp  immediately  west  of  Wampatuck  Hill  is  another  bowl- 
der of  quartz  porphyry,  which  is  nearly  a  fifteen-foot  cube. 
One  very  large  bowlder  of  the  greenish  fluidal  felsite  was  noted 
on  the  northeast  side  of  Cedar  Swamp,  near  the  angle  in  the 
Braintree  and  Quincy  boundary.  Of  exceptionally  large  granite 
bowlders  in  the  Blue  Hills  several  may  be  mentioned.  Two 
of  these  occur  lying  side  by  side  a  little  south  of  Pine  Tree 
Brook  and  nearly  half  a  mile  east  of  Randolph  Avenue.  A 
granite  bowlder  twenty  feet  long  lies  on  the  north  side  of  the 
carriage  road  south  of  Rattlesnake  Hill ;  and  another,  fifty-six 
feet  in  circumference  and  eight  feet  high,  rests  upon  the  sum- 
mit of  Babel  Rock,  northeast  of  Rattlesnake  Hill. 

South  of  the  Blue  Hills,  on  the  coarse,  basal  conglomerate 


557 

of  the  Norfolk  Basin,  are  many  large  and  handsome  bowlders, 
which  are  to  a  large  extent  practically  hi  situ  • —  residuary 
masses  and  not  glacial  erratics.  They  are  commonly  from 
fifteen  to  twenty-five  feet  or  more  in  diameter  ;  and  in  some 
cases,  as  in  the  woods  north  of  Ponkapog  Pond,  they  are  so 
closely  grouped  as  to  constitute  very  typical  and  picturesque 
"  rock  cities." 

East  of  the  Blue  Hills  there  are  but  few  large  bowlders  ; 
and  the  only  one  demanding  farther  mention  is  that  giant 
amono^  the  bowlders  of  Massachusetts  —  House  Rock.  This 
majestic  erratic  stands,  a  conspicuous  feature  of  the  landscape, 
about  half  a  mile  northwest  of  Whitman  Pond,  in  the  angle 
east  of  Essex  Street  and  south  of  Broad  Street  (see  the  map), 
at  the  northwestern  base  of  a  low  hill  of  the  biotitic  normal 
granite  (PL  28,  Fig.  1),  while  near  by  on  the  north  rises  a 
small  but  very  typical  sand  plain  of  the  Whitman  series  (95 
feet),  (PI.  25).  The  top  of  the  bowlder  overlooks  both  of 
these  topographic  features.  It  rests  upon  a  northward  sloping 
and  distinctly  glaciated  ledge  of  granite  :  and  it  is,  therefore,  a 
true  erratic  and  not  in  any  sense  a  merely  residuary  mass. 
Viewed  from  the  northeast  (PI.  28,  Fig.  2),  the  bowlder  pre- 
sents a  rather  striking  profile,  first  noted  by  Mr.  John  J.  Loud, 
to  whom  I  am  indebted  for  this  illustration,  which  has  caused 
it  to  be  known  locally  as  the  Weymouth  Sphinx.  The  max- 
mium  horizontal  dimensions  of  the  bowlder  are  :  north  side,  37 
feet ;  east  side,  25  feet ;  south  side,  42  feet ;  and  west  side,  42 
feet.  The  circumference,  obtained  by  applying  a  tape  line  at 
a  height  of  six  feet,  is  126  feet.  The  height,  determined 
by  a  hand  level,  is  37  feet,  the  top  of  the  bowlder  rising  3 
feet  above  the  summit  of  the  hill  at  the  base  of  which  it  lies. 
In  volume,  House  Rock  must  approximate  a  cube  35  feet  on 
an  edge,  or  over  40,000  cubic  feet.  At  170  pounds  per  cubic 
foot,  the  weight  is  about  7,000,000  pounds  (3,500  tons).  It 
is  thus  nearly  half  as  large  as  the  great  Madison  bowlder  of 
New  Hampshire.  Lithologically,  House  Rock  is  indistinguish- 
able   from  the  ledofe  on  which  it  rests  and  the  orranite  which  is 


558 

quarried  on  the  hill  above ;  and  similar  granite  occurs  in 
ledges  to  the  northward  for  three  quarters  of  a  mile,  or  nearly 
to  Commercial  Street.  Hence,  although  the  great  bowlder  has 
clearly  not  been  derived  from  the  hill  on  the  southeast,  and  is 
an  undoubted  glacial  erratic,  the  probabilities  are  very  great 
that  it  is  not  far-travelled.  In  fact,  the  parent  ledge  is  quite 
certainly  one  of  the  group  of  ledges  on  the  west  side  of  Essex 
Street  north  of  the  almshouse  ;  and  the  Weymouth  Sphinx  is 
now,  probably,  between  one  half  and  three  fourths  of  a  mile 
from  its  birthplace. 

The  power  of  the  ice-sheet  to  rend  the  solid  ledges  and  bear 
away  great  blocks  of  stone  is  well  illustrated  by  many  half 
disrupted  ledges  on  the  lee  slopes  and  in  the  chasms  of  the 
Blue  Hills  ;  but  the  most  striking  example  of  all  is  that  to 
which  my  attention  was  first  directed  by  Mr.  John  J.  Loud. 
It  is  in  the  woods  on  the  east  side  of  Whitman  Pond,  just  south 
of  the  southern  border  of  the  diorite,  where  the  biotite  granite 
is  well  sprinkled  and  in  part  crowded  with  the  segregations  and 
inclusions  of  diorite.  The  western  face  of  a  hig-h,  bold  ledo^e 
of  the  granite  has  been  sufficiently  disrupted  by  the  last  efforts 
of  the  ice,  to  form  what  is  virtually  a  roofless  cavern  which  may 
be  entered  through  a  narrow  crevice  about  twenty-five  feet 
long  and  closed  above.  Another  example,  equally  instructive 
but  less  impressive,  occurs  on  the  south  side  of  Washington 
Street  about  one  fourth  of  a  mile  east  of  Lovell  Corners.  In 
view  of  these  facts  it  is  plain  tfiat  in  the  production  of  its 
freight  of  bowlders  the  ice-sheet  must  in  some  instances  have 
quarried  away  whole  hills  of  solid  rock,  so  that  it  is  not  sur- 
prising that  in  the  case  of  such  a  giant  as  House  Rock  the 
parent  ledge  was  destroyed  in  its  making,  and  nothing  remains 
to  mark  the  site. 

In  closing  this  section,  we  may  notice  an  interesting  perched 
bowlder  of  granite  beyond  the  southern  limit  of  the  map.  It 
rests  (PI.  29)  on  the  sloping  surface  of  a  bold  ledge  of  granite 
on  the  south  side  of  Oak  Street,  near  the  Hingham  boundary. 
It  is  13  feet  long,  0  to  8  feet  wide,  and  7  to  8  feet  high. 


Occas. Papers, Boston  Soc.Nat.Hist.,Vol.Iv: 


Plate  ?.B. 


THE    HELIOTYPE    PRINTING    CO..     BOSTON. 


HOUSE  ROCK,  WEYMOUTH. 


Occas.  Papers,  Boston  Soc.  Nat.  Hist.,  Vol.  IV. 


Plate  29. 


PERCHED  BOWLDER  ON   OAK  STREET,   WEYMOUTH. 


559 


GLACIAL  GEOLOGY  — RETREATAL  PHENOMENA. 

The  retreatal  phenomena  of  the  ice-sheet  are  recorded  with 
greater  or  less  clearness  in  the  varied  phases  of  the  washed  or 
modified  drift,  including  :  first,  the  eskers  and  kames,  marking 
the  courses  of  glacial  streams  ;  second,  the  over-wash  plains  and 
marginal  plains  or  terraces  ;  third,  the  delta  plains,  or  the  de- 
posits of  glacial  lakes  and  the  kettles ,  by  which  they  are  diversi- 
fied ;  and  fourth,  the  glacial  lakes  themselves,  with  their  succes- 
sive outlets  and  shores.  For  this  area,  however,  the  other 
phenomena  are,  practically,  all  comprised  in  a  study  of  the  gla- 
cial lakes,  and  do  not  demand  separate  consideration. 

Besides  Lake  Bouve,  w^hich  occupied  the  entire  sweep  of  the 
southern  watershed  of  the  Boston  Basin  for  nearly  a  dozen 
miles,  from  Randolph  Avenue  south  of  the  Blue  Hills  to  the 
eastern  boundary  of  Hingham,  a  much  smaller  lake  —  a  glacial 
lakelet  —  nestled  in  the  concave  curve  of  the  northern  slope  of 
the  main  range  of  the  Blue  Hills  during  the  -time  when  the  mar- 
gin of  the  ice-sheet  formed  the  chord  of  this  arc. 

The  Glacial  Lake  of  the  Blue  Hills.  —  This  little  lake, 
which  was  probably  contemporaneous  with  and  tributary  to  the 
earlier  stages  of  Lake  Bouve,  left  its  record  in  the  modified 
drift  on  the  northern  slope  of  the  main  range,  midway  of  its 
length,  or  between  Hancock  Hill  on  the  west  and  Chickatawbut 
on  the  east.  The  extreme  length  of  the  free  ice-barrier  was 
about  one  and  a  half  miles.  But  at  no  time,  probably,  did  the 
water  area  much  exceed  a  mile  in  length  by  half  a  mile  in 
breadth,  following  approximately  the  contour  of  230  feet  (see 
PI.  14) .  The  highest  outlet  of  the  lake  is  the  one  which,  in 
the  general  northeastward  recession  of  the  margin  of  the  ice- 
sheet,  was,  naturally,  uncovered  first, — viz.,  Ponkapog  Pass 
between  Hancock  Hill  (510  feet)  and  Tucker  Hill  (449  feet), 
with  an  elevation  of  about  230  feet.  The  deepest  line  or  axis 
of  the  pass  lies  close  along  the  base  of  the  abrupt  southeastern 


560 

slope  of  Hancock  Hill,  some  six  hundred  feet  northwest  of  Hill- 
side Street.  During  the  existence  of  this  outlet  the  water  was 
tributary  southward,  down  the  valley  of  Marigold  Brook  to 
Hoosicwhisick  Pond  (165  feet),  and  thence  eastward  by  the 
valley  of  Monatiquot  Stream  to  Lake  Bouve.  The  Hoosic- 
whisick plain  (175  to  180  feet)  enclosing  this  large  kettle  pond, 
indents  the  southern  margin  of  the  Blue  Hills  and  borders  the 
valley  of  Marigold  Brook  as  a  well-defined  terrace  northward 
for  about  one  third  of  a  ,mile,  beyond  which  the  valley  rises 
fifty  feet  in  another  third  of  a  mile  to  the  summit  of  the  pass. 

This  outlet,  although  quite  certainly  the  principal  one,  was 
probably  short-lived  :  for,  unless  the  ice  margin  were  stationary 
for  a  time,  its  recession  must  soon  have  uncovered  Randolph 
Pass  (250  feet),  and,  a  little  later,  Braintree  Pass  (220  feet) 
at  the  head  of  Blueberry  Swamp  and  the  southern  base  of 
Chickatawbut  Hill  ;  and  the  latter  event  involved,  necessarily, 
the  abandonment  of  the  Ponkapog  outlet,  the  glacial  waters 
becoming  tributary  to  Monatiquot  Stream  and  Lake  Bouve,  east- 
ward through  Ced'ar  Swamp,  instead  of  southward  thi'ough 
Hoosicwhisick  Pond. 

Ponkapog  Pass  was  an  ideal  outlet  of  the  temporary  moun- 
tain lake  :  and  adjacent  to  it  on  the  north  we  find  the  chief  de- 
velopment of  deposits  formed  in  that  lake  by  the  glacial  streams 
tributary  to  it.  The  small  hill  between  the  pass  and  Hillside 
Street  (271  feet)  is  covered  with  till ;  but  immediately  north 
of  this,  in  the  angle  between  Hancock  Hill  and  Hillside  Street, 
the  modified  drift  is  abundant,  taking  the  form,  chiefly,  of  dis- 
continuous eskers,  kame-like  knolls  and  immature  plains.  Two 
systems  of  eskers  appear  to  be  recognizable  :  a  low,  more  or 
less  indistinct  and  apparently  older  line  trending  in  a  south- 
southwest  direction  toward  the  pass,  but  not  reaching  it ;  and 
a  strongly  marked  and  typical  northwest-southeast  series,  cross- 
ing the  first  and  tributary  to  an  irregular  and  distinctly  imma- 
ture delta  plain  lying  northeast  and  east  of  Hillside  Pond.  This 
series  really  embraces  two   eskers  —  one  on  either  side  of  the 


5G1 

brook  flowing  from  the  pond,  and  converging  on  tlie  nortliwest 
corner  of  the  phiin.  The  western  esker  begins  on  the  northeast 
slope  of  Hancock  Hill,  and,  after  following  this  for  several 
hundred  feet,  turns  abruptly  to  the  eastward,  ending  on  the 
line  of  the  supposed  earlier  south  westward  esker;  beyond  this 
it  continues  as  a  hioh  and  bold  embankment  extending;  some  six 
hundred  feet  to  Hillside  Street.  The  second  esker  of  this  series 
is  closely  parallel  with  this  last  link  of  the  first  one,  with  only 
the  brook  between  ;  but  it  cannot  be  follow^ed  beyond  or  north- 
west of  the  brook.  The  ice  margin  and  delta  front  of  the  plain 
are  normally  developed  :  but  the  surface  of  the  plain  is  very 
uneven,  with  knobs  and  bowlders  of  the  underlying  till  project- 
ing through  it ;  and  bowlders  are  also  a  feature  of  the  eastern 
esker.  Both  the  plain  and  its  tributary  eskers  are  diversified 
by  kettles,  and  closely  correspond  in  height  with  the  Ponkapog 
outlet.  The  less  distinct  line  of  esker  with  the  southwestward 
trend  may  be  supposed  to  date  from  the  time  when  the  ice  al- 
most completely  filled  this  basin,  and  the  subglacial  stream  was 
necessarily  diverted,  at  the  last,  from  its  normal  southeasterly 
course  toward  Ponkapog  Pass,  which  is  fully  eighty  feet  lower 
than  the  more  direct  pass  between  Tucker  and  Boyce  Hills. 

With  the  birth  of  the  glacial  lake,  however,  the  course  of 
the  glacial  stream  and  the  location  of  its  mouth  became  inde- 
pendent of  the  outlet,  although  the  latter  determined  even 
more  rigidly  than  before  the  upper  limit  or  maximum  elevation 
of  the  resulting  deposits. 

East  of  the  eskers,  on  the  north  side  of  Hillside  Street,  is  a 
second  immature  plain  with  a  strongly  undulating  surface  and 
a  high  ice  margin  on  the  edge  of  the  deep  valley  to  the  north. 
Northward  down  the  valley  of  Hillside  Pond,  is  also  much 
modified  drift ;  but  the  retreat  of  the  ice  margin  was,  appar- 
ently, too  rapid  to  permit  the  'development  of  a  normal  delta 
plain  in  water  from  fifty  to  over  a  hundred  feet  deep.  About 
one  fourth  of  a  mile  northeast  of  Hillside  Pond  a  small  brook 
flows  northward  across  Hillside  Street  from  the  pass  between 

OCCAS.  PAPERS  B.  S.  JT.  H.  IV.  36. 


562 

Boyce  and  Tucker  Hills  ;  and  farther  to  the  northeast  there  are 
onlv  slio;ht  mdications  of  modified  drift  over  the  o-ently  undu- 
lating  surface  of  till  until  we  have  crossed  the  valley  of  Blue- 
berry Swamp.  This  intervening  ground,  it  may  be  noted,  is 
mainly  above  the  level  of  Braintree  Pass,  which  was  probably 
opened  soon  after  the  ice  retreated  from  the  vicinity  of  Hillside 
Pond. 

The  only  noteworthy  development  of  modified  drift  which 
may  be  correlated  with  the  Braintree  Pass  outlet  is  that  on  the 
line  of  Randolph  Avenue  near  Hillside  Street ;  and  the  most 
notable  feature  of  this  occurrence  is  the  completely  isolated 
mound  of  gravel  rising  twenty-five  to  thirty  feet  above  the 
general  level  between  the  avenue  and  the  street.  The  origin 
•of  this  solitary  hillock  is  by  no,  means  clear.  Its  slopes  are 
steep  ;  it  is  elongated  in  an  east-west  direction  ;  and  it  is  com- 
posed in  part  of  quite  coarse,  angular  and  ill-assorted  gravel, 
with  an  occasional  large  stone.  The  elevation  of  its  summit 
must  be  about  220  feet,  agreeing  not  only  with  Braintree 
Pass,  but  also  with  the  narrow  development  of  modified  drift 
bordering  the  higher  till  slope  on  the  east  side  of  Bandolph 
Avenue.  The  hillock  might  be  regarded  as  an  erosion  outlier 
of  this  plain  or  terrace  ;  or  as  a  remnant  of  an  esker  tributary 
to  the  plain  ;  or,  perhaps,  better  still,  as  a  deposit  formed  in 
such  a  basin  or  hole  in  the  stagnant  margin  of  the  ice-sheet 
as  Russell  has  described  in  the  stagnant  margin  of  the  Mala- 
spina  glacier  in  Alaska.^ 

The  comparative  meagerness  of  the  deposits  in  the  eastern 
part  of  the  basin,  while  possibly  attributable  to  the  absence 
of  glacial  streams  of  adequate  power,  is,  I  think,  more  satis- 
factorily accounted  for  on  the  supposition  that,  soon  after  the 
opening  of  the  Braintree  Pass  outlet,  the  ice  completely 
relaxed  its  hold  on  the  western  end  of  the  range  and  the  im- 
pounded waters  subsided  to  the  level  of  Lake  Neponset,  which 
was  then  discharging  eastward  across  the  Hoosicwhisick  plain 

1  Jouni.  geol.,  vol.  l.pp.  230-233. 


563 

at  an  elevation  of  about   170  feet.     This  abandonment  of  its 
■site  was  the  closing  episode  in  the  histoiy  of  the  glacial  lakelet 
of  the  Blue  Hills. 

Although  we  seem  to  have  in  the  broad  and  gently  sloping 
plain  northwest  of  Hancock  and  Hemenway  Hills  a  consider- 
able surface  development  of  modified  drift,  referable,  perhaps, 
to  the  Hoosicwhisick  stage  of  Lake  Neponset,  it  is  probably,  in 
the  main,  of  slight  volume,  a  thin  deposit  of  sand  and  gravel 
over  a  plain  of  till.  And  the  almost  complete  absence  of 
modified  drift  from  the  upper  valley  of  Pine  Tree  Brook, 
seems,  at  first,  to  show  that  the  ice  retired  rather  rapidly  from 
this  part  of  the  hills.  It  is  more  probable,  however,  that  at 
this  stage  of  the  glacial  retreat  the  glacial  streams,  upon 
which  the  formation  of  the  modified  drift  was  mainly  depend- 
ent, were  diverted  to  the  east  or  west  by  the  high  relief  of  the 
Blue  Hills.  In  accordance  with  this  suggestion  we  find  that 
the  eskers  along  the  northern  base  of  the  hills  agree  closely 
in  trend  with  the  maro-in  of  the  northern  range,  the  clearest 
example  being  the  beautiful  esker  on  the  Cunningham  estate 
south  of  Edge  Hill  Road  in  Milton.  These  eskers  and  the 
plains  accompanying  them  may  be  most  conveniently  described 
in  connection  with  the  history  of  Lake  Neponset  in  Part  4. 


564 


LAKE  BOUVE,  AN  EXTINCT  GLACIAL  LAKE  IN  THE  SOUTH- 
ERN PART  OE  THE  BOSTON  BASIN.' 

Br  AMADEUS  W.  GEABAU. 

Introduction, 

Among  the  glacial  deposits  of  eastern  Massachusetts,  the 
frontal  accumulations  of  washed  sands  and  gravels  hold  a  pi-e- 
eminent  position,  not  alone  on  account  of  their  extensive 
development,  but  also  because  they  furnish  the  best  criteria  by 
which  the  succession  of  events  in  the  glacial  history  of  this 
region  may  be  determined.  J.  B.  Woodworth,  in  a  recent 
paper, ^  has  given  a  comprehensive  general  account  of  these  de- 
posits in  southern  New  England,  and  has  applied  to  them  the 
name  of  glacial  wash-plains.  He  recognizes  four  types  of 
glacial  wash-plains,  dependent  for  distinction  on  their  relation 
to  the  ice-margin.  These  he  describes  as  follows  :  (a)  "  Fron- 
tal moraine  terraces,  with  an  ice-contact  slope,  charged  with  till 
and  bowlders,  a  true  morainal  deposit."  (b)  "Frontal  ter- 
races, like  the  preceding  but  lacking  the  till-coating  along  the 
ice  contact."  (c)  "  Esker-fans,  small  plains  of  gravel  and 
sand  built  at  the  mouth  of  subglacial  tunnels  and  channels  in 
the  ice  ;  associated  with  an  esker  or  esker-like  chain  of  deposits 
made  in  the  ice-sheet  at  the  same  time."  (d)  "Wash-cones, 
steeply  sloping  deposits,  with  ice-contact  slope  on  the  iceward 

1  The  existence  of  the  glacial  lake  now  known  as  Lake  Bo^uve  was  clearly  recog- 
nized by  Professor  Crosby  in  Part  2  of  this  work;  and  he  has,  on  pages  274  to  286, 
given  a  general  account  of  its  features —  eskers,  delta-plains  and  outlets  —  so  far  as 
they  are  developed  in  the  area  covered  by  that  Part.  My  purpose,  therefore,  has 
been  to  extend  this  stiidy,  in  greater  detail  and  in  the  light  of  the  later  advances  of 
glaciology,  over  the  entire  basin  of  the  lake.  It  affords  me  pleasure  to  acknowledge 
the  cordial  cooperation  of  Professor  Crosby  in  the  prosecution  of  this  work,  and  the 
valuable  assistance  of  Mr.  M.  L.  Fuller  in  the  study  of  the  Monaticinot  Valley. 

"-  Bull.  Essex  inst.,  vol.  29,  1899,  pp.  71-119. 


565 

side  culminating  in  a  liigh  point,  with  gentler  slope  outward, 
in  the  manner  of  alluvial  cones." 

Of  these  four  types  the  frontal  terraces  are  by  far  the  most 
important^  because  most  frequent  and  most  extensive.  They 
naturally  fall  into  two  groups,  the  glacial  sand-plain  or  delta, 
and  the  frontal  apron  plain,  the  former  a  subaqueous,  the 
latter  a  subaerial  deposit.  Esker-fans,  in  so  far  as  they  fall 
in  either  the  one  or  the  other  of  these  groups,  are  best  con- 
sidered with  the  frontal  terraces. 

When  most  typically  developed,  the  glacial  sand-plain  pre- 
sents three  types  of  topographic  elements  which  are  strongly 
contrasted,  and  which  in  the  detail  of  their  features,  and  in 
their  combination,  serve  to  distinguish  the  glacial  sand-plain 
from  sand  and  gravel  deposits  of  similar  character  though 
different  origin.  These  three  elements  are  the  top,  front,  and 
back  slopes.  In  all  cases,  these  slopes  are  of  constructional 
origin,  i.  e.,  their  characteristics  are  due  to  the  deposition 
of  materials  derived  from  the  ice  sheet. 

The  top  slope  is  characterized  by  a  low  gradient,  which, 
though  variable,  probably  never  exceeds  the  limit  of  a  few 
degrees.  It  may  be  a  nearly  plane  surface  descending  in  one 
direction,  or,  what  is  perhaps  more  frequent,  it  may  have  the 
form  of  an  exceedingly  flat  semi-cone,  whose  apex  forms  the 
head  of  the  delta. 

The  frontal  slope  normally  bounds  the  glacial  delta  on  three 
sides,  describing  a  curve,  varying  from  semicircular  to  elon- 
gately  semi-oval  in  outline,  with  the  longer  axis  parallel  to  the 
ice-front.  This  slope  may  have  a  vei\y  moderate  gradient,  or 
it  may  be  as  high  as  twenty  degrees  or  even  more.  In  all 
cases,  however,  it  has  a  much  steeper  gradient  than  the  top 
slope,  audit  usually  joins  that  slope  so  as  to  form  with  it  a 
pronounced  obtuse  angle.  In  all  but  the  very  smallest  sand- 
plains  the  outline  of  the  front  is  not  a  continuous  curve,  but 
rather  a  combination  of  curves,  giving  a  lobate  character  to 
the  outer  margin.      (Plate  30.) 


566 

The  back  slope,  when  typically  developed,  is  distinguished 
from  the  frontal  slope  by  its  steeper  gradient,  this  being  nor- 
mally the  maximum  angle  of  repose  of  the  unconsolidated 
material,  and  by  the  absence  of  convex  lobation,  there  being 
often  instead  a  pronounced  concave  lobation,  M^ith  more  or  less 
prominent  cusps  between  the  concavities.  The  line  of  junction 
between  the  back  and  top  slopes  is  typically  a  sharp  one,  the 
angle  between  these  slopes  being  usually  much  less  obtuse 
than  that  between  the  top  and  frontal    slopes. 

A  perfect  development  of  the  three  types  of  slopes  in  a  given 
glacial  sand-plain  is  relatively  uncommon,  the  form  being 
modified  to  a  greater  or  less  extent.  The  most  frequent  modi- 
fication of  the  top  slope  consists  in  the  presence  of  kettle  holes, 
which,  when  perfectly  developed,,  are  sub-circular  or  elongate 
depressions,  with  steep  slopes  comparable  to  the  back  slopes  of 
the  sand-plains.  These  depressions  must  not  be  confounded 
with  the  elongated,  often  narrow  and  shallow  depressions  or. 
fosses,  which  mark  the  imperfect  junction  of  two  successive 
sand-plains.  In  these  latter  depressions,  the  slopes  are  not 
similar,  but  present  on  one  side  the  characters  of  a  frontal, 
and  on  the  other  those  of  a  back  slope.  Other  modifications 
of  the  noi'mal  top  slope  consist  in  longitudinal  creases  which 
are  merely  the  backward  extensions  of  the  interlobate  depres- 
sions of  the  frontal  slopes,  and  may  commonly  be  regarded  as 
due  to  subsequent  erosion.  The  modifications  of  the  frontal 
slopes,  unless  affected  by  erosion,  are  chiefly  in  the  angle, 
which,  as  already  stated,  may  vary  within  certain  limits.  The 
back  slopes  are  most  subject  to  variation.  This  often  aflPects 
the  outline,  which  may  range  from  straight  to  cuspate  or  very 
irregular;  or  it  may  aflPect  the  inclination,  which  may  be  steep, 
or  gentle  and  more  or  less  uniform,  or  very  irregular,  the 
surface  becoming  hummocky  or  kame-like.  Finally,  an  esker, 
or  a  series  of  eskers,  may  join  the  back  slope,  as  is  habit- 
ually the  case  in  esker  fans.  A  not  infrequent  though  acci- 
dental   modification  of  the  slopes,   and    one  which    affects  all 


5G7 

three  types,  is  found  in  pre-existing  i-ock  or  till  masses  against 
or  around  which  the  plain  has  been  built.  These  masses  some- 
times entirely  replace  one  or  the  other  of  the  side  slopes,  but 
they  rai'ely  cause  more  tha«  an  interruption  of  the  top  slope. 

In  structure  and  in  the  character  of  the  component  materials, 
the  glacial  sand-plains  exhibit  significant  features.  The  char- 
acter of  the  o-ravel  is  that  of  sub-ano-uhir  or  well-rounded  water- 
worn  and  water-laid  material,  sharply  angular  pebbles  and  those 
exhibiting  glacial  markings  being  generally  absent  except  where 
the  former  are  produced  by  the  subsequent  splitting  of  water- 
Avorn  pebbles.  The  stratification  is  usually  well  marked  and  is 
divisible  into  topset  beds,  or  those  approximately  parallel  to 
the  top  slope,  and  foreset  beds,  or  those  approximately 
parallelto  the  frontal  slope.  Truncation  of  the  foreset  beds, 
and  deposition  of  the  topset  beds  upon  the  truncated  edges  of 
the  foreset  beds,  is  a  feature  of  common  occurrence.  Irreg- 
ular bedding  with  thinning  and  thickening  of  layers,  and 
minor  cross-bedding,  and  ripple  marks,  are  among  the  other 
structural  features  usually  found.  What  have  been  denom- 
inated backset  beds,  parallel  to  the  back  slope  have  been 
described,^  but  they  are  of  slight  thickness  and  in  most  cases 
differ  genetically  from  the  topset  and  foreset  beds.  In  grade  of 
texture  the  material  of  a  sand-plain  varies  considerably,  but 
in  practically  all  cases  a  decrease  in  coarseness  is  observable 
from  the  head  to  the  front  of  the  deposit.  The  topset  beds 
commonly  exhibit  a  coarser  texture  than  the  foresets. 

The  glacial  origin  of  the  type  of  sand-plain  above  described 
is  today  probably  unquestioned,  and  their  interpretation  as 
deltas  built  into  a  body  of  standing  water  against  a  temporarily 
stationary  ice-front,  will  probably  be  questioned  by  few.  That 
they  are  subaerial  deposits  is  negatived  by  the  gentle  gradient 
of  the  top  slope,  the  abrupt  change  from  this  to  the  frontal 
slope,  the  lobate  outline  of  the  latter,  and  the  relation  of  top- 
set  and  foreset  beds.     A  subaerial  fan  of  glacial  origin  may 

1  Davis,  Bull.  geol.  soc.  Amer.,  vol.  1,  p.  197. 


568 

exhibit  the  steep  back  slopes  of  the  normal  sand-plain,  but  the 
frontal  slope  is  absent,  the  top  slope  being  continuous  from  the 
head  to  the  foot  of  the  plain.  In  like  manner  the  combination 
of  the  topographical  and  structural  features  of  the  sand-plains 
serves  to  distinguish  them  from  any  other  known  type  of  glacial 
deposits. 

These  deltas  are  most  nearly  related  to  normal  stream  deltas 
built  in  a  standing  body  of  water.  They  share  the  combination 
of  top  and  frontal  slopes  and  internal  structure,  but  the  former 
are  at  once  distinguished  by  the  characteristic  back  slopes, 
which,  in  the  case  of  the  normal  stream  deltas,  are  replaced 
by  the  shore  of  the  body  of  water  into  which  the  delta  is  built. 
This  characteristic  back  slope  is  one  of  the  most  significant 
topographical  features.  It  marks  the  contact  with  the  front 
of  a  now  vanished  ice-sheet,  against  which  the  deposit  was 
banked  and  on  the  removal  of  which,  through  melting,  the 
characteristic  slope  was  assumed  by  a  slumping  of  the  unsup- 
ported material,  this  at  the  same  time  producing  the  appear- 
ance of  backset  beds,  as  suggested  by  Upham.  This  ice- 
contact  slope  is  not  confined  to  this  type  of  deposit,  but  is 
normally  characteristic  of  all  ice-bound  extra-glacial  deposits. 
(See  Woodworth  :  The  ice  contact  in  the  classification  of  gla- 
cial deposits.  Amer.  geol.,  vol.  23,  1899,  pp.  80-86.)  It 
therefore  becomes  a  criterion  by  which  we  can  determine  the 
successive  temporary  stages  of  the  ice-front,  when  it  remained 
stationary  long  enough  to  permit  of  a  frontal  accumulation  of 
"•lacial  detritus. 

While  the  delta  character  of  the  type  of  sand-plain  here 
xiescribed,  and  its  intimate  relation  to  the  front  of  the  waning 
ice-sheet,  admit  of  little  doubt,  opinions  differ  as  to  the  char- 
acter of  the  body  of  water  in  which  the  delta  was  formed. 
We  cannot  conceive  that  deltas  of  this  class  can  form  on  the 
o[)en  sea- coast,  because  any  deposits  there  made  will  be  con- 
tinually modified  by  wave  action,  and  the  perfect  constructional 
slopes,  so  characteristic  of  these  deltas,  will  not  be  retained. 


569 

We  might,  of  course,  assume  with  Simler  ^  that  these  deposits 
were  formed  at  a  depth  below  the  surface  to  wliich  wave  action 
does  not  reach,  but  in  that  case  we  could  not  exj)ect  to 
obtain  the  characteristic  angle  between  the  top  and  frontal 
slopes,  which,  as  far  as  observation  and  experiment  extend, 
marks  the  average  level  of  the  water  in  which  the  delta  was 
forming.  Nor  can  we  expect  the  characteristic  lobate  front, 
indicative  of  a  shifting  or  forking  stream  mouth  with  distrib- 
utaries spreading  out  in  all  directions,  each  building  its  own 
independent  lobe. 

Even  if  the  delta  could  be  formed  without  being  modified  by 
wave  action  while  building,  it  could  not  survive  the  attack  of 
the  waves  during  the  process  of  uplifting,  unless  Ave  postulate 
a  catastrophic  elevation,  as  Professor  Shaler  has  done  in  the 
case  of  the  Martha's  Vineyard  and  Nantucket  plains,  vv'hich  are 
now  generally  regarded  as  subaerial  over-wash  or  apron  plains, 
but  which  he  supposed  to  have  been  formed  at  a  considerable 
depth  below  sea-level.  As  Professor  Shaler  says  :  "  We  may 
be  sure  that  if  they  [the  sand-plains]  were  exposed  even  for  a 
few  days  to  the  action  of  the  Atlantic  surf  and  tides  they  would 
bear  the  unmistakable  marks  of  water  action"  (loc.  cit., 
p.  45).  ~  , 

It  thus  appears  that  the  formation  of  perfect  glacial  deltas  on 
an  exposed  shore,  and  their  subsequent  preservation,  is  a  mat- 
ter of  extreme  improbabili-ty.  Most  of  the  objections  urged 
against  their  formation  on  open  shores  do  not,  however,  hold 
in  partially  enclosed  and  protected  embayments  ;  and  in  such 
basins  sand-plains  may  be  formed,  provided  tidal  action  is 
counterbalanced  by  a  sufficient  influx  of  fresh  water  from  the 
melting  ice.  Fuller^  has  endeavored  to  explain  the  origin  of 
the  Great  Barrington  sand-plain  in  the  Narragansett  Basin  in 
this  way;  but  Woodworth^  holds  that  a  flooded  estuary,  kept 

1  Geology  of  Nantucket,  Bull.  53,  U.  S.  geol.  surv.,  p.  44, 

2  Amer.  geol.,  vol.  21,  1899,  pp.  310-321. 

s  Amer.  geol.,  vol.  18, 1896,  pp,  150-168,  391,  392. 


570 

full  by  excessive  melting  of  the  ice,  and  probably  blocked  by 
residual  ice  masses  in  its  lower  course,  produced  a  fresh-water 
basin  in  which  the  plains  were  deposited  above  the  present  sea- 
level. 

Recognizing  the  possibility  of  the  formation  of  glacial  deltas 
in  protected  basins  at  sea-level,  the  probability  of  their  being 
thus  formed  becomes  very  slight  when  we  deal  with  a  series  of 
plains  of  different  altitudes.  For  in  that  case  we  must  postu- 
late differential  elevation  or  subsidence,  to  account  for  the 
varying  heights  of  the  plains.  If,  as  is  usually  the  case  with 
serial  plains,  successively  lower  series  are  met  with  as  Ave  pro- 
ceed northward,  we  must,  if  we  hold  to  the  submarine  view, 
postulate  successive  slight  and  rapid  elevations,  interrupted  by 
periods  of  relative  stability  ;  and  if,  as  is  also  often  the  case, 
such  series  of  deltas  in  distinct,  though  not  widely  se|)arated, 
areas  along  the  ice-front  show  no  correspondence  in  elevation, 
we  must  further  postulate  a  differential  movement  for  each 
period,  between  the  two  localities.  Thus  the  successive  series 
of  plains  of  the  Weymouth-Hingham  area  bear  no  relation  to 
the  series  in  the  closely  adjoining,  though  separated,  IS^eponset 
Valley,  such  as  we  should  expect  to  find  were  the  elevations  of 
their  top  slopes  determined  by  a  temporarily  higher  sea-level. 
If,  finally,  as  can  be  shown  in  the  majority  of  cases  in  eastern 
Massachusetts,  the  area  in  which  well-developed  sand-plains 
occur  is  rimmed  around  by  high  land  of  rock  and  till,  except 
where,  following  the  indication  of  the  ice-contact  slope  Ave  can 
readily  postulate  the  existence  of  an  ice  dam,  and  if,  further- 
more,, notches  in  this  rim  can  be  correlated  Avith  the  successive 
plains,  the  necessity  for  the  considei'ation  of  the  sea-level  and  its 
relation  to  the  basin  thus  enclosed  falls  away  entirely,  and  the 
theory  of  a  fresh-water  lake,  ponded  behind  an  ice  dam,  and  of 
successive  levels,  as  determined  by  the  uncovering  of  succes- 
sively lower  outlets  in  the  rim,  becomes  the  only  one  Avhich  Avill 
account  satisfactorily  for  the  facts  in  question. 

Lake  Bouve   is  the  first  of  such  vanished    crlacial  lakes  in 


571 

Massachusetts  to  which  a  name  has  been  applied,  and  of  which 
the  outline  and  extent  and  the  succession  of  events  in  its  his- 
toiy  have  been  established.  (See  Part  2,  p.  274  et  seq.)^ 
The  name,  as  stated  by  Professor  Crosby,  was  chosen  in  honor 
of  the  late  Mr.  T.  T.  Bouve,  a  former  President  of  the  Boston 
Society  of  Natui'al  History,  a  life-long  resident  of  Hingham,  and 
an  enthusiastic  student  of  the  glacial  geology  of  that  region. 


Extent  and  Outline  of  Lake  Bouve. 

Lake  Bouve  was  situated  in  what  are  now  Plymouth  and 
Norfolk  Counties,  Massachusetts,  on  the  southern  watershed  of 
Boston  Bay.  The  area  covered  at  one  time  or  another  by  the 
waters  of  this  lake  comprises  nearly  the  whole  of  the  present 
townships  of  Hingham,  Weymouth  and  Braintree,  the  eastern 
half  of  Quincy,  and  the  adjoining  lower  portions  of  Randolph, 
Holbrook  and  Rockland.  This  area  forms  a  basin  open  on  the 
north,  but  surrounded  on  the  east,  south  and  west  by  a  rim  of 
rock  and  till.  Following  this  rim  from  its  northwestern  end  in 
the  Blue  Hills  at  West  Quincy,  to  a  point  a  mile  or  more  north 
of  Prospect  Hill  in  Hingham,  a  distance  of  nearly  thirty  miles, 
we  find  no  pass  below  the  130  to  140  foot  contours.  About  a 
mile  north  of  Prospect  Hill,  however,  the  rim  becomes  de- 
pressed to  95  feet  above  sea-level,  and  from  that  point  north- 
ward to  Turkey  Hill  in  Hingham  it  is  very  irregular,  becoming 
at  one  point  depressed  to  the  60  foot  contour.  North  of  Tur- 
key Hill  the  rim  of  the  basin  quickly  descends  to  the  level  of 
the  salt  marshes. 

The  basin  of  Lake  Bouve  drains  northward  into  Boston  Bay  ; 
and  the  principal  streams  of  this  watershed,  beginning  on  the 
west,  are :  first,  Monatiquot  River,  which  reaches  the  bay 
through  the  estuary  known  as  Weymouth  Fore  River  ;  second, 
and  third.  Mill  River  and  Old  Swamp  River,  both  of  which 

1  Amer.  geol.,  vol.  17, 1896,  pp.  128-130;  Science,  n.  s.,  vol.  3, 1896,  pp.  212,  213. 


572 

are  tributary  to  Whitman  Pond  and  through  that  to  the  estuary 
of  Weymouth  Back  Eiver  ;  and,  fourth  and  fifth,  Plymouth 
River  and  Beechwood  River,  the  two  principal  branches  of 
Weir  River,  which  was  tributary  to  Hingham  Harbor  in  pre- 
fflacial  times,  but  now  reaches  the  bav  through  Nantasket 
Harbor.  The  upper  valley  of  the  Monatiquot  River  is  largely 
of  a  swampy  character.  The  upper  end  of  the  valley  of  Mill 
River  is  occupied  by  Weymouth  Great  Pond,  out  of  which  the 
river  flows.  The  valley  of  Old  Swamp  River,  again,  as  the 
name  implies,  is  of  a  decidedly  swampy  character.  The  valley 
of  Plymouth  River  is  clogged  and  truncated  by  drift ;  but  its 
original  upper  course  is  probably  represented  by  the  basin  of 
Accord  Pond,  which  is  now,  through  the  accident  of  drift 
accumulation,  tributary  to  Beechwood  River.  And  on  the 
water-parting,  in  the  direct  line  of  Beechwood  River,  lies  Val- 
ley Swamp.  At  the  heads  of  these  streams  we  find,  naturally, 
the  lowest  depressions  or  passes  of  the  southern  water-parting 
or  rim  of  the  basin.  The  western  valleys  are,  in  their  upper 
courses,  the  most  deeply  incised  ;  but  this  more  strongly  ac- 
centuated topographic  character  is  compensated  by  the  increased 
elevations  of  the  water-parting  westward  ;  so  that,  in  spite  of 
their  varying  depths,  we  find  a  surprising  uniformity  in  the 
elevations  of  the  passes.  The  first,  third  and  fifth  passes,  or 
those  at  the  head,  respectively,  of  Monatiquot  River,  Old 
Swamp  River  and  the  direct  line  of  Beechwood  River  (Valley 
Swamp  Pass) ,  are  tl>e  lowest.  They  vary  but  little  in  eleva- 
tion from  140  feet,  and  agree  so  closely  that,  apparently,  tliey 
served  simultaneously  as  outlets  of  Lake  Bouve.  The  some- 
what greater  elevations  of  tlie  Weymouth  Great  Pond  and 
Accord  Pond  passes,  is  probably  due  in  part,  at  least,  to 
greater  depths  of  drift. 

The  elevation  of  the  water  surface  at  Weymouth  Great 
Pond  is  slightlv  aljove  150  feet.^     The  pass  at  the  head  of  tlie 

'  Mr.  Frank  A.  Bates,  Secretary  of  the  Boston  Scientific  Society,  kindly  furnished 
me  with  the  levels  of  the  pipe-line  from  Weymouth  Great  Pond  to  Weymouth  town 
hall.    The  water-level  of  the  l'on<l  was  aivew  in  the  notes  as  152.611  feet  at  the  outlet. 


pond  has,  therefore,  an  elevation  of  possibly  160  feet,  and 
could  have  served  as  an  outlet  for  the  impounded  waters  only 
while  the  ice  still  occupied  the  main  basin  and  closed  the  north- 
ern end  of  this  valley.  The  shallowness  of  the  Accord  Pond 
pass,  on  the  other  hand,  prevented  the  development  of  any 
considerable  area  of  impounded  water  tributary  to  it  before 
free  communication  was  established  bistween  this  trough  and 
the  Old  Swamp  River  and  Valley  Swamp  passes. 

South  of  Mayflower  Park  the  Monatiquot  Valley  is  fully  one 
hundred  feet  in  depth  ;  and  the  lateral  slopes  rise,  at  some 
points,  rather  abruptly  from  its  swampy  floor.  On  both 
sides  may  be  traced,  at  intervals,  lateral  terraces,  due, 
probably,  to  marginal  streams,  and  consisting  chiefly  of 
coarse,  water-worn,  glacial  detritus,  bowlders,  cobbles  and 
pebbles,  with  a  moderate  admixture  of  sand.  Small  plains  of 
fine  material  occur  on  the  western  side.  At  Brookville,  near  the 
county  line,  is  situated  the  present  divide  between  the  northward- 
flowing  Monatiquot  and  the  southward-flowing  Salisbury  Plain 
Rivers.  As  previously  stated,  the  elevation  of  this  divide  is 
neai-ly  140  feet  above  tide  level.  North  of  Mayflower  Park 
the  valley  merges  with  the  northward-sloping  lowland  of  the 
basin  of  Lake  Bouve.  At  South  Brain  tree  Monatiquot  River 
receives  a  tributary  (Monatiquot  Stream)  carrying  a  large 
part  of  the  southern  drainage  of  the  Blue  Hills.  Near  Hoosic- 
whisick  Pond,  at  the  head  of  Monatiquot  Stream,  is  the  lowest 
pass  in  the  water-parting  between  the  basin  of  Lake  Bouve 
and  the  upper  Neponset  Valley,  which  during  the  lifetime  of 
Lake  Bouve  was  occupied  by  the  waters  of  a  similar  glacial 
lake.  This  pass  has  an  elevation  of  about  160  feet  above  sea- 
level.  Northward  the  basin  of  Lake  Bouve  is  continuous  with 
the  main  area  of  the  Boston  Basin,  which,  during  at  least 
the  later  stages  of  Lake  Bouve,  was  occupied  by  the  waters  of 
the  glacial  Lake  Shawmut. 


574 


^skers  of  Lake  Bouve. 

While  the  ice-front  rested  on  the  southern  water-parting  of 
the  basin  of  Lake  Bouve,  the  drainage  must,  obviously,  have 
been  reversed,  the  subglacial  streams  naturally  finding  their 
readiest  escape  southward  through  the  passes  at  the  heads  of 
the  northward-sloping  valleys.  As  the  pent-up  waters  emerged 
from  the  ice,  they  rapidly  deposited  their  burden  of  gravel  and 
sand,  and  thus  gave  rise  to  the  extensive  over-wash  or  apron 
plains  which  now  clog  the  southward-sloping  valleys.  Evi- 
dence that  the  glacial  streams  followed  the  courses  here  indi- 
cated is  found  in  the  eskers,  which  presumably  mark  the  main 
lines  of  subglacial  drainage,  and,  as  a  rule,  coincide  in  position 
and  trend  with  the  northward-sloping  valleys,  and  especially 
with  the  upper  courses  of  the  valleys.  It  is  certainly  very 
interesting  to  note  this  regard  of  the  glacial  streams  for  the 
main  features  at  least  of  the  topography,  and  the  more  so  since 
this  very  fact  may  fairly  be  considered  an  indication  that  the 
streams  were  subglacial. 

As  might  be  conjectured,  the  maximum  elevation  of  the 
crests  of  the  eskers  corresponds  approximately  to  that  of  the 
divide  across  which  the  subglacial  drainage  discharged  at  the 
time  of  their  formation.  The  average  elevation  of  the  crests 
of  the  Valley  Swamp  eskers  is  about  140  feet  or  that  of  the 
Valley  Swamp  pass.  As  already  noted  by  Crosby,  the  eskers 
of  South  Hingham  agree  in  elevation  with  Accord  plain  —  an 
over- wash  plain  sloping  southward  from  the  divide,  where  its 
surface  is  160  feet  above  sea-level.  The  eskers  of  Monatiquot 
Valley  agree  in  elevation  with  the  plains  of  that  valley,  or, 
what  amounts  to  the  same  thing,  with  the  elevation  of  the  pass 
at  Brookville.  In  general  the  eskers  in  the  northern  part  of 
the  basin  can  be  correlated  with  the  various  outlets  of  the  later 
stages  of  Lake  Bouve,  agreeing  with  them  in  the  elevation  of 
their  crest-lines.  They  have,  however,  no  constant  or  even 
general  relation  to  the  sand-plains  of  the  same  summit  eleva- 


575 

tion,  and  this   is   characteristic  of  most  of  the  eskcrs  of  this 
region . 

The  majority  of  the  eskers  in  the  basin  of  Lake  Bouve  have 
a  perfection  of  form  and  regularity  of  sloping  sides  which  de- 
mand the  assumption  that  they  were  formed  where  they  now 
occur,  and  Avere  not  let  down  from  a  superglacial  or  englacial 
bed  by  the  melting  of  the  ice-floor.  Assuming  this  to  be 
true,  there  have  been  suggested  the  following  hypotheses  to  ac- 
count for  the  eskers  of  the  basin  of  Lake  Bouve.  ( 1 )  They  may 
represent  deposits  of  drift  in  crevasses  in  the  ice  ;  (2)  they  may 
represent  drift-filled  river  gorges  cut  into  the  ice  down  to  bed- 
rock ;  or  (3)  they  may  represent  aggraded  stream  beds  in  sub- 
glacial  tunnels.  Some  of  the  more  irregular  esker-like  deposits 
of  drift  may  possibly  repi^esent  the  first  type,  though  it  would 
be  very  difficult  to  distinguish  such  deposits  from  shaken-up 
eskers  deposited  in  englacial  tunnels  or  superglacial  channels, 
and  subsequently  let  down.  The  striking  correspondence  in 
summit  elevation  between  the  eskers  and  the  notches  in  the 
rim  of  the  basin,  however,  forbids  the  assumption  that  many 
of  these  eskers  have  had  such  an  origin.  The  ice  gorges  and 
the  subglacial  tunnels  are,  therefore,  the  types  of  channels 
which  will  best  explain  eskers  of  this  class  ;  and  in  both  cases 
the  summit  of  the  esker  must,  in  general,  be  regarded  as  repre- 
senting the  stream  bed  at  the  time  the  channel  was  abandoned. 
In  the  basin  of  Lake  Bouve,  hoAvever,  ice  gorges  are  for  the 
most  part  ruled  out  because  in  nearly  all  cases  the  bed-rock  of 
the  stream  channel  is  everywhere  lower  than  the  sill  across 
which  the  discharge  of  the  stream  took  place.  The  subglacial 
-Stream  alone  is  capable  of  carving  out  a  tunnel  the  mouth  of 
Avhich  may  be  much  higher  than  the  main  part  of  its  course. 
Being  under  a  great  head  of  water  and  confined  in  a  narrow 
passage  with  an  unbroken  roof,  the  stream  may  discharge  across 
a  rock  sill  whose  elevation  is  limited  only  by  the  elevation  of 
water  head.  Even  in  the  early  stages  of  the  formation  of  such 
;a  subglacial  channel  aggradation  of  the  stream  bed  will  occur, 


576 

which  will  continue,  until  the  bed  is  aggraded  to  the  level  of 
the  sill  or  slightly  above  it,  provided  the  channel  is  not  aban- 
doned before  this  stage  is  reached.      At  any  stage  previous  to 
this  final  one  in  the  aggradation  of  the  stream  bed,  there  will 
be  a  continuous  and  tolerably  uniform  rise  in  the  bed  toward 
the  sill  across  which  discharge  takes  place.      As  the  roof  of  the 
tunnel  is  more  and  more  cut  awav,  a^o-radation  of  the  bed  will 
progress  to  a  corresponding  extent.     The  rock  sill,  which  de- 
termines the  level  of  discharge,  need  not  be  at  the  mouth  of  the 
subglacial  stream.     A  body  of  standing  water  held  up  against 
the  ice  front,  and  having  its  level  determined  by  the  distant  sill 
will  produce  the  same  effect.      In  this  case,  however,  it  is  to  be 
expected  that  the  eskers  will  terminate  in  a  fan-like  deposit  of 
sand  and  gravel,  having  all  the  characters  of  a  normal  sand- 
plain.      If  the  channel  is  abandoned  before  the  whole  stream 
bed  has  been  aggraded  to  the  level  of  the  sill,  an  esker  will 
remain   which   rises   in   the   direction  in  which   the   subglacial 
stream   flowed.      Such   an   esker   occurs   on    the  west   side    of 
Weymouth   Back    River.      It  is  one  of  the  fine  series  which 
extends  for  several  miles  along  this   side  of  the  estuaiy,  and 
unlike  most  of  them,  it  terminates  in  a  high  delta-fan,  which  is  a 
very  prominent  topographical  feature  north  of  East  Weymouth 
Station.      The  esker  is  separated  from  the  others  of  the  group 
by  a  narrow  notch  cut  down  to  sea-level,  and  probably  of  late 
origin.      This  notch  permits  the  discharge  of  the  drainage  from 
a  flat  meadow,  enclosed  on  all  sides  by  higli  ground.     From 
the  notch,  where  the  esker  is  less  than  fifty  feet  in  height,  it 
rises  steadily  southward,  at  first  rather  gently,  and  then  more 
steeply,    until    it    reaches   an    elevation    of  about    a    hundred 
feet  above  the  adjoining  marsh  lands.      Tlien  it  quickly  drops 
oif,  ending  in  a  hummocky  tract  of  ground  which  connects  it 
Avith  the  East  Weymoutli  delta  fan.      This  depression  between 
the  esker  and  the  fan  is  to   be   accounted  for  by  a  residual  ice 
mass  which  lingered  in   this  spot  and  over  which   part  of  the 
delta  was  built.      The  summit  of  the  esker  is  everywhere  strewn 


577 

with  coarse  blocks,  partly  embedded,  giving  the  appearance  of 
the  bed  of  a  stream  with  a  strong  current. 

If  at  any  time  subsequent  to  the  aggradation  of  the  sub- 
glacial  stream  bed,  a  lower  pass  in  the  rim  is  uncovered,  and 
the  level  of  the  water  held  up  in  front  of  the  ice  lowered  in 
consequence,  those  stream  beds  which  were  aggraded  to  a 
higher  level,  will  be  degraded  again  until  they  correspond  to 
the  elevation  of  the  newly  opened  pass.  This  of  course  assumes 
that  the  stream  continues  to  flow  in  its  old  channel.  Such 
degradation  will  result  in  the  building  up  of  an  esker  fan,  if 
none  existed,  or  in  increasing  the  area  of  one  previously  built, 
which  of  course  will  also  be  degraded  to  the  new  water  level. 
Thus  it  will  be  seen  that,  whether  built  up  to  the  new  level  or 
degraded  from  a  higher  level,  the  eskers  and  sand-plains  of  any 
stage  of  the  lake  agree  essentially  with  each  other  in  summit 
elevation,  and  with  the  height  of  the  outlet  which  determined 
the  water  level.  If  the  stream  abandons  its  old  channel  prior 
to   or   durino-  the  chanore  from  a  hioher  to  a  lower  level,  an 

coo  ' 

abnormally  high  esker  will  remain.  Examples  of  such  are 
found  in  the  hio-h  eskers  of  South  Hino-ham,  which  rise  above 
the  surrounding  Liberty  Plain,  and  in  the  East  Weymouth 
esker,  the  higher  part  of  which  is  considerably  above  the  sum- 
mits of  the  adjoining  plains  and  eskers. 

The  crest-lines  of  many  eskers  show  local  depressions  or 
interruptions  which  are  often  abrupt  and  deep.  Where  these 
cannot  be  explained  by  subsequent  erosion,  they  probably  rep- 
resent a  portion  of  the  channel  which  was  partially  filled  by 
ice,  either  from  the  caving  in  of  the  roof  of  the  tunnel,  or  from 
the  lingering  of  a  residuary  mass  of  ice  in  the  channel.  If  the 
ice  masses  are  deeply  covered  by  the  sands  and  gravels  of  this 
stream  bed,  the  subsequent  melting  will  produce  but  a  slight 
depression.  If,  however,  the  ice  mass  is  extensive  and  fills  the 
greater  part  of  the  tunnel,  the  depression  in  the  esker  resulting 
from  the  subsequent  melting  of  this  mass  will  be  extensive  and 
deep.     The  width  of  the  channel  has  a  marked  influence  on  the 

OCCAS.  PAPERS  B.  S.  N.  H.      IV.      37. 


578 

height  of  the  esker,  as  shown  by  Woodworth  (Proc.  Boston 
soc.  nat.  hist.,  vol.  26,  p.  203).  If  the  channel  is  narrow  the 
amount  of  material  sliding  down  to  form  the  side  slopes  on 
removal  of  the  retaining  ice  walls,  may  be  sufficient  to  mate- 
rially diminish  the  height  of  the  esker :  while  in  a  broad 
channel  the  esker  is  likely  to  retain  its  original  height  even  if  a 
considerable  amount  slides  down  to  form  the  side  slopes. 

The  easternmost  series  of  eskers  in  Lake  Bouve  is  mainly 
confined  to  the  Valley  Swamp  area,  though  several  short 
eskers  in  the  vicinity  of  Falling  Mill  Pond  and  farther  north 
in  Hingham  and  in  the  Hockley  district  probably  belong  to 
this  series.  Prof.  Crosby  holds  that  the  two  remarkable 
parallel  eskers  north  of  Grreat  Hill  in  Hingham  belong  to  this 
series,  their  peculiar  direction  being  due  to  the  influence  of  the 
drumlins  in  deflecting  the  course  of  the  stream  (Part  2,  p. 
284) .  It  is  not  unlikely  that  eskers  of  this  series  were  more 
extensive  before  the  free  drainage  eastward,  which  came  into 
existence  during  some  of  the  later  stages  of  the  lake,  taused 
their  being  more  or  less  modified,  if  not  destroyed.  The 
main  line  of  eskers  runs  parallel  to  Prospect  Street,  and 
consists  of  a  fine  double  esker  opposite  Prospect  Hill,  and  a 
partially  buried  single  esker  forming  tlie  eastern  border  of 
the  swamp.  The  double  esker  is  undoubtedly  due  to  an  ice- 
floor  in  the  tunnel,  which,  on  subsequently  melting,  lowered 
the  centre,  and  produced  a  double  ridge,  as  explained  by 
Crosby  in  Part  2.  A  remarkable  branched  esker  runs  parallel 
to  the  stream  draining  Accord  Pond  and  points  in  the  direction 
of  the  pond,  suggesting  a  stream  which  discharged  across  the 
divide  at  that  point,  in  company  with  the  streams  which  formed 
the  two  high  Soutli  Hingham  eskers.  This  esker  ends  in  a 
small  esker  fan,  beyond  which  lies  low  swampy  land.  If  the 
discharo-e  of  the  stream  wliich  formed  this  esker  was  across  the 
divide  at  Accord  Pond,  as  is  indicated  by  the  direction  of  the 
esker,  the  esker  fan  which  terminates  it  may  be  the  product  of 
the  deirradation  of  the  esker  from  a  formerlv  ii'reater  heio'lit, 


579 

i.  e.,  that  of  Accord  Pond  pass,  to  that  of  Valley  Svvainp  pass, 
on  the  establishment  of  free  drainasje  in  that  direction.  Near 
Queen  Anne  Corners  are  several  other  eskers  pointing  in  the 
direction  of  Accord  Pond,  so  that  we  may  assume  that  at  one 
time  a  considerable  drainage  passed  over  the  divide  in  that 
vicinity.  The  esker-like  ridge  of  coarse  material  extending 
parallel  to  the  road  southeastward  from  Queen  Anne  Corners, 
is  probably  a  part  of  the  heavy  moraine  v/hich  lies  to  the  south 
of  that  region,  and  which  is  characterized  by  ridges  and  deep 
longitudinal  depressions. 

The  Accord  Pond  eskei-s  are  continued  northward  in  the 
high  eskers  of  Liberty  Plain  and  Cashing  Street ;  and  these 
mark  a  line  of  drainage  the  northward  continuation  of  which  is 
probabl}^  recorded  in  the  eskers  east  of  Weymouth  Back  River, 
including,  in  the  opinion  of  Professor  Crosby,  those  of  Stod- 
dard Neck. 

The  finest  series  of  eskers  in  the  basin  of  Lake  Bouve  lies  in 
the  Old  Swamp  River  drainage  line.  These  begin  in  North 
Weymouth,  and  extend  along  the  west  side  of  Weymouth 
Back  River,  in  a  series  of  finely  developed,  nearly  parallel,  and 
steep-sided  ridges,  which  are  very  continuous.  In  East  Wey- 
mouth the  series  is  broken,  but  it  can  still  be  traced  in  dis- 
continuous ridges  to  about  a  mile  south  of  Lovell  Corners.  In 
North  Weymouth  these  eskers  correspond  in  elevation  to  the 
Hingham  plains,  but  in  East  Weymouth  they  generally  rise 
to  the  height  of  the  Whitman  plains. 

The  subglacial  drainage  of  the  central  portion  of  the  district 
is  preserved  in  the  somewhat  interrupted  esker  which  extends 
from  Weymouth  southward,  west  of  Whitman  Pond,  almost 
to  South  Weymouth.  The  esker  begins  in  Weymouth,  but  is 
interrupted  along  the  line  of  the  East  Weymouth  road  (Broad 
Street).  South  of  this  it  appears  again,  and  crosses  the  South 
Weymouth  road  (Main  Street)  near  its  junction  with  the  Lovell 
Corners  road  (Washington  Street)  ,  where  it  receives  an  eastern 
branch  or  tributary.      This  branch  is  perfect  only  for  a  short  dis- 


580 

tance,  being  frequently  interrupted  by  ledges.  The  combined 
esker  appears  to  end  in  a  little  plain  near  the  southern  boundary 
of  this  stage  of  the  lake.  On  the  eastern  side  of  this  plain, 
however,  and  skirting  the  western  border  of  the  South  Wey- 
mouth road,  is  a  partially  buried  esker  with  a  somewhat  crescen- 
tic  outline,  which  can  be  traced  to  Mill  River.  South  of  this 
stream  it  is  again  continued  until  it  finally  dies  out  near  South 
Weymouth.  The  general  direction  of  this  esker  indicates  a 
line  of  drainage  by  way  of  Weymouth  Great  Pond,  although 
the  direction  of  the  southernmost  portion  of  the  esker  would 
indicate  a  deflection,  at  least  in  part,  of  the  drainage  towards 
the  valley  of  Old  Swamp  River. 

The  southern  portion  of  this  esker  is  higher  than  the  northern 
part,  which  agrees  in  general  with  the  elevation  of  the  Whit- 
man plains. 

The  westernmost  series  of  eskers  belongs  to  the  valley  of 
the  Monatiquot  River.  They  are  distinctly  developed  as  far 
north  as  South  Braintree,  though  it  is  not  improbable  that  the 
series  is  represented  in  Quiney  and  possibly  ftirther  north.  The 
best  development,  however,  is  in  the  Monatiquot  Valley, 
where  a  main  line,  with  a  number  of  branches,  crosses  and 
recrosses  the  stream  at  frequent  intervals.  It  is  probable  that 
all  the  eskers  of  this  valley  can  be  relegated  to  a  single  sub- 
glacial  stream,  discharging  over  the  divide  at  Brookville,  and 
spreading  sands  over  the  Brockton  area.  The  relations  of  some 
of  these  eskers  to  the  sand-plains  of  the  valley  will  be  noticed 
more  in  detail  below. 


Sand-Plains  of  Lahe  Bottve. 

Eificht  distinct  series  of  sand-plains  are  recognizable  within 
the  basin  of  Lake  Bouve.  All  of  these,  except  the  highest 
and  tlie  lowest,  can  be  correlated  with  passes  in  the  rim  of 
the  basin. 


581 

The  Randol'ph  Plains.  —  These  are  the  highest  pLiins  of 
Lake  Bouve,  having  a  summit  elevation  of  about  170  feet  al)ove 
sea-level.  They  are  situated  wholly  along  the  western  margin 
of  the  Monatiquot  Valley,  and  appear  to  have  been  formed 
in  marginal  embayments  of  the  ice  lobe  which  occupied  the 
valley  and  -which  closed  the  lower  outlet  at  Brookville.  These 
plains  are  built  up  against  the  rock  or  till  wall  of  the  valley,  in 
the  form  of  lateral  terraces  with  steep  ice-contact  slopes  on  the 
valley  side.  Where  sectioned  in  railroad  cuts,  the  material  is 
often  seen  to  be  fine  and  well  stratified. 

The  best  development  of  these  plains  is  north  of  Randolph, 
though  small  ones  occur  along  the  western  wall  of  the  valley  at 
'various  points.  The  great  iri-egular  sand-plain  near  Brook- 
ville belongs,  in  part  at  least,  to  this  series. 

The  Mo7iatiquot  Plains.  —  The  plains  of  this  series  have 
an  approximate  elevation  of  140  feet  above  the  sea,  which 
agrees  with  that  of  the  divide  at  Brookville.  The  most  exten- 
sive plain  referred  to  this  series  lies  north  and  northeast  of 
Braintree  Great  Pond.  It  ends  northward  in  a  well-defined 
ice-margin,  but  the  lateral  and  frontal  slopes  are  indistinct, 
owing  no  doubt  to  the  irregularity  of  the  previous  topography 
of  this  area. 

The  most  perfect  plain  of  this  series  lies  southeast  of  South 
Braintree.  It  is  crossed  by  Plain  Street,  and  the  Abington  and 
Plymouth  Branch  of  the  Old  Colony  Railroad.  Steep  ice-con- 
tact slopes  sharply  define  the  plain  on  the  north  and  northeast, 
and  the  railroad  section  shows  good  stratification  and  fine  mate- 
rial. In  the  same  latitude,  southward  of  South  Braintree  and 
Little  Pond,  are  two  smaller  plains,  and  in  the  narrow  valley 
of  the  Monatiquot  River,  south  of  Mayflower  Park,  are  several 
small,  but  well-defined  and  isolated  plains.  The  four  most 
important  ©f  these  lie  between  Holbrook  and  Avon  Stations 
and  may  be  designated  respectively  the  first,  second,  third  and 
fourth  Holbrook  plains.  (The  fourth  of  these  and  two  smaller 
additional  ones  are  not  shown  on  the  map.) 


582 

The  first  Holbrook  plain  lies  immediately  southeast  of  the 
railroad  station  and  is  being  dug  away  for  sand.  The  compo- 
nent material  of  this  plain  is  fairly  coarse  near  the  head  of  the 
plain,  which  is  toward  the  northwest.  Here  bowlders  a  foot 
or  more  in  diameter  are  not  uncommon,  and  the  surface  of  the 
plain  is  pitted  by  kettles.  Away  from  the  head  the  strata, 
which  dip  gently  to  the  southeast,  become  finer,  and  are  often 
beautifully  ripple-marked.  The  surface  slopes  with  the  sti'ata  ; 
and  kettle  holes  occur  occasionally  in  the  finer-grained  portion. 
One  of  these,  when  freshly  sectioned,  showed  the  fine-grained 
ripple-marked  strata  sloping  gently  toward  the  centre  of  the 
depression,  which  is  underlain  by  a  core  of  large  bowlders. 
These  were  probably  embedded  in  the  ice  block  which  gave  rise 
to  the  kettle.  The  eastern  face  of  this  plain  is  formed  by  a 
partly  buried  esker  of  the  Monatiquot  Valley  series  ;  and  the 
steep  eastern  face  of  this  esker  gives  the  plain  the  appearance 
of  having  a  steep  ice-contact  slope  on  the  east  side.  The  sec- 
tion shows  an  abrupt  and  striking  contrast  between  the  fine, 
stratified  and  ripple-marked  material  of  the  plain,  and  the  coarse 
rudely  stratified  material  of  the  esker. 

Between  this  plain  and  the  railroad  is  an  esker  composed  of 
very  coarse  material  which  is  more  or  less  dug  away  ;  and  in 
line  with  this,  and  undoubtedly  a  continuation  of  it,  is  a  group 
of  eskers  north  of  Holbrook  Station.  The  southern  end  of  the 
former  esker  consists  chiefly  of  bowlders,  which  range  up  to 
two  or  three  feet  in  diameter.  They  are  all  much  water-worn, 
and  no  striations  have  been  observed  on  them.  This  esker  ter- 
minates in  a  small  level-topped  plain  of  the  normal  height  of 
the  Monatiquot  series.  The  esker,  however,  breaks  down  and 
becomes  more  or  less  irregular  before  it  reaches  the  plain,  be- 
tween which  latter  and  the  esker  a  distinct  depression  occurs. 
This  plain  apparently  consists  wholly  of  fine  material,  there  be- 
ing no  coarse  gravel  or  cobbles  on  top.  Its  frontal  slope  is 
very  steep,  and  its  back  slope  is  a  normal  ice-contact  margin. 
A  branch  from  the  coarse  esker  north  of  this  plain  appears  to 


583 

have  been  formerly  connected  witli  another  esker  to  the  east  of 
the  Holbrook  Station  pond.  The  direction  of  this  branch  and 
its  rehition  to  the  hirge  plain  south  of  it,  indicate  that  the  eskers 
were  formed  previous  to  the  plains.  The  line  of  eskers  is  con- 
tinued a  little  farther  south  in  the  esker  lying  west  of  the 
swampy  hind  bordering  Monatiquot  River.  The  fine  esker 
farther  south,  on  the  east  side  of  the  swampy  border  of  the 
Monatiquot,  is  probably  a  continuation  of  this  line  of  drainage, 
though  it  may  represent  an  independent,  parallel,  subglacial 
stream,  of  which  the  buried  esker  on  the  east  side  of  the  first 
Holbrook  plain  is  a  remnant. 

The  second  Holbrook  plain  lies  south  of  the  small  esker  fan 
before  described,  and  between  the  railroad  and  the  swampy 
border  of  the  Monatiquot.  It  is  a  well-defined,  prominent 
plain  of  moderately  fine  material,  with  a  steep,  kamy,  ice- 
contact  slope  on  the  northeast,  and  a  somewhat  lobate  delta 
slope  on  the  south  and  southwest.  It  is  crossed  by  the  Ran- 
dolph-Brookville  road.  A  small  esker,  probably  a  buried 
branch,  runs  out  from  the  eastern  face  of  the  plain  and  joins 
the  main  ridge  between  the  plain  and  the  swamp.  Less  than 
half  a  mile  south  of  this  is  the  third  plain,  which  is  also  well- 
defined  and  even-topped,  and  consists  of  fine  material.  This 
has  been  largely  cut  away  for  sand  and  gravel.  A  good  ice- 
contact  margin  defines  it  on  the  northwest  and  north,  there 
being  a  considerable  area  of  low  land  between  this  and  the 
hio;h  orround  on  the  west.  When  the  sections  were  fresh, 
some  interesting  structural  features  were  shown,  among  which 
were  foreset  beds  pointing  in  northerly  and  easterly  direc- 
tions. The  delta  in  this  case  seems  to  have  grown  in  a 
northeasterly  direction,  parallel  to  the  ice-front.  Still  farther 
south,  and  not  fixr  north  of  Avon  Station,  is  a  fourth  plain, 
lying  between  the  railroad  and  the  high  land  on  the  west. 
This  is  of  the  same  type  as  the  others,  but  is  easily  overlooked 
because  it  is  in  the  woods.  The  surface  is  in  places  bowlder- 
strewn,  but  no  regular  frontal  or  back  slopes  have  been 
observed. 


East  of  the  first  Holbrook  plain  and  south  of  the  road  lead- 
ing from  the  station  to  Holbrook  Village,  is  a  somewhat 
irregular  frontal  deposit  with  a  steep  ice-contact  slope  on  the 
north.  It  contains  much  coarse  material  and  is  more  or  less 
pitted,  and  is  continued  northward  across  the  road  in  an  irregu- 
lar esker-like  ridge  which  is  in  part  utilized  as  a  burying 
ground. 

The  Liberty  Plains.  —  The  plains  of  this  series  are  con- 
fined to  the  southeastern  portion  of  the  lake  basin,  the  name 
being  derived  from  the  largest  member  of  the  series  —  Liberty 
Plain  in  South  Hingham  (see  map).  They  agree  in  elevation 
with  the  Monatiquot  plains  ;  although  they  must  be  correlated 
with  entirely  distinct  outlets,  and  probably  are  not  strictly  con- 
temporaneous with  the  more  western  series,  representing  a  later 
and  broader  development  of  Lake  Bouve.  Furthermore,  two 
outlets  must  be  recoo-nized  for  this  stage,  —  one  at  the  head  of 
Old  Swamp  River  and  the  other  in  Valley  Swamp  at  the  head 
of  Beechwood  River,  the  pass  at  the  head  of  Weymouth  Great 
Pond  being  somewhat  higher  than  these.  The  best  develop- 
ment of  the  Liberty  plains  is  in  South  Hingham,  w4iere  all  the 
features  are  shown  in  various  degrees  of  perfection.  An  ice- 
contact  line  extends  obliquely  northeast  and  southwest  across 
Main  Street.  This  is  not  always  of  the  normal  type,  but  often 
very  kamy,  with  deep  re-entrants  and  strong  projecting  cusps, 
while  sometimes  it  is  represented  by  a  gentle  descent.  In 
this  latter  case  the  ice-front  appears  to  have  been. sloping,  with 
the  sand  spread  over  it.  Ledges  occasionally  obscure  the 
character  of  the  plain.  A  nearly  continuous,  more  or  less 
well-marked,  southward-descending  delta  slope  faces  Mill  River, 
which  heads  in  Accord  Pond.  Several  large  kettle  holes  in 
the  northern  part  of  the  plain,  and  linear  depressions  parallel 
with  the  ice-front,  partially  subdivide  the  plain.  The  southern 
sides  of  these  depressions  usually  present  the  characters  of  a 
more  or  less  steep  ice-contact  slope,  Avhile  the  northern  face 
often  presents  a  more  or  less  lobate  delta  slope.      This  indicates 


585 

that  these  depressions  represent  the  unfilled  fosse  between'two 
successive  stages  of  the  plain,  recording  a  recession  of  the  ice- 
margin  during  the  development  of  the  plain.  The  texture  of 
the  material  becomes  coarser  near  the  ice-contact  slope. 

Two  strongl}^  developed  but  smaller  and  isolated  plains  — 
the  Break  Neck  Hills  — lie  west  of  the  Liberty  Plain.  These 
are  characterized  by  level  summits  and  steep   marginal  slopes. 

Directly  south  of  these  is  a  third  minor  plain,  which  is 
crossed  by  Derby  Street  near  Whiting  Street,  and  is  joined  on 
the  south  and  east  to  the  slopes  of  ledge  and  till  which  here 
formed  the  shore  of  Lake  Bouve  during  the  Liberty  stage. 

Farther  west  along  Derby  and  Oak  Streets,  crossing  the 
Weymouth  line  and  extending  well  into  the  valley  of  Old 
Swamp  River,  is  the  very  perfect  plain,  also  bordering  the 
southern  rim  of  the  basin,  which  has  been  known  locally  as 
Huckleberry  Plain  and  Mosquito  Plain.  The  southwestern 
margin  of  this  plain  is  a  finely  developed  lobate  frontal  margin 
facing  the  valley  of  Old  Swamp  River,  and  showing,  where 
sectioned,  sloping  foreset  beds.  On  the  map  this  frontal 
slope  is  not  differentiated  from  the  abrupt  northwestern  and 
northern  ice-margin.  As  already  stated,  this  plain,  the  north- 
western portion  of  which  is  singularly  bold  and  perfect,  is  built 
up  in  the  southeastern  portion  against  the  high  land.  Not 
infrequently  isolated  ledges  rise  above  the  level  of  the  plain. 
Much  of  the  northern  margin  of  this  plain  is  poorly  defined  on 
account  of  projecting  granite  ledges  against  and  over  which  the 
sand  was  deposited.  This  portion  of  the  plain  is  also  broken 
by  numerous  kettle  holes  ;  while  between  the  ledges  winding 
eskers  are  frequent. 

The  Whitman  Plains.  — These  plains,  with  a  maximum 
summit  elevation  of  95  to  100  feet  above  the  sea,  lie  almost 
wholly  in  Weymouth,  being  confined  to  the  central  portion  of 
the  old  lake  basin.  They  characterize  especially  the  region 
about  Whitman  Pond  and  East  Weymouth,  and  only  two 
small  plains  of  this  series  occur  in  Hingham.      The  largest  of 


586 

these  lies  in  the  midst  of  a  rocky  area,  but  is  distinct  and  well 
developed.  It  is  perhaps  half  a  mile  in  east  and  west  extent,  but 
is  narrow  from  north  to  south.  It  lies  about  half  a  mile  south 
of  Ward  Street,  and  is  distinctly  elevated  above  Glad  Tidings 
plain,  a  lobe  of  which  lies  a  short  distance  to  the  north.  The 
northern  slope  is  a  steep  ice-contact  margin ;  the  southern 
slope  is  also  steep,  and  forms  one  side  of  a  deep  and  narrow 
fosse  which  separates  this  plain  from  a  high  ridge  of  rock.  A 
discontinued  road  crosses  the  plain. 

Where  this  road  crosses  the  western  branch  of  Plymouth 
River,  lies  the  other  and  much  smaller  plain  of  this  series  in 
Ilingham.  This  also  has  a  steep  northern  face.  In  the 
vicinity  of  this  plain  are  numerous  esker-like  ridges  and  irregu- 
lar gravel  and  sand  deposits  bordering  the  swampy  ground 
about  Plymouth  River.  These  are  not  readily  differentiated  ; 
but  they  may  represent  an  irregular  plain,  or,  more  probably, 
a  group  of  eskers  of  the  Accord  Pond  series. 

In  addition  to  these  two  plains,  there  is  an  irregular  ter- 
race-like deposit  bordering  Liberty  Plain  south  of  Fulling 
Mill  Pond  in  South  Ilingham,  between  Main  and  Charles 
Streets,  which  corresponds  in  general  to  the  elevation  of  the 
Whitman  plains.  This  probably  represents  the  more  or  less 
modified  ice-contact  margin  of  Liberty  Plain,  which  suffered 
some  erosion  during  the  time  that  the  waters  of  this  and  the 
succeedino-  stag-e  drained  eastward  across  the  divide  between 
Prospect  and  Turkey  Hills. 

In  the  area  lying  between  the  east  branch  of  Plymoutli 
River  and  the  northern  margin  of  Liberty  Plain,  on  both  sides 
of  Gushing  Street,  is  an  irregular  plain  with  a  tolerably  level 
surface.  This  probably  belongs  to  this  series.  From  it  rise 
the  two  hio'h  eskers  of  the  Accord  Pond  series,  which  show 
good  sections  on  Gushing  Street.  The  plain  extends  to  the 
margin  of  Liberty  Plain,  but  is  very  irregular  in  that  vicinity. 
It  is,  throughout,  heavily  wooded. 

The  best  development  of  the  Whitman  plains  is,  as  stated. 


587 

about  Whitman  Pond,  in  Weymouth.  The  plain  east  of  the 
pond  is  much  broken  up  by  ledges  which  j)rotrude  above  its 
surface.  South  of  Lovell  Corners,  however,  the  plain  is  well 
developed,  level-topped,  and  heavily  wooded.  It  has  a  steep 
margin  facing  the  pond  and  a  variously  lobate  delta  margin  on 
the  east  and  soutli,  the  latter  face  being  bordered  by  the 
swamp  land  of  Old  Swamp  River.  West  of  the  pond  are  two 
well-developed  plains ;  the  southern  embraces  Whortleberry 
Pond,  and  is  separated  from  the  northern  by  Mill  River.  This 
latter  plain  is  bounded  on  the  north  by  an  arm  of  Whitman 
Pond,  and  by  a  small  stream  which  drains  into  this  arm.  Steep 
ice-contact  margins  border  both  plains  on  the  northwest.  An- 
other extensive  plain  occurs  in  East  Weymouth  north  of  the 
pond.  This  plain  has  dammed  the  valley  of  which  Whitman 
Pond  is  a  part,  and  has  foi'ced  the  pond  to  find  an  outlet  over  the 
granite  ledges  to  the  east  of  the  buried  valley.  Several  small 
plains  of  this  series  occur  to  the  west  and  northwest  of  those 
immediately  encircling  Whitman  Pond. 

The  Glad  Tidings  Plains. — The  plains  of  this  series, 
with  a  maximum  elevation  of  70  feet,  are  confined  to  Hingham, 
with  the  exception  of  the  plain  partially  encircling  the  Quincy 
Reservoir  in  Braintree.  In  Hingham,  the  Glad  Tidings  plains 
are  found  in  the  western  central  portion  of  the  town,  noi'th  of 
Liberty  Plain  and  northeast  of  the  plains  of  the  Whitman  series. 
The  series  is  named  for  its  easternmost  member,  —  the  beauti- 
ful and  typical  delta  plain  which  encircles  the  bold  ledge  of 
granite  known  as  Glad  Tidings  Rock,  and  on  which  the  village 
of  South  Hingham  stands.  The  non-distinctive  name  "Upper 
Plain  "  appears  on  the  map,  copied  from  the  government  topo- 
graphic sheet.  The  original  Glad  Tidings  Plain  is  bounded  by 
Gushing  Pond  and  Plymouth  River  on  the  west  and  north, 
and  by  Fulling  Mill  Pond  and  the  west  branch  of  Plymouth 
River  on  the  east.  It  is  separated  from  Liberty  plain  on  the 
north  by  a  well-marked,  wide,  but  rather  shallow  fosse. 

Northwest  of  Plymouth  River  and  Gushing  Pond  is  a  very 


588 

extensive  plain  of  this  level,  which  encloses  and  nearly  covers 
Nutty  Hill,  a  small  clrumlin  lying  north  of  the  East  Weymouth 
road  (High  Street).  This  plain  is  characterized  by  numerous 
irregularities,  some  of  which  are  of  true  kettle-hole  type. 
Pigeon  Plain,  which  has  a  pitted  surface,  and  several  other 
smaller  plains  of  this  level  lie  in  the  vicinity  of  Great  Hill, 
Hingham.  These  plains  usually  exhibit  good  delta  and  ice- 
contact  slopes  ;    and  sections  show  southward-sloping  foresets. 

Great  Hill  is  a  drumlin  which  is  nearly  surrounded  by  eskers 
and  sand-plains.  On  its  lateral  slopes  are  several  good  erosion 
terraces  which  correspond  in  elevation  to  the  adjoining  plains. 

The  plain  partially  encii'cling  the  Quincy  Reservoir  in  the 
northwestern  part  of  Braintree  has  approximately  the  Glad 
Tidings  level.  This  is  a  mere,  coincidence,  however,  for  its 
elevation  was  not  directly  determined  by  the  Glad  Tidings  out- 
let, but  rather  by  a  low  place  in  the  ridge  forming  the  east 
side  of  the  reservoir  valley,  and  on  which  the  village  of  North 
Braintree  stands.  This  means  that  it  is  not  necessarily  con- 
temporaneous with  the  Glad  Tidings  plains  of  Hingham  ;  and 
it  might,  therefore,  very  properly  be  referred  to  a  distinct 
series,  the  facts  being  essentially  similar  to  those  which  have 
determined  the  differentiation  of  the  Monatiquot  and  Liberty 
plains.  The  ice-contact  margin  in  the  northwest,  and  the 
delta  margin  on  the  southeast,  although  not  strongly  accentu- 
ated, are  on  the  whole  well  developed.  An  extensive  sand  pit 
has  been  excavated  in  the  northeastern  end  of  this  plain,  show- 
ing the  material  to  be  fine  and  well  stratified,  with  abundant 
ripples  and  cross -bedding. 

The  Hingham  Plains.  —  The  plains  corresponding  to  the 
outlet  along  the  north  side  of  the  Turke}^  Hills,  and  on  the 
largest  and  most  northerly  of  which  the  village  of  Hingham 
Centre  stands,  have  an  average  elevation  of  50  feet  above  sea- 
level.  They  are  confined  to  the  northern  part  of  Hingham, 
with  the  exception  of  one  small  member  in  North  Weymouth, 
north  of  King  Oak  Hill  and  near  Weymouth  Heights  Station. 


589 

This  latter  plain  is  one  of  the  best  sand-plains  in  the  whole 
area.  It  shows  a  rather  steep,  lobatc,  delta  front,  deeply 
incised  between  the  lobes,  and  showing  in  sections  radiately 
sloping  foreset  beds  in  eacli  lobe.  A  large,  completely  enclosed 
kettle  hole  modifies  the  summit,  which  is  otherwise  level 
except  for  the  deeply  incised  interlobate  portions  of  the  frontal 
margin,  which  are  extended  some  distance  back  from  the  front. 
The  material  is  coarser  toward  the  head  of  the  plain, 
which  is  mostly  A^ery  steep  and  has  in  places  finger-like  cusps 
extending  northward  from  it.  These  projections  probably 
mark  crevices  in  the  front  of  the  ice  which  were  filled  by 
tongues  of  sand.  Several  of  these  prolongations  have  an 
esker-like  form,  but  they  are  not  very  continuous.  Westward 
this  plain  ends  in  a  narrow  strip  which  has  all  the  aspect  of 
an  east  and  west  esker.  This  portion  of  the  plain  is  occupied 
by  the  cemetery,  and  parts  of  it  have  been  much  modified  in 
form.  In  the  section  on  Norton  Street  an  esker-like  form  is 
shown,  but  the  material  is  fine,  the  stratification  perfect,  and 
ripple-marks  are  common. 

The  main  plain  of  the  Hingham  series  underlies  the  village 
of  that  name,  and  is  quite  extensive,  though  not  of  great  regu- 
larity of  surface  or  side  slopes.  The  best  development  is  south 
of  the  Home  Meadow  marshes,  as  described  by  Crosby  on 
pp.  281  and  282  of  the  Geology  of  Hingham. 

The  Quincy  Plains. — The  plains  of  this  series  are  exten- 
sively developed  in  the  northern  part  of  the  basin  of  Lake 
Bouve,  from  West  Quincy  to  the  Hockley  district  in  North 
Hingham.  They  have  a  maximum  elevation  of  40  feet.  Good 
ice-contacts  are  frequently  shown  along  their  northern  margins, 
though  where  they  border  the  salt  marshes  they  are  frequently 
eroded.  Well-developed  members  of  this  series  are  found 
throughout  Quincy,  the  village  of  that  name  being  built  upon 
some  of  them.  A  plain  of  this  series  also  is  well  developed  in 
North  Weymouth,  where  a  gentle  slope  carries  it  toward  Wey- 
mouth Fore  River,  which  it  borders  with  a  well-marked  lobate 


590 

delta  slope,  where  erosion  has  not  destroyed  the  origmaloutlme. 
(See  Plate  30.)  This  plain  has.  its  highest  part  in  Hunt's 
Hill  Point  at  the  mouth  of  Weymouth  Fore  River.  This 
portion  is  an  esker-like  prolongation  of  the  plain,  and  appears 
to  be  of  the  nature  of  a  feeding  esker.  The  material  increases 
in  coarseness  northward,  being  exceedingly  coarse,  but  well 
rounded,  in  the  sectioned  portion.  Rude  stratification  obtains ■ 
here,  as  shown  in  the  gravel  pit  at  the  water  margin.  Prof. 
Crosby  has  calculated  the  percentage  of  material  in  this  esker 
as  follows  :  ^  50  per  cent,  came  from  the  slate  and  conglomer- 
ate belt  immediately  to  the  north,  and  thirteen  miles  wide  ;  40 
per  cent,  came  from  a  belt  of  hornblendic  granites,  diorites, 
and  felsites,  with  some  Cambrian  slates  and  quartzites,  lying 
between  thirteen  and  twenty-three  miles  north  of  the  plain  ; 
and  10  per  cent,  came  from  a  third  belt,  consisting  of  mica 
schists,  muscovite  granites  and  gneisses,  pegmatite,  etc.,  and 
extending  from  twenty-three  miles  north  of  the  plain  into  New 
Hampshire. 

The  region  known  as  the  Hockley  district  in  West  Hing- 
ham  contains  irregular  plains,  some  of  wliich  are  referable  to 
the  Grlad  Tidings,  some  to  the  Hingham,  and  some  to  the 
Quincy  series.  The  deposits  are  of  considerable  irregularity, 
the  sand  in  this,  as  in  some  other  portions  of  the  basin,  appar- 
ently forming  an  uneven  veneer  over  an  irregular  rock 
topography.  It  is  probable  that  ice  masses  lingered  between 
the  ledges  of  this  region,  after  they  had  disappeared  from  the 
ledges  themselves.  The  sand  and  gravel  probably  were  spread 
over  this  in  a  more  or  less  uiiiform  plain,  the  iri-egularities  now 
found  being  due  mainly  to  the  melting  of  the  residual  ice 
masses  and  the  caving  in  of  the  deposits. 

Tlie  Coastal  Plains.  — The  plains  of  this  series  are  the 
lowest,  never  rising  above  the  twenty-five-foot  contour.  They 
are  distinct  in  height  from  those  of  the  preceding  stage,  and  are 
found   here   and   there    north   of  the  other  series,   and    almost 

1  Technology  (luarterly,  vol.  0,  p.  142. 


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591 

invariably  bordering  the  marshes  or  open  water.  Small  ex- 
amples of  this  series  occur  in  Quincy  and  Hingham. 

Irregular  Sand  De2^osits.  —  In  addition  to  the  more  reg- 
ular deposits,  of  washed  material  referable  to  plains  of  distinct 
levels,  there  occur  numerous  irregular  accumulations  of  the 
kind  already  noted  as  occurring  in  the  Hockley  district. 
These  are  not  referable  to  any  particular  series.  The  plains 
themselves  are  very  local,  and  indicate  an  irregular  recession 
of  the  ice-front. 

The  only  other  class  of  deposits  contemporaneous  with  the 
sand-plains  of  this  area  are  morainal  ridges  which  probably 
formed  under  the  attenuated  margin  of  the  ice,  while  sand-plains 
were  forming  elsewhere  at  its  front.  The  best  example  of  these 
moraines  occupies  the  area  east  of  Glad  Tidings  and  north  of 
Liberty  Plains  in  Hingham.  It  is  characterized  by  a  bowldery 
and  more  or  less  irregular  surface.  Some  of  the  drumloid 
hills  of  the  Prospect  Hill  group  probably  belong  to  this 
moraine. 

History  of  Laic  e  Boiive. 

The  history  of  Lake  Bouve  appears  to  have  been  somewhat 
as  follows.  While  the  ice-front  occupied  the  summit  of  the 
divide,  and  streams  from  it  were  discharo-ins;  south  throug-h 
Valley  Swamp,  and  at  Accord  Pond,  eroding  Prospect  Hill  in 
the  one  case,  and  building  the  Accord  apron  plain  in  the  other, 
a  long  narrow  lobe  of  ice  occupied  the  valley  of  Old  Swamp 
Hiver.  From  its  southern  end,  where  was  building  the 
moraine,  streams  discharged  southward  over  Rockl&nd,  wash- 
ing away  the  fine  material,  and  leaving  the  bowlders  which 
are  now  piled  up  in  fences  and  scattered  about  the  fields, 
while  at  the  same  time  the  sand  deposits  to  the  south  were 
spread  out.  The  flow  of  water  over  this  area  probably  con- 
tinued until  the  close  of  the  Liberty  stage  of  Lake  Bouve.  No 
important  deposits,  however,  were  formed  in  the  valley  of  the 


592 

river,  even  after  the  Liberty  stage  of  the  lake  had  come  into 
existence. 

A  second  narrow  lobe  of  ice  occupied  the  valley  that  now 
holds  Weymouth  Great  Pond.  Streams  from  this  lobe  dis- 
charged southward,  past  North  Abington,  into  the  valley  of 
Shuraatuscacant  River,  Avhile  at  the  same  time  the  sand  de- 
posits south  of  the  pond  were  formed. 

A  third,  and  by  far  the  largest  lobe  of  ice- of  this  stage, 
occupied  the  valley  of  the  Monatiquot  River.  From  its  south- 
ern end  streams  discharged  over  the  area  where  Brockton  now 
stands,  many  of  the  sand  and  gravel  deposits  of  that  region 
being  due  to  these  streams.  Throughout  this  district  bowlders 
are  very  common  immediately  south  of  the  divide.  They  usu- 
ally present  a  more  or  less  washed  and  rounded  appearance, 
though  many,  owing  to  atmospheric  disintegration,  show  rough 
surfaces.  Many  have  also  been  more  or  less  frost-shattered. 
It  seems  most  probable  that  many  if  not  most  of  these  bowl- 
ders were  exposed  through  the  washing  away  of  the  finer 
material  by  the  streams  which  came  from  the  ice. 

During  this  time  the  lateral  morainal  deposits  on  the  eastern 
margin  of  the  valley  of  the  Monatiquot  w^ere  laid  down  ;  these 
are,  however,  of  a  distinct  type  from  the  sand-plains  on  the 
western  margin.  A  line  of  these  deposits,  consisting  of  very 
coarse,  rounded  material,  extends  on  both  sides  of  the  road 
between   Holbrook  and  Brookville. 

At  the  time  of  the  maximum  extension  of  the  Monatiquot 
lobe,  the  ice  probably  rested  firmly  against  the  southern  face  of 
the  Blue  Hill  Range.  When  the  ice  melted  away  from  tliis 
face  of  the  Blue  Hills,  an  outlet  for  the  waters  of  the  glacial 
lake  in  the  Charles-Neponset  basin  was  established  eastward, 
along  the  line  of  Monatiquot  Stream.  This  drainage  neces- 
sarily turned  southward  between  the  high  land  west  of  the 
Monatiquot  valley  and  the  ice  lobe  lying  in  that  valley,  and  the 
elevation  of  this  marginal  river  was  clearly  about  170  feet.^ 
Indications  of  flowing  water  at  that  level  are  plentiful  along  the 


593 

western  margin  of  the  valley.  They  consist  in  tlie  stripping 
of  the  till  from  the  rocky  ledges  ;  in  bowlders,  often  well 
rounded,  which  were  left  when  the  finer  material  was  washed 
away  ;  in  the  accumulation  of  sand  in  depressions,  and  in  the 
building  of  the  marginal  sand-plains  of  the  Randolph  series, 
with  ice-contact  slopes  facing  the  valley,  as  has  been  already 
described. 

As  soon  as  the  ice  lobe  in  the  Monatiquot  valley  had  melted 
sufficiently  to  uncover  the  divide  at  Brookville,  the  exposed 
part  of  the  valley  became  a  small  lake,  with  an  elevation  at  its 
surface  of  about  140  feet  above  the  present  sea-level.  During 
the  continued  shrinking  of  the  ice  lobe,  the  plains  between  Avon 
and  Holbrook  Stations  were  formed.  The  fact  that  these  plains 
are  all  confined  to  the  western  side  of  the  valley  leads  to  the 
supposition  that  the  ice  lingered  longest  in  the  eastern  half, 
and  that  the  drainage  continued  in  the  direction  now  followed 
by  the  railroad.  This  seems  to  be  borne  out  by  the  bowldery 
character  of  the  valley  bottom  near  Avon  Station,  which  is  not 
unlike  an  ancient  stream  bed. 

While  these  plains  were  forming  in  the  Monatiquot  valley, 
the  ice  lobes  in  the  other  two  valleys  were  likewise  shrinking. 
That  of  the  Great  Pond  valley  allowed  drainage  along  its  west- 
ern maro;in,  which  resulted  in  the  buildingf  of  small  marginal 
plains  against  the  old  land  on  the  west.  The  ice  lobe  of  Old 
Swamp  River  built  no  important  plains  after  it  had  formed  the 
morainal  deposits  at  the  head  of  the  valley.  The  Liberty  stage 
of  the  lake  may  have  come  into  existence  while  the  ice  lobes 
still  remained  in  the  valleys.  The  ice-contact  slopes  point  to  a 
northwesterly  recession  of  the  ice-front  during  the  earlier 
periods  of  this  stage.  The  large  South  Hingham  plain  and  the 
Break  Neck  Hills  were  probably  formed  while  the  drainage 
passed  over  the  Valley  Swamp  divide.  The  large  western  plain 
of  this  series  was,  however,  built  while  Old  Swamp  River 
valley  was  free  from  ice,  but  filled  with  water,  as  testified  to 
by  the  frontal  lobes  of  this  plain,  which  point  to  a  drainage 

OCCAS.  PAPERS  B.  S.  N.  H.     IV.     38. 


594, 

across  the  divide  north  of  Rockland.  Probably  by  this  time 
the  ice  had  relinquished  its  hold  on  the  upland  of  till 
east  of  Mayflower  Park  and  the,  up  to  this  time,  independent 
Monatiquot  and  Liberty  lakes  were  joined  into  a  long  and 
narrow  body  of  water  stretching  from  the  Blue  Hills  to  Pros- 
pect Hill  in  Hingham.  This  lake  drained  southward  by  the 
three  outlets  at  Brookville,  Rockland,  and  Valley  Swamp,  the 
pass  at  the  head  of  Weymouth  Great  Pond  being  abandoned 
on  account  of  its  slightly  greater  elevation.  During  all  this 
time  the  waters  from  the  higher  Charles-Neponset  lake  on 
the  west,  entered  Lake  Bouve  south  of  the  Blue  Hills,  there 
being  no  lower  outlet  for  these  waters  as  lono^  as  the  ice 
retained  its  hold  on  the  northern  face  of  the  Blue  Hills  at  West 
Quincy. 

Toward  the  close  of  the  Liberty  stage  a  heavy  moraine 
had  been  built  along  the  line  of  South  Pleasant  Street  in  South 
Hino;ham,  between  Main  Street  and  the  eastern  border  of  the 
township.  This  moraine  can  be  traced  southeastward,  Mt. 
Blue  in  Scituate  being,  at  least  in  part,  of  morainal  character. 
Black  Pond  Hill  and  Buttonwood  Hill  are  also  probably  parts 
of  this  moraine.  As  long  as  the  ice  held  on  to  this  moraine, 
eastward  drainage  was  impossible  ;  but  with  the  further  reces- 
sion of  the  ice-front,  the  ninety-iive-foot  outlet  east  of  Union 
Street  in  Hingham  was  opened,  and  the  surface  of  the  lake 
sank  to  a  corresponding  level. 

As  long  as  the  valleys  of  Beech  wood  River  and  Bound 
Brook,  which  drain  northeastward  into  Scituate  Harbor,  were 
filled  by  the  ice,  drainage  in  that  direction  was  impossible.  The 
waters,  therefore,  which  left  the  lake  through  the  outlet  about 
a  mile  and  a  half  northeast  of  Prospect  Hill,  were  forced  to  turn 
southward  along  a  narrow  channel  between  Prospect  Hill  and 
Mt.  Blue,  across  the  one  hundred  foot  divide  west  of  Otis  Hill, 
into  Burnt  Plain  Swamp,  and  thence  by  way  of  Hoop  Pole 
Swamp  into  Wildcat  Brook  and  North  River.  Abundant  evi- 
dence  of  the  flow   of  water  in  this  direction   is  furnished  by 


595 


the  cliffed  slopes  of  the  morainic  hills  bordering  this  channel . 
Following  the  one  hundred  foot  contour,  north  of  Union  Street 
in  Hingham,  we  find  that  the  northern  face  of  the  drumloidal 
hill,  whose  base  it  skirts,  has  a  very  distinct  erosion  cliff, 
traceable,  with  some  interruptions,  for  several  miles  southeast- 
ward. The  drumloid  at  the  outlet  has  an  elevation  of  175  feet, 
and  slopes  gently  northeastward,  its  surface  heavily  bowlder- 
strewn.  Facing  the  flat  lowland  north  of  it  is  a  cliff  twenty 
feet  or  more  in  height,  Vvith  a  slope  of  thirty  degrees,  and 
making  an  abrupt  angle  with  the  normal  slope  of  the  hill,  as 
shown  in  the  following  diagram  (Fig.  47). 


Fig.  47. 

Diagram   of   tlie   cliff  at  the  Wliitman  outlet,  sliowing  the  relation  of  the 
■slopes  to  the  bottom  of  the  outlet.     Scale,  one  inch=20  feet. 


Bowlders  frequently  project  from  the  surface  of  the  cliff: 
while  from  its  base  a  flat,  wooded,  swampy  tract,  thickly 
strewn  with  rounded,  moss-covered  bowlders,  extends  north- 
ward. This  bowlder-covered  tract  has  all  the  aspect  of  an 
ancient  stream  channel  from  which  the  fine  material  has  been 
washed  away  by  a  strong  current,  only  bowlders  remaining. 
The  cliff  facing  this  bowlder-strewn  bed  is  an  unmistakable 
erosion  scarp.  In  some  places  the  angle  of  the  cliff  decreases  to 
twenty-eight  or  even  twenty-six  degrees,  and  in  one  place  is  as 


596 

low  as  twenty  degrees,  having  been,  perhaps,  modified  by  subse- 
quent creep  and  rain-wash.  Wherever  the  cliiFis  recognizable, 
however,  the  change  from  the  relatively  gentle  normal  slope  of 
the  hill  to  the  steep  slope  of  the  cliff  is  an  abrupt  and  marked 
one. 

The  cliff  can  be  seen  on  Beech  wood  Street,  where  this  street 
crosses  the  one  hundred  foot  contour.  Occasionally  the  cliff  is 
divided  into  two  parts,  with  a  terrace  between  ;  but  in  most 
cases  it  is  a  single  cliff,  facing  a  flat,  swampy  and  bowlder- 
strewn  lowland.  The  southern  portion  of  this  channel,  as  far 
as  Burnt  Plain  Swamp,  is  occupied  by  Beech  wood  River.  The 
sides  of  the  valley,  which  is  perhaps  a  third  of  a  mile  in  width, 
in  many  places  present  a  cliffed  appearance,  this  being  particu- 
lai'ly  noticeable  on  the  western  slope  of  the  Mt.  Blue  moraine. 

While  the  ice  retained  its  hold  on  Booth  Hill  in  Scituate,  no 
lower  pass  than  that  at  Burnt  Plain  Swamp  was  uncovered.  A 
line  drawn  from  the  northern  face  of  the  Blue  Hills  at  West 
Quincy  to  Booth  Hill  near  North  Scituate  Station  on  the  Ply- 
mouth branch  of  the  Old  Colony  Railroad,  has  a  direction  a 
few  degrees  south  of  east  and  a  length  of  about  thirteen  miles. 
While  the  ice-front  rested  along  this  line,  or  south  of  it,  the 
discharge  of  the  great  body  of  water  in  Lake  Bouve,  to- 
gether with  that  of  the  Charles-Neponset  lake  on  the  west, 
was  by  way  of  Burnt  Plain  Swamp.  Even  after  the  ice  re- 
laxed its  hold  on  the  northern  face  of  the  Blue  Hills,  and  the 
waters  of  the  Boston  area  (Lake  ShaAvmut)  became  tributary 
to  Lake  Bouve,  the  discharge  was  across  this  pass,  until  the  ice 
receded  from  the  moraine  of  Booth  Hill.  Meanwhile  a  small 
body  of  water  was  held  up  by  the  ice  on  the  lowland  about 
Beechwood  village,  and  in  this  body  of  standing  water  were 
built  up  some  of  the  sand-[)lains  found  in  that  vicinity.  Some 
of  the  others  were  probably  built  wlien  this  water  stood  at  a 
lower  level  and  the  discharge  took  place  north  of  Booth  Hill. 

As  the  ice-front  receded  northward  along  the  eastern  margin 
of  Lake  Bouve,  many  deposits  of  morainal  material  remained 


597 

on  the  lowland  north  of  the  outlet.  These  were  generally 
clifFed  to  a  greater  or  less  extent  by  the  current  which  flowed 
eastward  past  them.  The  eastern,  ice-contact  border  of  Liberty 
Plain  in  South  Hinghani  was  also  somewhat  modified  during 
the  undoubtedly  prolonged  standing  of  the  water  at  this  level. 
The  terrace-like  margin  of  this  plain,  already  spoken  of,  is 
probably  an  erosion  terrace  formed  at  this  time.  If  any  de- 
posits were  formed  over  the  area  now  occupied  by  Glad 
Tidings  Plain,  they  must  have  been  so  modified  after  the  low- 
ering of  the  water  to  the  Glad  Tidings  level  as  to  become  re- 
ferable to  this  lower  stage. 

Drainage  was  finally  established  north  of  Booth  Hill,  along 
the  line  of  the  railroad,  in  a  southeasterly  direction.  For  a 
time  it  must  have  passed  between  the  ice  and  the  northeastern 
face  of  Booth  Hill,  as  indicated  by  the  terraces  so  prominently 
developed  on  the  northeastern  face  of  the  hill.  These  have 
been  described  in  detail  by  Crosby  on  pp.  165—166  of  Part  1. 
He  recognizes  four  distinct  terraces,  the  elevations  of  which 
above  sea-level  show  a  striking  correspondence  to  the  elevations 
of  the  lower  plains  of  Lake  Bouve.  The  lowest  is  25  feet 
above  the  marshes  and  is  "several  hundred  feet  wide  midway 
of  the  leno;th  of  the  hill,  ....  but  narrows  toward  either  end." 
Fifteen  or  twenty  feet  higher,  i.  e.,  40-45  feet  above  the 
marshes,  is  a  second  "well-defined  terrace  and  bowlder  pave- 
ment from  one  hundred  to  two  hundred  feet  wide."  Thirty 
feet  higher,  or  about  seventy  feet  above'  the  marsh,  is  the 
third  terrace,  at  least  three  •  hundred  feet  wide  and  thickly 
strewn  with  bowlders.  Like  the  other  two,  this  terrace  dies  out 
toward  the  western  end  of  the  hill.  Again,  thirty  feet  higher, 
or  one  hundred  feet  above  the  marsh  "  we  come  to  a  broad, 
level  plain,  which  seems  at  first  to  form  the  summit  of  the 
hill  ;  but,  on  tracing  it  eastward  across  the  highway  the  hill 
is  found  to  rise  abruptly  some  twenty  feet  above  the  plain  to 

the  true  summit The  plain  is  here  at  least  three  hundred 

feet  wide ;  and  at  all  points,  save  where  it  has  been  cleared  for 
cultivation,  it  is  strewn  with  bowlders"  (Crosby). 


598 

This  high  terrace  probably  indicates  that  toward  the  end  of 
the  Whitman  stage  a  discharge  of  water  took  place  across  the 
top  of  this  hill,  along  the  ice-front,  cutting  the  cliff,  and  leav- 
ing the  bowlders  behind.  The  seventy-foot  terrace  clearly 
corresponds  to  the  next  lower  stage,  i.  e.,  the  Glad  Tidings. 
The  immediate  discharge  from  Lake  Bouve  was  south  of  Tur- 
key Hills,  where  the  divide  sinks  to  something  below  the 
seventy-foot  contour.  Along  this  line  the  waters  of  the  Glad 
Tidings  stage  joined  those  of  the  Beech  wood  lakelet,  and 
together  they  discharged  along  the  ice-front  northeast  of  Booth 
Hill. 

As  long  as  the  ice-front  held  on  to  the  northern  face  of 
Turkey  Hills,  the  water  level  of  Lake  Bouve  could  not  fall 
below  the  sixty-foot  contour  line,  this  being  approximately  the 
elevation  of  the  lowest  pass  in  the  eastern  rim  of  the  basin, 
south  of  Turkey  Hills.  To  allow  the  waters  of  the  lake  to  sink 
to  a  lovver  level,  the  ice  had  to  relinquish  its  hold  on  Turkey 
Hills,  thus  allowing  passage  to  the  water  north  of  this  drum- 
lin  along  the  line  followed  by  the  Old  Colony  Railroad.  The 
well-marked,  bowlder-strewn  channel  along  this  line  and  the 
sand  deposits  in  Cohasset  resulting  from  the  drainage  in  this 
direction  have  been  described  by  Crosby.  The  forty  to  forty- 
five  foot  erosion  terrace  on  the  northern  face  of  Booth  Hill 
corresponds  to  the  Hingham  plain,  and  indicates  that  the  drain- 
age still  passed  southeastward  between  the  ice  and  the  hill  at 
this  late  stage  of  the  lake. 

It  required  the  opening  of  a  still  lower  pass  to  permit  the 
waters  to  sink  to  the  level  at  which  the  Quincy  plains  were 
built.  That  this  was  lower  than  the  forty-foot  contour,  is 
shown  by  the  fact,  that  in  the  majority  of  cases  the  elevation  of 
this  plain  above  the  sea,  is  much  less.  It  is  not  improbable 
that  the  twenty-five-foot  terrace  on  Booth  Hill  corresponds  to 
this  plain,  though  it  may  correspond  to  the  somewhat  lower 
coastal  plain  which  is  sparingly  developed  in  the  Quincy  and 
Hingham  area.     At  all  events,  the  drainage  of  the  lake  during 


599 

the  formation  of  the  Qiiincy  plains  could  only  have  been  north 
of  Turkey  Hills, — and  probably  was  along  the  line  now  fol- 
lowed by  Weir  River. 

While  at  any  stage  of  the  lake  a  considerable  body  of  water 
was  held  up  in  front  of  the  ice,  the  size  and  outline  of  the  lake 
was  changed  with  each  successive  uncovering  of  a  lower  outlet. 
As  the  water  level  subsided  to  the  level  of  the  new  passes,  the 
lake  contracted  and  parts  formerly  under  water  were  added  to 
the  shore.  The  plains  of  the  higher  levels  became  either  a 
part  of  the  shore  or  remained  as  islands  during  the  succeeding 
lower  stages.  Rock  and  till  ridges  and  the  tops  of  drumlins 
commonly  projected  above  water  as  islands  during  every  stage 
of  the  lake,  and  over  extensive  districts  the  water  was  often 
very  shallow.  Nevertheless,  the  lake  covered  such  an  extensive 
area,  and  the  changes  which  marked  each  successive  stage  of 
its  history,  have  such  a  direct  bearing  on  the  present  topog- 
raphy and  surface  geology  of  the  southern  part  of  the  Boston 
Basin,  that  it  well  deserves  to  stand  as  an  example  of  an  ice- 
barrier  lake  of  varied  but  perfectly  determinable  history. 

Note  on  the  Mcqj,  Plate  25. — Since  the  printing  of  the 
map,  several  years  ago,  a  more  careful  study  of  portions  of  the 
old  lake  basin  has  shown  some  inaccuracies  in  detail.  The 
"  shore  lines  "  outlined  in  blue  are  not  to  be  regarded  as  actu- 
ally recognized  beaches  showing  shore  features,  though  in 
some  places  these  are  indicated.  They  represent  rather  the 
contour  lines,  transcribed  from  the  Government  topographical 
map,  and  corrected  in  places,  though  not  everywhere.  If 
these  contours  were  accurate  they  would  represent  the  areas 
flooded  during  the  successive  stages  of  the  lake,  and  they 
would  show  the  places  where  shore  features  are  to  be  looked 
for.  The  westernmost  lobe  of  the  Liberty  plains  at  the  mouth 
of  the  narrow  valley  of  Old  Swamp  River  should  show  an  ice- 
contact  margin  only  on  the  northwest.  The  western  and 
southern  margin  is  a  lobate  delta  front.  A  small  plain  of  the 
Monatiquot  series,  just  north  of  Avon  Station  on  the  main  line, 


600 

is  not  shown  on  the  map.  The  hills  northeast  of  Prospect  Hill 
in  Hingham  should  be  delineated  as  drumloids  rather  than 
drumlins.  They  belong  to  the  moraine  of  South  Pleasant 
Street.  The  two  high  eskers  of  South  Hingham  should  be 
continued  across  Liberty  Plain  in  the  direction  of  Accord 
Pond.  In  the  western  part  of  Quincy  are  several  small  plains 
of  the  Quincy  series  which  are  not  shown  on  the  map. 


601 


PALAEONTOLOGY  OF  THE  CAMBRIAN  TERRANES 
OF  THE  BOSTON  BASIN. 

BY  AMADEUS  W.  GRABAU. 

In  the  following  pages  it  is  proposed  to  give  a  complete  ac- 
count of  the  Cambrian  fossils  which  have  been  found  up  to  the 
present  time  within  the  area  of  the  Boston  Basin,  with  such 
additional  notices  of  the  Cambrian  fossils  from  other  parts  of 
eastern  Massachusetts  as  will  give  a  comprehensive  view  of  the 
Cambrian  palaeontology  of  this  region.  It  is  believed  that  by 
bringing  the  descriptions  of  these  fossils  together  the  present 
state  of  our  knowledge  of  this  subject  is  more  clearly  set  forth, 
and  a  basis  for  future  investigations  is  furnished. 

One  new  genus  and  several  new  species  are  described,  and 
the  range  of  several  well-known  species  has  been  somewhat  ex- 
tended. The  whereabouts  of  the  types  and  other  important 
specimens  from  this  region  is  given  as  fully  as  could  be  ascei'- 
tained. 


HISTORICAL. 

The  first  fossil  described  from  this  region  was  the  trilobite 
Paradoxides  hai'lani,  now  known  to  occur  in  the  middle 
Cambrian  slates  of  the  townsliip  of  Braintree.  The  species 
was  described  by  Dr.  Jacob  Green  in  1834  in  the  American 
Journal  of  Science  (vol.  25),  and  the  specific  name  was  given 
in  honor  of  Dr.  Richard  Harlan  who  brought  the  first  speci- 
men to  Green's  notice.  This  specimen  was  obtained  from  the 
collection  of  Mr.  Francis  Alger  of  Boston,  who  had  purchased 
it  from  the  old  Columbian  Museum,  of  Boston,  on  the  breaking 


602 

up  of  that  institution.  The  angularity  of  the  fragment  of 
rock  containing  the  fossil,  indicates  that  it  had  been  taken  in 
the  neighborhood  of  the  parent  ledge,  and  was  not  a  fragment 
from  the  drift.  In  lithic  character  it  appeared  identical  with 
the  Braintree  argillites,  in  which  other  specimens  were  subse- 
quently found.  The  precise  locality  from  which  the  first 
specimen  came  is  unknown,  but  the  probabilities  are  that  it 
was  taken  in  the  vicinity  of,  if  not  from,  the  quarry  on  the 
south  bank  of  Hayward  Creek  in  the  extreme  northeastern 
part  of  Braintree.  This  specimen  has  been  in  the  possession  of 
the  Boston  Society  of  Natural  History  for  many  years.  It 
was  not  labeled,  but  a  cast  of  it,  also  in  the  collection  of  the 
Society,   was   labeled,   "Cast    o^  Paradoxides   harlani  .from 

Braintree,  Mass., more, than  30  years  old,  presented 

by  C.  T.  Jackson,  June  28,  1868." 

According  to  this  label  the  cast,  which  is  one  of  several, 
was  made  befoi'e  1838.  Green's  description,  as  has  just  been 
stated,  appeared  in  1834,  and  it  was  not  until  1856  that  the 
next  specimens  of  this  trilobite  were  announced  by  Prof.  Wm. 
B.  Rogers.  The  specimen,  furthermore,  is  the  only  one  from 
which  numerous  casts  were  made,  and  as  it  corresponds  to 
Green's  description,  its  identification  as  the  type  is  unquestion- 
ably correct.  The  specimens  of  this  trilobite  mentioned  by 
Prof.  Rogers  as  the  first  found  in  situ,  were  obtained 
by  him  from  the  quarry  on  Hayward  Creek,  his  attention  hav- 
ing been  called  to  the  occurrence  of  these  fossils  at  that  local- 
ity, by  Mr.  Peter  Wainwright  of  Boston.  They  liad  previ- 
ously been  noticed  by  Mr.  Hayward,  the  proprietor  of  the 
quarry,  who,  however,  was  unacquainted  with  their  scientific 
value.  The  specimens  obtained  by  Prof.  Rogers  were 
exhibited  at  the  meeting  of  the  Boston  Society  of  Natural. 
History,  on  August  6,  1856  ;  and  an  account  of  them  was 
published  in  the  Proceedings  of  that  meeting.  Almost  simul- 
taneously with  this  account,  a  letter  from  Prof  Rogers  to 
Pi'of.   J.    D.   Dana,  dated    August  13,  1856,  appeared  in  the 


603 

American  Journal  of  Science,  2d  series,  vol.  22,  p.  296,  in 
which  the  discovei'y  of  these  specimens  was  announced. 

Another  specimen  of  this  trilobite  was  presented  to  the  So- 
ciety by  Mr.  Charles  Stodder,  at  the  meeting  of  April  21, 
1858  ;  and  after  that,  a  considerable  number  of  specimens 
was  obtained  from  the  original  locality,  the  most  important  of 
which  are  noticed  below. 

Not  until  1892,  was  Paradoxides  harlani  known  to  occur 
outside  of  the  type  locality  on  Hayward  Creek.  In  that  year 
Mr.  Thomas  A.  Watson  discovered  a  specimen  in  the  slate 
ledge  on  his  land  on  the  north  side  of  Weymouth  Fore  River, 
in  Brain  tree,  and  about  a  mile  southeast  of  the  quarry  on 
Hayward  Creek.  This  specimen  is  now  in  the  collection  of  the 
Boston  Society  of  Natural  History.  Previous  to  this,  how- 
ever, in  the  summer  of  1872,  while  digging  a  cistern  on  his 
land  in  East  Braintree  Village,  Mr.  Albert  Hobart  found 
several  specimens  of  Paradoxides  harlani  in  a  green  shaly 
argillite.  The  discovery  was  not,  however,  announced  until 
nearly  twenty-five  years  later.  This  place  is  three  fourths  of  a 
mile  from  Mr.  Watson's  locality,  and  nearly  a  mile  and 
a  quarter  due  south  from  Hayward  Creek.  Only  one  speci- 
men was  preserved  ;  and  this  was  brought  to  the  notice  of  the 
Society  in  1896  by  Mr.  Watson,  who  pointed  out  the  close 
lithic  resemblance  of  the  rock  fragment  to  the  argillite  of 
the  neighboring  ledges. 

In  1860  Mr.  Albert  Ordway  found  a  fragment  of  a  small 
trilobite  in  the  Paradoxides  beds  of  Hayward  Creek,  which  he 
referred  to  the  genus  Ellipsocephalus.  This  was  the  first 
fossil  other  than  Paradoxides  which  had  been  found  in  those 
strata.  Mr.  Ordway  published  his  discovery  in  the  Proceed- 
ings of  the  Boston  Society  of  Natural  History,  vol.  8,  p.  5, 
but  did  not  propose  a  specific  name,  nor  did  he  give  a  satis- 
factory description.  What  ultimately  became  of  this  specimen 
is  not  known.  It  may  be  the  specimen  now  in  the  collection 
of  the  society  labeled  Conocephalus,  from   Dr.   W.   E.   Rice 


604 

(No.  3  of  the  list,  under  Agrattlos  qiiachxtngularis) ,  or  it 
may  be  the  type  specimen  of  Whitfield's  Arionellus  quad- 
rangularis.  This  latter  specimen  was  for  many  years  in  the 
collection  of  the  late  Prof.  Jules  Marcou,  who  presented  it  to 
the  American  Museum  of  Natural  History  in  New  York  City. 

Mr.  Ordway  also  found  in  the  Braintree  argillites  "  a  dis- 
tinct fucoidal  impression  which  shows  three  branches,  each 
about  four  inches  long,  but  not  sufficiently  Avell  marked  to 
aiford  any  evidence  with  regard  to  its  nature  "  (Proc.  Boston 
soc.  nat.  hist.,  vol.  8,  p.  6,  1862).  Similar  markings  have 
since  been  found. 

The  next  discoveries  in  the  Braintree  argillites  were  by 
Prof.  N.  S.  Shaler,  and  the  specimens  found  by  him  were 
described  by  Mr.  Charles  D.  Walcott  in  1884.  The  new 
species  added  Avere  Ptychoparia  rogersi  and  Hyolithes 
shaleri.  In  his  paper  ^  Mr.  Walcott  gives  a  full  account  of 
the  specimens  discovered  up  to  that  time  in  the  Braintree 
argillites,  with  critical  remarks,  and  with  a  large  number  of 
illustrations  which  are  here  reproduced. 

At  that  time  the  beds  from  which  these  remains  were 
obtained  were  reo-arded  as  of  Lower  Cambrian  ao-e.  This 
mistake  was  rectified  in  1888,  when  Mr.  Walcott  published 
his  re-classification  of  the  Cambrian  rocks  of  North  America, 
in  which  he  showed  the  Braintree  beds  to  be  of  Middle 
Cambrian  age.^ 

Numerous  specimens  of  trilobites,  as  well  as  remains  of 
other  organisms,  liave  been  obtained  from  the  quarry  on  Hay- 
ward  Creek,  within  recent  years.  Among  them  should  be 
mentioned  especially  the  collection  of  Mr.  W.  W.  Dodge  and 
that  of  the  U.  S.  Geological  Survey.  The  most  important  of 
these  specimens  will  be  noted  below.  The  species  added  to 
those  long  known  from  that  locality  are  :  Acrofhele  gamagei 

1  Bull.  10,  U.  S.  geol.  surv. 

2  Nature,  vol.  38. 


605 

Hobbs,  Parmophorella    (cf. )    acadica,    and    Hyolithes   (  ?) 
Jiaytoardensis.     Most  of  the  specimens  are  imperfect. 

Fossils  from  the  Lower  Cambrian  series  of  eastern  Massa- 
chusetts were  first  described  in  1888,  by  Prof.  N.  S.  Shaler 
and  Dr.  August  Foerste,  the  discovery  of  the  first  specimen 
dating  back  to  1883  (Bull.  mus.  comp.  zool.,  vol.  16,  no.  2). 
Twenty-three  species  were  described,  a  number  of  which  were 
new.  The  locality  where  these  fossils  were  discovered  is  near 
the  village  of  North  Attleboro,  in  Bristol  County,  Massa- 
chusetts, all  the  fossils  having  been  obtained  from  three  outcrops 
in  tlie  vicinity  of  Hoppin  Hill.  In  the  next  year,  1889,  Dr. 
Foerste  described  Hyolithes  from  the  limestone  near  East 
Point,  Nahant,  and  correlated  these  beds  with  those  at  North 
Attleboro,  and  at  Mill  Cove,  Weymouth,  which  latter  beds 
were  not  then  known  to  be  fossiliferous.  Previous  to  this,  in 
1887,  Mr.  John  H.  Sears  had  found  specimens  of  Hyolithes  in 
the -Nahant  limestone,  but  their  organic  nature  was  questioned. 
Mr.  Sears,  however,  believed  them  to  be  fossils,  and  to  him 
belongs  the  credit  of  being  the  first  one  to  find  these  obscure 
fossils  at  Nahant,  and  to  recoo;nize  their  oro-anic  nature.  Dur- 
ing  the  next  year  (1888)  Mr.  Alfred  C.  Lane  found  similar 
remains  in  the  Nahant  limestone.  His  original  specimen  is 
now  in  the  collection  of  Harvard  University,  and  represents  the 
second  discovery  of  fossils  at  Nahant.  Mr.  Sears'  original 
specimen  is  in  the  collection  of  the  Peabody  Academy  of  Sci- 
ences at  Salem.  It  is  interesting  to  note  in  this  connection 
that  as  early  as  1850  Prof.  Louis  Agassiz  published  a  note  in 
the  Proceedings  of  the  Boston  Society  of  Natural  History 
(vol.  3,  p.  341)  to  the  effect  that  he  had  discovered  what 
appeared  to  be  corals  in  the  Nahant  argillites,  which  he 
referred  to  the  Carboniferous  age.  Between  1887  and  1890 
Mr.  Sears  made  extensive  collections  from  the  Nahant  lime- 
stone, and    in    1890    he    announced  ^  the  occurrence  of  three 

1  Bull.  Essex  inst.,  vol.  22,  p.  32. 


606 

species  of  Hyolithes  {H,  princeps,  H.  communis,  also  var. 
emmonsi,  and  H.  impar)  and  a  species  of  Stenotheca  {S. 
riigosa)  in  the  limestones  of  ISTahant,  the  identification  being 
made  by  Mr.  Walcott.  Mr.  Sears  also  announced  a  third 
locality  for  Lower  Cambrian  fossils  in  eastern  Massachusetts. 
This  is  in  the  valley  between  Prospect  Hill  and  Hunslow's 
Hill,  in  Rowley,  where  a  limestone  occurs  which  has  "  nearly 
all  become  altered  to  chert  and  epidote,  but  fragments  of 
the  Hyolithes  fossils  are  still  to  be  found  in  it"  (loc.  cit.,  p. 
33).  Mr.  Sears  further  noted  that  on  the  fishing  grounds 
known  as  Jeffrey's  Ledge,  twenty  miles  east-northeast  from 
Thatcher's  Island,  fishermen  have  pulled  up  large  masses  of 
the  Olenellus  Lower  Cambrian  chert  and  limestone  identical 
with  that  of  Nahant  and  Rowley. 

More  recently  Mr.  Sears  has  discovered  traces  of  fossils  in  a 
dark  limestone  from  Topsfield,  Mass.,  in  which  sections  of 
Hyolithes  of  several  species  are  distinguishable.  The  next 
important  discoveries  in  the  Lower  Cambrian  of  eastern  Massa- 
chusetts are  due  to  Mr.  Thomas  A.  Watson,  whose  interest  in 
the  local  geology  has  been  so  productive  of  results.  In  1891 
Mr.  AYatson  found  a  small  bowlder  of  red  limestone  on  Pleas- 
ant Beach,  Cohasset,  a  second  at  Sandy  Cove,  and  in  1894  a 
third  one  of  the  same  character  at  Bass  Point,  Nahant.  The 
finding  of  these  bowlders  leads  to  the  supposition  that  a  fifth 
locality  for  Lower  Cambrian  fossils  occurs  somewhere  in  east- 
ern Massachusetts,  as  the  probabilities  are  against  the  source  of 
these  bowlders  being  in  Rowley,  where  the  prevailing  matei-ial 
is  grayish  limestone  similar  to  that  of  Nahant,  while  the  red 
beds  are  of  a  more  shaly  character.  The  possibility  that  these 
bowlders  were  carried  here  from  some  more  northern  locality, 
either  by  ice  or  as  ballast,  must  not  be  overlooked. 

The  first  Cohasset  bowlder  was  forwarded  to  Mr.  Walcott 
for  examination,  and  he  found  in  it  Hyolithes  commioiis, 
small  var.,  and  Strajjarolliiia  remota.  Mr.  Walcott  pub- 
lished a  notice  of  this  bowlder  in  the  Proceedinofs  of  the  Bioloo-- 


607 

ical  Society  of  Washington,  vol.  7,  p.  155,  July,  1892. 
Besides  the  two  species  identified  by  Walcott,  twelve  other 
species  have  been  found  in  the  three  bowlders  found  by  Mr. 
Watson.  A  mass  of  similar  rock  has  since  been  found  by 
Mr.  Watson  at  Weymouth.  In  1894  Mr.  Watson  found  a 
pebble  of  dark  reddish  brown,  calcareous  slate  at  Nahant,  west 
of  the  point  known  as  John's  Peril.  On  the  bedding  plane 
occurs  a'  fragmentary  cephalon  of  a  trilobite  here  referred  to 
Agraulos  (Strenuella)  strenuus.,  and  on  the  fracture  face 
occur  several  cross-sections  of  the  thorax. 

The  most  recent  discoveries  in  the  Lower  Cambrian  of 
eastern  Massachusetts  have  been  made  by  Mr.  H.  T. 
Burr  near  Mill  Cove  at  North  Weymouth.  Several  years  ago 
Prof.  W.  O.  Crosby  noticed  a  cross-section  of  a  trilobite  on  a 
fracture  face  of  the  red  slate  at  Mill  Cove.  In  1897  Messrs. 
J.  B.  Woodworth  and  Aug.  F.  Foerste  noticed  Hyolithes  in 
the  red  slates  and  associated  limestones  at  Mill  Cove.  Neither 
of  these  discoveries  has  been  published.  In  the  spring  of  1899 
the  writer  revisited  the  locality  in  company  with  Messrs.  Burr 
and  R.  E.  Burke.  Several  specimens  of  slender  hyolithids 
were  obtained,  which  are  referred  to  Orthotheca  cylindrica. 
Later  in  the  year  some  glaciated  pebbles  of  dark  slate,  carry- 
ing trilobite  fragments,  were  found  by  the  writer  in  the  drift 
about  a  mile  east  of  Weymouth  Heights  Station.  Their  hox'izon 
was  not  determined,  but  they  have  since  been  recognized  as 
identical  with  the  rock  on  Pearl  Street.  The  ledges  on  Pearl 
St.,  North  Weymouth,  were  carefully  examined  by  Mr.  Burr 
in  the  autumn  of  1899  and  were  found  to  contain  a  rich  Olen- 
ellus  fauna.  A  large  number  of  specimens,  representing  some 
seven  or  eight  species  of  Lower  Cambrian  fossils  has  already 
been  obtained.  A  summary  of  the  discoveries  has  been  pub- 
lished by  Mr.  Burr  in  the  American  Geologist,  vol.  24,  pp. 
41—50.  The  rock  is  identical  with  the  pebble  from  Nahant 
carrying  Strenuella. 

No  undoubted  Upper  Cambrian  strata  have  as  yet  been  found 


608 

in  situ  in  eastern  Massachusetts,  but  many  pebbles  and  frag- 
ments of  rock  bearing  fossils  of  Upper  Cambrian  or  Lower 
Ordovician  age  have  been  discovered.  The  first  discovery  of 
such  fossiliferous  pebbles  was  announced  by  Prof.  W.  B. 
Rogers  in  1861,  in  the  Proceedings  of  the  Boston  Society  of 
Natural  History  (vol.  7,  1861,  pp.  389-391).  Professor 
Rogers  thought  that  two  distinct  species  of  Lingula  were  recog- 
nizable in  the  rock,  resembling  Lingula  prima  and  Lingula 
antiqua  of  the  Potsdam  sandstones  of  New  York.  The  pebbles 
were  found  in  the  Carboniferous  conglomerates  north  of  Fall 
River,  Massachusetts.  Similar  pebbles  of  gray  quartzite  with 
obscure  impressions  of  Lingula  were  found  by  Professor  Rogers 
in  the  Carboniferous  conglomerate  of  Newport,  Rhode  Island, 
of  which  an  account  was  given  in  1875,  in  the  Proceedings  of 
the  Boston  Society  of  Natural  History  (vol.  18,  p.  100). 
More  recently,  Mr.  J.  B.  Woodworth,  of  the  U.  S. 
Geological  Survey,  has  found  pebbles  bearing  Lingula  in  the 
conglomerate  near  Attleboro,  and  at  various  points  eastward 
and  southward.  In  these  Mr.  Walcott  has  discovered  three 
species  of  linguloid  shells. 

A  pebble  of  quartzite  carrying  Scolithus  indistinguishable 
from  8.  linearis  of  the  New  York  Potsdam  was  recently 
found  by  Prof.  W.  O.  Crosby  on  Easton's  Beach  at  Newport, 
R.  I.  It  probably  came  from  a  neighboring  ledge  of  the  Car- 
boniferous conglomerate.  Specimens  of  quartzite  carrying 
Scolithus  were  also  found  by  Mr.  Woodworth  on  Martha's 
Vineyard,  on  Nantucket,  and  near  Duxbury,  Mass. 


609 

STRATIGRAPHIC    SUMMARY. 

Lotoer   Cambrian. 

The  table  on  page  610  shows  the  distribution  of  the  fossils 
from  strata  lower  than  the  Paradoxicles  beds  in  eastern  Massa- 
<ihusetts.     The  localities  are  as  follows  : 

1.  Station  No.  1,  North  Attleboro. 

2.  "       No.  2,      " 

3.  "       No.  3,      " 

4.  Bowlder  from  Barrier  Beach,  Bass  Point,  Nahant. 

5.  Bowlder  from  Pleasant  Beach,  Cohasset. 

6.  Bowlder  from  Sandy  Cove,  Cohasset. 

7.  Pebble  from  Nahant. 

8.  White  limestone  at  Nahant. 

9.  E.ed  slates  and  limestones  at  Mill  Cove,  Weymouth. 

10.  Dark  purplish  slates  of  Pearl  Street,  North  Weymouth. 

11.  Siliceous  limestone  in  Rowley. 

12.  "  "  "  Topsfield. 

(Localities  11  and  12  have  furnished  only  indeterminable 
fossils,  hence  they  are  omitted  from  the  table.) 

Twenty-three  species  were  originally  described  from  North 
Attleboro  by  Shaler  and  Foerste,  nine  of  these  being  described 
as  new.  Of  these  nine,  Paradoxides  walcotti  was  considered 
by  Walcott  ^  to  be  generically,  if  not  specifically,  identical  with 
Olenellus  asai^hoides  Emmons,  of  the  Lower  Cambrian  of 
Troy,  New  York. 

Ptychoparia  "mucronatua  is  regarded  by  Walcott  as  identi- 
cal with  Agraulos  (SfrenueUa)  streuims  Billings."  Four 
of  the  twenty-three  species  found  at  North  Attleboro  have  so 
far  been  found  only  in  eastern  Massachusetts.  These  are 
Stenotheca    ahrupta,   Pleurotomaria    (Pajyliistoma)    attle- 

1  loth  ann.  rep.  U.  S.  geol.  surv.,  part  1,  p.  636. 

2  Ibid,  p.  653. 

OCCAS.  PAPERS  B.  S.  N.   H.   IV.  39. 


610 


Locality  number 

1 

2 

3 

4 

5 

6 

— 

8 

1 
1 

9 

10 

c 

Beachiopoda 
1     Iphidea  bella  (^)         .         

2     Obolella  crassa 

c 

1 

C 

3     Obolella  atlantica  

Pelectpoda 
4.    Fordilla  troyensis  (?) 

1 
1 

Gastropoda 

1 

7     Scenella  (?)     sp 

1 

c 

1 
1 

r 
r 
r 

9     Platyceras  deflectuni         

10    Watsonella  crosbyi 

1 
r 
c 

11.   Raphistoma  attleborensis 

12     Straparollina  remota 

1 

r 

r 

13    Stenotlieca  abrupta          

c 
r 

C 

1 

r 
r 

15    Stenotheca  pauper         

c 

1 

Htolithidae 
17     Hyolithes  princeps 

c 
r 

18    Hyolithes  excellens 

19    Hyolithes  ciuadricostatus 

r 
1 

20    Hyolithes  americanus 

1 

21     Hyolithes  billinffsi         

r 

22     Hyolithes  searsi,    

r 

r 
r 
r 
c 

23     Hyolithes  cominunis 

9 

25    Orthotheca  cylindrica 

c 

1 

r 

C 

r 
r 

r 

26    Orthotheca  emiiiousi     

27    Orthotheca  (^)  foerstei.         

C 
C 

28    Hyolithellus  micans 

r 

29    TJrotheca  pervetus   

1 

30    Salterella  curvatus 

c 

\  .  Crustacea 

1 

.r. 

32    Olenellus  (Holmia)  broiigeri 

c 

r 
r 
r 

.33     Olpnellns  sn                 

34.   01enellus(  Mesonacis)  asaphoides  (?) 

1 
c 
1 

37.  Microdiscus  bellimarginatus 

C 

39    Microdiscus   of   M  helena 

1 
c 

c 
c 

C 

1 

41.   Ptychoparia  (?)  attleborensis 

42    Trails 

r 

Table  showing  the  distribution  of  species  in  rocks  below  the 
Paracloxides  beds  in  eastern  Massachusetts,  (c  =  common, 
r  =  rare  ;   numerals  indicate  number  of  specimens  found.) 


611 

borensis,  Orthotheca  foerstel,  and  Salterella  CAcrvatus. 
The  first  of  these  was  also  found  in  the  Nahant  bowlder,  and 
the  second  in  the  two  Cohasset  bowlders.  The  third  was 
found  in  the  bowlder  from  Pleasant  Beach,  Cohasset,  but  the 
fourth  has  not  been  noticed  in  eastern  Massachusetts  outside 
of  North  Attleboro. 

Fordilla  troyensis  (  ?)  is  probably  a  fragmentary  specimen 
of  Watsonella  crosbyi^  and  Hyolithes  princei-ys  Shaler  and 
Foerste  is  here  separated  as  Orthotheca  foerstei. 

From  the  table  it  appears  that,  while  the  specimens  found  at 
station  3  all  occur  at  station  2,  these  two  stations  and  station  1 
have  not  a  single  species  in  common.  Making  allowance  for 
unrecorded  specimens  of  the  commoner  forms,  there  still  re- 
mains a  decided  difference.  Similar  differences  are  shown  by 
the  association  of  species  in  the  Nahant  and  Cohasset  bowlders. 
The  three  bowlders  found  at  Nahant  and  Cohasset  agree  so 
closely  in  texture,  color,  and  fossil  contents  with  the  North 
Attleboro  rock,  that  from  hand  specimens  it  is  impossible  to 
distinguish  the  two.  The  precise  locality  of  the  bed-rock 
which  has  furnished  these  bowlders  has  not  been  ascertained, 
but  it  is  very  probable  that  this  is  somewhere  to  the  north 
of  Nahant,  and  possibly  beneath  the  sea  or  neighboring- 
marshes. 

The  pebble  of  dark  reddish  brown  calcareous  shale  from 
Nahant  has  furnished  only  Agraidos  {Strenuella)  strenuus. 
This  pebble  agrees  closely  with  the  dark  purplish  slates  of 
Pearl  Street,  North  Weymouth,  in  which  the  same  variety  of 
Strenuella  strenua  has  been  found.  The  mode  of  preserva- 
tion is  also  similar  in  both  cases,  the  fossils  being  coated  with 
a  soft,  greenish,  chloritic  minei'al.  The  occurrence  of  this  peb- 
ble at  Nahant  leads  to  the  supposition  that  beds  of  the  North 
Weymouth  type  occur  somewhere  north  of  Nahant.  In  the 
white  limestone  of  Nahant  Orthotheca  cylindrica  predomi- 
nates, this  being  also  a  common  species  in  the  limestone 
bowlders,    and    at    station    1,    North    Attleboro.      Hyolithes 


612 

impar,  II.  prhiceps,  H.  excellens,  and  H.  searsi  have  not 
been  recognized,  in  eastern  Massachusetts,  outside  of  the 
Nahant  rocks,  nor  is  Iphidea  hella  known  from  any  other 
deposit  in  this  region. 

A  number  of  specimens  of  the  white  limestone  from  Nahant, 
obtained  by  Mr.  J.  B.  Woodworth,  show  black  shining  gra- 
phitic lines  resembling  lead-pencil  marks.  These  often  outline 
the  hyolithids,  but  sometimes  show  no  definite  relation  to  those 
shells.  What  the  organic  matter  was  which  furnished  the 
carbon,  and  what  the  condition  of  its  preservation,  is  a  subject 
for  investigation. 

The  only  fossils  so  far  found  in  the  red  slates  and  limestones 
of  Mill  Cove,  Weymouth,  are  hyolithids  ( Orthotheca  cylin- 
drical, and  indeterminate  fragments)  and  the  cross-section  of  a 
trilobite  above  referred  to.  More  extended  search,  however, 
will  undoubtedly  reveal  other  types  similar  to  those  occurring 
at  Nahant.^  The  fossiliferous  beds  of  Pearl  Street  are  strati- 
graphically  above  the  red  beds  of  Mill  Cove.  The  fauna  of 
the  Pearl  Street  beds  is  essentially  a  trilobite  fauna  and  re- 
sembles most  nearly  the  fauna  of  the  Olenellus  beds  of  Con- 
ception Bay,  Newfoundland,  though  Olenellus  asaphoides, 
which  appears  to  be  represented  in  the  Pearl  Street  beds,  has 
not  been  found  as  yet  at  Conception  Bay. 

The  dark  gray  limestone  of  Jeffrey's  Ledge,  off  Rockport, 
contains  sections  referred  by  Sears  to  Salterella  rugosa  (  ?) 
and  hyolithids.  This  rock  closely  resembles  the  Nahant  lime- 
stone and  is  probably  to  be  correlated  with  it.  The  dark  lime- 
stone from  Topsfield,  Mass.,  and  the  gray  and  red  limestone 
of  Rowley,  contain  traces  of  fossils,  of  which  hyolithids  alone 
have  so  far  been  identified. 

1  Mr.  Burr  has  since  found  trilobites  in  these  rocks. 


613 


Middle   Cambrian. 

The  only  productive  locality  for  Middle  Cambrian  fossils  is 
still  the  quarry  on  Hay  ward  Creek  in  Braintree.  The  follow- 
ing have  been  found  there  up  to  the  present  time  : 

1.  Fucoids  ?  or  trails. 

2.  Acrothele  gamagei. 

3.  Parniojjhorella  acadica. 

4.  Hyolithes  shaleri. 

5.  Hyolithes  (?)  hayiioardensis. 

6.  A-graiilos  quadrangularis . 

7.  Ptychoparia  rogersi. 

8.  Paradoxides  harlani. 

The  only  two  relatively  common  forms  are  Paradoxides  har- 
lani and  Agraulos  quadrangularis . 

Only  two  other  Massachusetts  localities  furnishing  undoubted 
Paradoxides  Cambrian  fossils  are  known.  These  are  Mr. 
Watson's  and  Mr.  Hobart's  ledges  in  East  Braintree,  both  of 
which  have  furnished  specimens  of  Paradoxides  harlani. 
Professor  Shaler,  however,  states  ^  that  "  a  number  of  distinct 
remains  occur  near  the  Neponset  River  in  beds  having  much 
the  same  aspect,  and  apparently  at  about  the  same  distance 
from  the  syenites,  as  those  at  Braintree." 

Upper   Cambrian. 

The  following  species  have  been  recognized  from  the 
quartzite  pebbles  of  the  Carboniferous  conglomerate,  which 
alone,  so  far  as  known,  represent  this  horizon  in  eastern 
Massachusetts  : 

1.  Scolithus  linearis. 

2.  Lingulobolus  affinis. 

3.  Sphaerobolus  spissus. 

4.  Obolus  {LingulelUi)  rogersi. 

1  Bull.  mus.  comp.  zool.  vol.  16,  p.  23. 


614 

The  three  brachiopods  are  found  associated  in  Lower  Ordovi- 
cian  quartzite  on  Great  Bell  Island,  Newfoundland,  and  Mr. 
Walcott  is  of  the  opinion  that  it  is  quite  probable  that  these 
indurated  sandstones  which  carry  these  brachiopods,  represent 
the  passage  beds  between  the  Cambrian  and  Lower  Ordovi- 
cian.  He  further  suggests  the  probability  that  the  fossiliferous 
pebbles  of  the  conglomerate  may  be  derived  from  the  sand- 
stones of  Great  Bell  Island,  a  fact  which,  if  proved,  could  only 
be  explained  by  the  theory  of  Professor  Shaler,  that  these  con- 
glomerates represent  the  deposits  of  Carboniferous  glaciers. 


DESCRIPTIONS   OF   THE    SPECIES. 

Annelida 
Genus   ScOLiTHUS  Haldemann 

SCOLITHUS,    cf.    S.   LINEARIS    Hall. 

Compare  Scolithus  linearis  Hall,  Pal.  N.  Y.,  1847,  vol.  1, 
p.  2. 

The  structures  referred  to  this  species  are  slender,  sub- 
cylindrical,  slightly  flexuous,  stem-like  bodies  passing  through 
the  quartzites,  and  having  a  circular  cross-section.  They  con- 
sist of  the  same  material  as  the  rock  matrix,  from  which  they 
are  distinguished  only  by  their  smooth  peripheral  surfaces. 
They  most  probably  represent  the  filling  of  worm  burrows, 
the  peripheral  surfaces  of  which  were  formed  of  agglutinated 
sand  grains. 

A  specimen  collected  by  Prof  Crosby  at  Newport,  R.  I., 
contains  tubes  measurins:  4.5  mm.  in  diameter,  bv  a  length  of 
over  70  mm.,  the  total  length  not  being  shown  in  tlie  specimen. 
A   pebble  from   Gay  Head    beach   shows   a  number   of  these 


615 

tubes  in  parallel  positions,  traceable  for  more  tlian  150  mm.  in 
length  and  varying  from  2  to  4  mm.  in  diameter.  A  pebble 
from  the  Duxbury  shore  shows  the  tubes  standing  vertical  to 
the  stratification. 

Horizon  and  Locality.  —  In  pebbles  of  quartzite  of  Upper 
Cambrian  or  Lower  Ordovician  age,  obtained  from  the  Car- 
boniferous conglomerate  of  Newport,  E,.  I.,  (Crosby  —  Coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  11,976)  and  found  as  pebbles 
and  bowlders  at  Martha's  Vineyard,  Nantucket  and  Duxbury 
(J.  B.  Woodworth  —  Coll.  Harv.  univ.  and  U.  S.  nat.  mus.). 


Brachiopoda. 
Genus  AcROTHELE  Linnarsson. 

ACROTHELE    GAMAGEI    (Hobbs). 

PI.  31,  Figs.  1,  a-d. 

1899.      Obolella  gamagei  Hobbs,  Amer.  geoL,  vol.   23,  p. 
114. 

Shell  subcircular,  slightly  wider  than  long.  Posterior  end 
truncate  ;  hinge-line  about  one  third  the  width  of  the  shell,  or 
less,  with  the  sides  of  the  shell  regularly  curving  to  it. 

Pedicle  valve  strongly  elevated  at  the  beak,  which  is  situated 
near  the  posterior  margin,  and  is  neatly  pointed.  Just  behind 
the  beak  the  shell  is  pierced  by  a  circular  foramen,  behind 
which  it  slopes  steeply  to  the  hinge  margin.  A  moderately 
well-defined,  triangular,  flat  area  ("false  area")  occupies  the 
greater  part  of  the  space  between  the  beak  and  the  hinge-line. 
The  flattening  of  the  posterior  end  is  not  abrupt,  the  sides  of 
the  shell  curving  toward  it  and  merging  into  it.  Anterior  and 
lateral  slopes  somewhat  abrupt  and  slightly  concave  in  the  first 
half,  becoming  gentle  and  flat  or  even  slightly  convex  in  the 


616 

second  half.  Brachial  valve  nearly  flat  or  gently  convex  in 
antero-posterior  diameter.  The  postero-lateral  margins  are 
gently  elevated,  leaving  a  concavity  on  each  side  which  passes 
forward  and  outward  from  the  beak  to  the  lateral  margins. 
Beak  minute,  not  elevated.  Posterior  margin  squarely  trun- 
cate, formino'  a  strais^ht  hino-e-line. 

Surface  marked  by  coarse,  sublamellose  lines  of  growth,  be- 
tween which  there  are  finer,  concentric  lines.  Fine,  closely 
set,  radiating  striae  are  visible  on  the  more  perfect  specimens. 

Interior  unknown. 

This  species  varies  somewhat  in  the  proportion  of  its  length 
and  width.  In  compressed  specimens  the  width  may  greatly 
exceed  the  length,  the  elevation  of  the  beak  being  increased  at 
the  same  time. 

The  following  measurements  (in  millimeters)  express  the 
variation  in  uncompressed  specimens  : 


PI  31, 
Figure 

Length 

Width 

Height 

Hinge 

1. 

la. 

10. 

11- 

1.5 

4. 

2. 

lb. 

8. 

9. 

1. 

3. 

3. 

— 

9. 

10. 

— 

4. 

4. 

Ic. 

8. 

9. 

1.5 

3. 

5. 

Id. 

7.5 

8. 

— 

— 

Nos.  1  to  4  are  pedicle  valves  ;    No.  5  a  brachial  valve. 

This  is  the  Massachusetts  representative  of  Acrothele  niat- 
thewi  (Hartt)  and  its  varieties  from  the  St.  John  group  of 
New  Brunswick.  It  differs  from  that  species  in  its  more 
nearly  circular  form,  its  longer  hinge-line,  and  in  the  radiating 
striae  of  the  surface.  The  anterior  flat  or  slightly  convex 
slope  also  distinguishes  this  species  from  the  New  Brunswick 
forms,  in  which  the  slope  continues  concave  to  the  front. 

An  imperfect  speciimen  was  described  by  W.  E.  Hobbs  under 
the  name  Oholella  gamagei.  A  careful  comparison  with  the 
specimens  here  described  shows  it  to  be  identical  with  them. 


617 


Consequently    Mr.    Hobbs'    specific   name   is   retained 
figure  is  here  reproduced  from  the  original  block. 

Horizon  and  Locality.  —  This 
species  has  been  found  in  the  sili- 
ceous argillites  of  Mesocambrian 
age  at  Hay  ward  Creek,  associated 
with  Paradoxides  harlani,  A.g- 
raulos  qtiadrangularis  and  other 
species.  Nine  specimens  were 
collected  in  1890  by  Mr.  W.  W. 
Dodge,  four  of  which  are  here- 
with illustrated.  Coll.  W.  W. 
Dodo-e. 


His 


"•i^  iUMJttJukil 


Fig.  48.  Acrothele  gamagei 
(Hobbs).  Figure  of  the  speci- 
men described  by  Mr.  Hobbs. 


Genus  Iphidea,  Billings. 


Iphidea  BELLA  Billings  (  ?) . 
Plate  31,  fig.  2. 

Compare  : 

1872.   Ijjhidea  bella  Billings,  Can.  nat.  and  geol.,  n.  s.,^ 

vol.  6,  p.  477. 
1874.    Iphidea  bella   Billings,  Pal.  foss.,  vol.  2,  pt.   1, 

p.  76. 
1886.    Iphidea    bella    Walcott,   Bull.    30,  U.   S.    geol. 

surv.,  p.  100,  pi.  7,  fig.  4. 
1890.    Iphidea  bella  Walcott,  10th  ann.  rep.  U.  S.  geol. 

surv.,  p.  608,  pi.  67,  fig.  6. 

Pedicle  valve  sub-conical,  elevated  at  the  beak  and  with  a 
straight  hinge-line  which  is  shorter  than  the  greatest  width  of 
the   valve.      Anterior  outline  regularly  curved,  the  curvature 


618 

comprising  more  than  a  semicircle.  Posterior  end  furnished 
with  a  flat  sub-triangular  ' '  false  area  "  with  a  central  eleva- 
tion or  "  pseudo-deltidium."  Surface  ornamented  with  rather 
distant  concentric  lines  of  growth,  and  with  faint,  radiating 
striae  which  are  rather  far  apart. 

Dimensions  :  width,  14.5  mm  ;  length,  10  mm.  ;  height,  less 
than  3  mm.  ;  length  of  hinge-line,  10  mm. 

The  valve  from  which  the  above  description  is  made,  seems 
to  be  referable  to  Iphidea  bella.  It  is  imperfect,  and  the  out- 
line and  surface  markings  are  rather  obscure  ;  nevertheless,  the 
characteristic  features  mentioned  are  visible  without  difficulty. 
Billings  remarks  that  the  genus  Iphidea  resembles  Acrotreta, 
"  but  differs  therefrom  in  havino-  a  large  convex  deltidium." 
This  is  very  marked  in  the  specimen  described.  The  dimen- 
sions of  the  ventral  (pedicle)  valve  of  the  type  specimen  of/. 
hella  are  given  by  Billings  as  follows  : 

Width,  7  lines     (=  14.5  mm.). 

Length,  5  lines    (=  10.5  mm.). 

Height,  2  lines  (=  4.  mm.). 
The  very  close  correspondence,  amounting  almost  to  identity, 
between  the  measurements  of  the  type  specimen  from  Quebec 
and  the  Nahant  specimen,  is  very  striking.  The  only  im- 
portant difference  is  in  the  height,  which  is  less  in  the  Nahant 
specimen.  As  the  beak  is  broken  away,  and  as  the  specimen 
is  otherwise  imperfect,  the  measurement  given  may  not  repre- 
sent the  true  heio-ht.  The  size  of  the  hino-e  area  of  the  Nahant 
specimen  seems  to  be  somewhat  less  than  that  of  the  type 
specimen,  and  the  area  in  general  less  sloping.  These  differ- 
ences might  be  regarded  as  sufficient  for  specific  differentiation. 
Horizon  and  Locality.  —  The  only  specimen  known  from 
the  Massachusetts  rocks  was  found  by  Mr.  John  H.  Sears  in 
the  white  limestone  of  East  Point,  Nahant.  Coll.  Peabody 
acad.  sci.,  Salem,  cat.  no.,  705. 


•  619 

Genus  Obolella.     Billings. 

Obolella  crass  a  (Hall). 
(Compare  PI.  31,  fig.  3.) 

1847.    Orbicula  (?)  crassa  Hall,  Pal.  N.  Y.,  vol.  1,  p.  290, 

pi.  79,  figs.  8a,  b. 
1847.    A.vimila    (?)   desquamata  Hall,   Pal.  N.  Y.,  vol.    1, 

p.  292,  pi.  80,  figs.  3a,  b. 
1871.     Obolella  {Orbicula  ?)  crassa  Ford,  Amer.  journ.  sci., 

3d  ser.,  vol.  2,  p.  33. 
1871.     Obolella  crassa;    O.  desquamata  Billings,   Can.  nat. 

and  geol.,  n.  s.,  vol.  6,  p.  218.     Idem,  1872,  Amer. 

journ.  sci.,  3d  ser.,  vol.  3,  p.  356. 
1873.    Dicellomus   crassa   Hall,   23d  rep.  N.  Y.   State  cab. 

nat.  hist.,  p.  246,  pi.  13,  figs.  6-9. 
1881.     Obolella   crassa    Ford,    Amer.    journ.    sci.,   3d    ser., 

vol.  21,  p.  131,  figs.  1,  2. 
1886.     Obolella  crassa  Walcott,  Bull.  30,  U.  S.  geol.  surv., 

p.  114,  pi.  10,  figs.  1,  la-f. 
1888.     Obolella  crassa  Hall,  var.,  Shaler  and  Foerste,  Bull. 

mus.  comp.  zool.,  vol.  16,  p.  27,  pi.  1,  figs,  la— f. 
1890.     Obolella  crassa  Walcott,  10th  ann.  rep.  U.  S.   geol. 

surv.,  p.  612,  pi.  71,  figs.  4,  4a-f. 
This  species   was  found  abundantly  by  Shaler  and  Foerste 
at  stations  2  and  3,  North  Attleboro,  Mass. 

A  specimen  of  a  single  valve,  obtained  from  the  white  lime- 
stone at  Nahant,  is  provisionally  referred  to  this  species  (PL  31, 
fig.  3).  Outline  sub-circular,  exact  form  not  fully  determin- 
able. Valve  elevated  posteriorly,  the  greatest  elevation  being 
about  a  third  or  a  fourth  of  the  length  of  the  shell  from  the 
beak ;  beak  incurved,  and  projecting  slightly  over  the  pos- 
terior marffin.      On  the  surface  of  the  valve  a  central  flatten- 


620 

ing  passes  forward  from  the  beak,  widening  rapidly  toward 
the  anterior  margin.  This  flattening  is  most  pronounced  near 
the  beak,  where  the  shell  slopes  abruptly  on  either  side  of  it. 
Anteriorly  the  flattened  space  is  merged  into  the  general  flat 
surface  of  the  valve.  Concentric  lines  of  some  strength  ap- 
pear to  have  occurred  at  intervals,  and  there  are  indications 
that  the  shell  was  ornamented  by  radiating  striae  as  well. 

Length,  about  9.5  mm.  ;  width,  about  9  mm.  ;  greatest  ele- 
vation, about  2.  mm. 

Horizon  and  Locality.  —  In  the  white  metamorphic  lime- 
stone at  East  Point,  Kahant.  Collected  by  A.  W.  Grabau, 
one  specimen.       Coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11,970. 

ObOLELLA    ATLANTIC  a  Walcott. 

PI.  34,  figs.  3a,  b. 

1888.  Oholella    ( ?)    sp.      Shaler    and  Foerste,    Bull.    mus. 

comp.  zooL,  vol.  16,  p.  27,  pi.  1,  figs.  2a-c. 

1889.  Oholella  atlantica  Walcott,   Proc.   U.  S.   nat.   mus., 

vol.  12,  p.  36. 

1898.  Oholella  atlantica  Walcott,  10th  ann.  rep.  U.  S.  geol. 

surv.,  p.  611,  pi.  71,  figs.  1,  la— c. 

1899.  Oholella  atlantica  Matthew,  Trans,  roy.  soc.  Canada, 

vol.  5,  sect.  4,  p.  70. 

1900.  Oholella    atlantica     Burr,     Amer.     geol.,    vol.     25, 

p.  47. 

"  Shell  almost  circular  in  outline,  moderately  convex,  with  no 
pi'ominent  beak.  The  exterior  surface  is  marked  by  concentric 
(exfoliated)  stri^  of  growth,  a  moderate  distance  apart,  and  dis- 
tinct. The  interior  cast  of  the  dorsal  valve  is  in  general  moder- 
ately convex,  at  the  edges  being  more  finely  and  less  distinctly 
striate  than  the  exterior  surface.  The  margin  along  the  beak  is 
flat.    The  cardinal  scars  in  the  cast  follow  the  outline  of  the  shell, 


621 

and  are  well  defined  along  their  exterior  outline,  but  not  along 
their  interior.  The  reverse  is  true  of  the  casts  of  the  lateral 
scars.  The  lateral  scars  unite  with  the  central  scars,  forminsT 
a  figure  comparable  with  that  of  a  reversed  ^,  which  is  dis- 
distinctly  outlined  along  the  outline  facing  away  from  the  beak, 
but  is  indistinct  along  the  outline  facing  the  cardinal  scars. 
The  diameter  of  the  shell  is  5  mm."      (Shaler  and  Foerste). 

A  number  of  specimens  of  this  species  were  discovered  by 
Mr.  Burr  in  the  Lower  Cambrian  slates  of  Pearl  Street,  near 
Mill  Cove.  Some  of  these  show  fine  radiating  striae  and 
occasional  rather  strong  concentric  growth  lines.  Several 
internal  moulds  show  faintly  some  of  the  muscular  scars. 

Dimensions:  Length,  6.  mm.,  and  3.5  mm.;  width,  6.1 
mm.,  and  4  mm.,  respectively. 

A  single  specimen  was  found  by  Shaler  and  Foerste  at 
station  2,  North  Attleboro,  Mass. 


Obolella  (?)  sp. 

1900.    OhoUUa  (  ?)  sp.,  Burr,  Amer.  geoL,  vol.  25,  p.  48. 

Several  specimens  from    the  Pearl  Street  Lower  Cambrian 
slates  indicate  the  presence  of   another  Obolelloid  brachiopod. 


Genus  LiNGULOBOLUS  Matthew. 
LiNGULOBOLUS  AFFiNis  (Billings). 

1872.    Lingulella    (?)    ajjinis  Billings,  Can.  nat.  and  geol., 

n.  s.,  vol.  6,  p.  468,  fig.  4. 
1874.    Lingulella  (?)    affinis  Billings,  Pal.   foss.,  vol.  2,  p. 
67,  fig.  35. 


622 

1889.  Lingulepis  affinis  Walcott,  Amer.  journ.  sci.,  3d  ser., 
vol.  37,  p.  381. 

1895.  Lingulobolus  a^nis  Matthew.  Trans,  roy.  soc.  Can- 
ada, 2d  ser.,  vol.  1,  sect.  4,  p.  261,  pi.  1, 
figs.   4a,  b. 

1898.  Obolus  {Lingulobolus)  ajjinis  Walcott,  Amer.  journ. 
sci.,  4th  ser.,  vol.  6,  p.  327. 

"  Ventral  valve  elongate,  conical  or  acutely  triangular. 
Apical  angle  about  45°  [55°?].  Front  margin  gently  convex 
in  the  middle,  rounded  at  the  angles  ;  sides  nearly  straight, 
uniformly  converging  from  the  anterior  angles  to  the  beak. 
Surface  with  very  fine  longitudinal  strias,  about  ten  in  the 
width  of  one  line. 

' '  This  species  is  founded  upon  the  single  specimen  of  a  ven- 
tral valve The  upper  two  thirds  is  pai"tly  worn  away  in 

the  middle,  leaving  only  the  outline  in  the  stone.  It  appears 
to  have  been,  when  perfect,  gently  convex,  the  rostral  portion 
near  the  beak,  semi-cylindrical.  Length  about  thirteen  lines, 
width  nine  lines. 

"  The  dorsal  valve  has  not  been  identified  "  (Billings). 

This  species  was  originally  described  from  the  Upper  Cam- 
brian (Lower  Ordovician?)  sandstone  of  Great  Bell  Island, 
Newfoundland.  Mr.  Walcott  has  identified  it  in  the  quartzite 
pebbles  of  the  Narragansett  basin. 


Genus  Sphaerobolus  Matthew. 

Sphaerobolus  spissus  (Billings). 

1872.    Lingulella   (?)   spissa  Billings,  Can.   nat.  and  geol., 

n.  s.,  vol.  6,  p.=  468,  figs.  5a— c. 
1874.    Lmgulella  (?)  sjnssa    Billings,    Pal.    foss.,    vol.    2, 

pt.  1,  p.  66,  figs.  36a-c. 


623 

1895.  Sphaerobohis  spissits  Matthew,  Trans,  roy.  soo.  Can- 
ada, 2d  ser.,  vol.  1,  pt.  4,  p.  263,  pi.  1,  figs.  5a-c. 

1898.  Obolus  (^Lingulobolus)  sjjissus  ^idaott,  Amer.  journ. 
sci.,  4th  ser.,  vol.  6,  p.  327. 

"  Shell  sub-pentagonal,  or  sub-ovate,  length  and  width 
about  equal,  sometimes  strongly  ventricose.  Dorsal  valve 
with  the  front  margin  straight  or  very  gently  convex  for  about 
two  thirds  of  the  width  in  the  middle  ;  anterior  angles  rounded  ; 
sides  straight  or  slightly  convex  and  sub-parallel  until  within 
one  third  or  one  fourth  the  length  from  the  beak,  then  con- 
verging to  the  apex,  where  they  form  an  obtuse  angle  which 
varies  from  100  to  about  110  deo-rees.  This  valve  is  opener- 
ally  very  convex,  sometimes  almost  hemispherical,  the  outline 
on  a  side  view  is  rather  abruptly  elevated  in  the  rostral  third, 
depressed  convex  for  a  short  space  in  the  middle,  and  then 
more  gently  descending  to  the  front  margin.  Most  of  the 
specimens  of  this  valve  are  eight  or  nine  lines  in  length,  and 
about  the  same  in  width. 

"  The  shell  which  is  supposed  to  be  the  ventral  valve  of  this 
species,  is  gently  convex,  with  usually  a  somewhat  flat  space 
extending  from  the  front  margin  upwards  towards  the  beak. 
The  apical  angle  appears  to  be  from  90  to  100  degrees.  Shell 
very  thick,  of  a  lamellar  structure,  dark  brown  or  nearly  black, 
and,  sometimes,  where  exfoliated,  of  an  ashy  gray  color.  Sur- 
face with  a  number  of  obscure  undulations  of  growth  and  with 
fine  longitudinal  stri*,  about  ten  in  the  width  of  one  line." 
(Billings). 

This  species  is  found  associated  with  the  preceding  in  the 
sandstones  of  Great  Bell  Island,  and  Mr.  Walcott  has  also 
identified  it  in  the  quartzite  pebbles  from  the  Carboniferous 
conglomerate  of  the  Narra^ansett  basin . 


624 
Genus  LiNGULELLA  Salter. 

LiNGULELLA  ROGERSI  Walcott. 

Plate  31,  fig.  4. 

1861.    Lingula    antiqua    Rogers,    Proc.     Boston    soc.    nat. 

hist.,  vol.  7,  pp.  389-391. 
1898.     Obolus   {Lhigiilella)   rogersi  Walcott,  Proc.   U.    S. 

nat.  mus.,  vol.  21,  pp.  413-415. 

"  General  form  elongate-ovate,  with  the  ventral  valve  sub- 
acuminate  and  the  dorsal  valve  ovate  in  outline.  There  is 
considerable  range  of  variation  in  the  outline  of  the  valves, 
owing  largely  to  distortion  apparently  produced  by  movement 
of  the  matrix.  The  convexity  of  the  valves  is  fairly  strong 
and  nearly  the  same  in  both,  except  that  the  dorsal  valve  curves 
more  abruptly  inward  toward  the  beak. 

' '  The  outer  surface  of  the  shell  usually  adheres  to  the 
matrix.  .  .  .  The  surface  is  formed  by  very  fine  concentric  lines 
and  strife  of  growth,  crossed  transversely  by  strong  undulating, 
slightly  lamellose  lines.  When  the  outer  layer  is  exfoliated 
the  inner  layers  are  marked  by  concentric  lines  of  growth  and 
fine  radiating  strias.  This  is  also  the  character  of  the  inner 
surface,  so  far  as  can  be  determined  from  the  specimens  in  the 
collection.  The  shell  is  rather  thick  and  built  up  of  a  thin 
outer  layer  and  several  inner  layers  or  lamellas,  the  latter  be- 
coming increasingly  numerous  toward  the  front.  The  largest 
dorsal  valve  in  the  collection  has  a  length  of  IS  mm.,  with  a 
width  of  11  mm.,  and  a  smaller  ventral  valve  with  a  length  of 
12  mm.  has  a  width  of  9  mm.  The  dimensions  of  most  of  the 
specimens  in  the  collection  average  less  than  those  here  given. 

"  The  area  of  the  ventral  valve  is  relatively  short  for  a 
species  of  this  type.  It  is  divided  midway,  as  seen  in  the  cast, 
by  a  strong  pedicle  furrow.      Owing  to  the  imperfection  of  the 


625 

material,  none  of  the  specimens  show  flexure  lines  or  stria?  of 
growth.  The  area  of  the  dorsal  valve  is  short,  and  extends 
but  a  short  distance  on  either  side  of  the  median  line.  The 
cast  of  the  interior  of  the  ventral  valve  is  very  much  like  that 
of  the  interior  of  O.  {L.)  cyane.  It  has  the  same  median 
ridge  and  the  transverse  trapezoidal  area,  which  includes  the 
central,  middle,  and  outside  lateral  muscle  scars  ;  the  main 
vascular  sinuses  are  indicated  by  slight  ridges.  The  cast  of 
the  interior  of  the  dorsal  valve  shows  a  narrow  median  septum, 
two  central  muscle  scars  of  average  size,  situated  a  short  dis- 
tance back  of  the  centre  of  the  shell,  and  two  small  anterior 
lateral  scars,  located  some  distance  in  advance  of  the  centre, 
which  gives  an  elongated  visceral  cavity  somewhat  like  that  of 
0.  {L-)  hayesi,  of  the  Middle  Cambrian,  and  0.  {L.)  lam- 
borni,  of  the  Upper  Cambrian  "  (Walcott). 

Horizon  and  Locality. —  This  species  was  discovered  by 
Walcott  in  the  quartzite  pebbles  of  Upper  Cambrian  or  Lower 
Ordovician  age  found  in  Rhode  Island  and  southern  Massa- 
chusetts. It  also  occurs  on  Great  Bell  Island,  Conception  Bay, 
Newfoundland. 

The  original  pebble  from  Fall  River  contains  a  number  of 
specimens  of  this  species.  They  are  in  the  collection  of  the 
Boston  Society  of  Natural  History  (cat.  no.,  2750).  The  slab 
from  the  Newport  conglomerate  referred  to  by  Professor  Rogers 
in  his  communication  of  1875  also  contains  this  species.  (Coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  20).  Type  in  coll.  U.  S.  nat. 
mus.,  no.  27,336. 

Gastropoda. 

Genus  Parmophorella  Matthew. 
Parmophorella  acadica  (Hartt) . 
.  Plate  31,  fig.  5. 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  40. 


626 

1868.    Discina  acadica  Hartt,   Dawson's   Acadian  Geology, 

2d  ed.,  p.  644,  fig.  222. 
1884.    Palceacmea  acadica  Whitfield,  Bidl.  Amer.  mus.  nat. 

hist.,  vol.  1,  p.  140. 
1884.    Palceacmea   ?  acadica  Walcott,  Bull.  10,  U.  S.  geol. 

surv.,  p.  19,  pi.  1,  fig.  6. 
1886.    8tenotheccc      {Parmo'phoTella')      acadica      Matthew, 

Trans,  roy.  soc.  Canada,  vol.  3,  sect.  4,  p.  59,  pi.  6, 

figs.  10,  10  a-c. 
"Shell  elliptical  in  outline;  sides  more  or  less  straight. 
Conical,  but  very  depressed.  Apex  apparently  central. 
Surface  marked  with  a  number  of  deep  concentric  irregular 
sharp  furrows,  not  always  continuous,  and  often  breaking  up 
into  smaller  grooves,  and  all  these  seem  at  times  to  be  im- 
pressed with  lighter  lines  running  nearly  parallel  with  them. 
Of  the  large  furrows,  from  nine  to  ten  can  usually  be  counted. 
The  whole  surface  of  the  shell  is  marked  with  a  great  number 
of  delicate  raised  lines  radiating  from  the  summit  to  the  cir- 
cumference, and  just  visible  to  the  naked  eye  "  (Hartt). 

This  species  was  originally  described  from  the  Saint  John 
formation  of  New  Brunswick.  A  much  crushed  and  broken 
specimen  from  the  Braintree  argillites  is  referred  to  this  species. 
The  sides  are  somewhat  more  curved  than  in  the  type  speci- 
men, and  the  outline  appears  consequently  somewhat  more  oval. 
The  apex  is  not  visible,  but  is  apparently  central  or  nearly  so. 
The  concentric  lines  are  sharp  and  irregular,  and  faint  radiat- 
ing lines  are  visible.  The  shell  is  compressed  and  flattened  on 
the  rock,  and  only  a  part  of  the  shell  substance  remains.  The 
margin  is  imperfect.  Length,  10  mm.  ;  width,  9  mm.  The 
proportions  of  the  type  specimen  are  the  same,  but  that  speci- 
men is  somewhat  larger. 

Horizon  and  Locality. — In  the  Middle  Cambrian  argil- 
lites of  Hay  ward  Creek,  Braintree,  Mass.,  collected  by  Mr.  J. 
B.  Woodvvorth,  1892  (student  pal.  coll.  Harvard  Univ., 
cat.  no.,  503). 


627 

Genus  ScENELLA  Billings. 

SCENELLA    RETICULATA  Billings. 

1872.    Scenella  reticulata,  Billings,  Can.  nat.   and  geol.,  2d 

ser.,  vol.  6,  p.  479. 
1886.    Scenella  reticulata  Walcott,  Bull.    30,  U.    S.   geol. 

surv.,  p.  125,  pi.  12,  figs.  6,  6  a. 
1888.    Scenella  reticulata    Shaler  and    Foerste,   Bull.    mus. 

comp.  zool.,  vol.  16,  p.  29,  pi.  1,  fig.  6. 
1890.    Scenella   reticulata   Walcott,    10th    ann.   rep.   U.    S. 

geol.  surv.,  p.  616,  pi.  73,  figs.  9,  9  a-d. 
This   species  was   found   by  Shaler  and  Foerste  in  the  red 
limestone    of  Lower    Cambrian   age  at  station  no.   2,  North 
Attleboro,  Mass. 


Scenella  ( ?)  sp. 

PI.  31,  figs.  6a-c. 

Shell  patelliform  elongate,  sub-ovoid,  moderately  elevated, 
the  greatest  elevation  a  little  posterior  to  the  middle.  Beak 
excentric,  about  half-way  between  the  centre  and  the  posterior 
margin.  The  longitudinal  contour  of  the  shell,  anterior  to  the 
beak,  is  a  uniform  curve  ;  posterior  to  the  beak  it  descends 
rather  more  abruptly.  Transverse  contour  sub-triangular, 
with  the  sides  gently  convex  and  diverging  at  an  angle  of  a 
little  over  ninety  degrees.  Summit  rounded.  Two  faintly 
impressed  lines  are  observed  in  certain  positions,  running  from 
the  beak  to  the  antero-lateral  margins.  Surface  marked  by 
concentric  striae.  The  specimen  is  scarcely  more  than  an 
internal  mould  with  the  shell  destroyed.  The  outline,  eleva- 
tion and  faint  concentric  striae  are  the  principal  features  pre- 


628 

served.  The  length  of  the  shell  is  3.4  mm.,  width  2.4  mm., 
and  the  maximum  elevation  about  1  mm.  The  shell  bears 
some  resemblance  to  specimens  of  Stenotheca  riigosa  figured 
by  Walcott,  but  it  is  probably  not  a  Stenotheca.  The  shell 
is  too  imperfect  to  admit  of  its  being  the  type  of  a  new  species, 
though  it  probably  is  undescribed. 

Horizon  and  Locality.  —  In  the  reddish  brown  Lower  Cam- 
brian limestone  bowlder  from  Pleasant  Beach,  Cohasset  (coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  11,971). 


Genus  Platycekas  Conrad. 

Platyceras  prim^vu3I  Billings. 
PI.  31,  figs.  7a,  b. 

1871.    Platyceras  2^rimcBvum^\\Y].r\gB,  Can.  nat.   and  geol., 

vol.  6,  p.  220. 
1886.    Platyceras  i:)rim<Bv%mi  Walcott,  Bull.  30,  U.  S.  geol. 

surv.,  p.  130,  pi.  12,  figs.  5,  5  a. 
1888.    Platyceras primcBvum  Shaler  and  Foerste,  Bull.  mus. 

comp.  zool.,  vol.   16,  p.  30,  pis.   1  and  2,  figs.  10 

a— c. 
1890.    Platyceras  lyrimcsvuni  Walcott,  10th  ann.  rep.  U.  S. 

geol.  surv.,  p.  618,  pi.  74,  figs.  11,  11  a. 
"  Shell  minute,  consisting  of  about  two  whorls,  which  as 
seen  from  above  are  ventricose,  but  most  narrowly  rounded  at 
the  suture ;  the  inner  whorl  scarcely  elevated  above  the  outer. 
The  under  side  is  not  seen  in  the  specimen.  Diameter,  meas- 
ured from  the  outer  lip  across  to  the  opposite  side,  one  line  ; 
width  of  last  whorl  at  the  aperture,  about  one  third  of  a  line  " 
(Billings). 

Walcott  states  that  a  specimen  from  Troy,  N.   Y.,  shows 


629 

that  the  left  side  is  more  ventricose,  and  that  tlie  surface  is 
marked  with  ' '  fine  stria?  and  lines  of  growth  that  arch  back- 
ward over  the  dorsum,  indicating  a  deep  dorsal  sinuosity  in  the 
peristome ;  a  second  series  of  fine  strife  cross  the  stria?  of 
growth  and  form  a  fine  reticulated  surface." 

A  specimen  from  Cohasset  answers  to  the  following  descrip- 
tion :  Shell  minute,  consisting  originally  of  perhaps  two  whorls, 
but  preserving  less  than  one  and  one  half  volutions,  the  central 
portion  being  destroyed.  Whorls  regularly  incurved,  the  coil- 
ing not  in  a  perfect  plane.  Earlier  whorls  laterally  compressed 
and  sub-angular  along  the  outer  margin.  Last  whorl  rapidly 
widening  toward  the  aperture,  increasing  more  on  the  right  ven- 
tral (inner)  side,  thus  giving  the  outline  when  looked  at  from 
above  (dorsally),  an  oblique  unsymmetrical  appearance. 

The  shell  is  exfoliated,  only  the  internal  mould  remainino-. 
This  shows  faint,  encircling  striae  which  arch  over  the  outer 
margin,  without  apparently  making  any  re-entrant  as  in  the 
specimens  observed  by  Walcott.  This  means  either  that  no 
such  sinuosity  existed  in  this  specimen,  or  else  that  it  is  not 
indicated  in  the  internal  mould.  Occasionally  somewhat 
thicker,  though  still  faint,  lines  encircle  the  shell.  The  char- 
acteristic features  are  :  the  regular  coil,  the  sub-angular  dorsum, 
and  the  irregular  expansion  of  the  younger  portion  of  the  shell. 
Greatest  diameter  of  shell,  1.6  mm.  ;  dorso-ventral  (antero- 
posterior) diameter  of  whorl  at  aperture,  8  mm.  ;  greatest 
lateral  diameter,  9  mm. 

This  species  was  described  by  Shaler  and  Foerste  from  North 
Attleboro.  In  their  specimens  the  second  whorl  enlarges 
rapidly,  and  all  are  evenly  rounded  throughout.  "  Seen  from 
below,  only  the  last  whorl  is  visible,  and  the  whorls  have  a 
somewhat  spiral  form  ;  the  ascent  of  the  spire  increases  rapidly 
towards  the  aperture,  at  that  point  partly  overlapping  the  first 
part  of  the  whorl."  In  this  latter  respect  the  North  Attleboro 
and  Cohasset  specimens  agree,  though  from  the  desci-iption, 
the  former  appear  somewhat  more  rounded. 


630 

Horizon  and  Loccdity. —  Lower  Cambrian,  station  2,  North 
Attleboro,  a  dozen  specimens  (Shaler  and  Foerste).  Lower 
Cambrian  limestone  bowlder,  Sandy  Cove,  Cohasset,  1  speci- 
men (coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11,960). 


Platyceras  deflectum  sp.  nov. 

PL  31,  figs.  8  a-b. 

Compare  Stenotheca  ( ?)  rugosu  Walcott,  Bull.  30,  U.  S. 
geol.  surv.,  pi.  12,  figs.  1  d-e,  1886,  and  10th  ann. 
rep.  U.  S.  Geol.  surv.,  pi.  74,  figs.  1  h,  i,  1890. 

Shell  minute,  oblique,  with  a  large  aperture.  Beak  pointed, 
curved  over  the  posterior  margin,  and  slightly  overhanging. 
Dorsum  rounded,  sloping  ofi^  to  the  sides,  abruptly  near  the 
beak,  and  more  gently  near  the  front.  Antero-posterior  con- 
tour an  unequal  curve,  most  convex  near  the  beak.  Transverse 
eontour  similarly  irregular,  most  convex  on  the  right  side. 
Aperture  obliquely  sub-ovate.  Of  the  surface  markings  only 
fine,  rather  distant  encircling  striae  remain,  with  extremely 
faint  indications  of  radiating  striae. 

The  shell  widens  rapidly  from  the  beak,  as  a  result  of  which 
the  dorsum  near  the  anterior  margin  becomes  much  flatter  than 
nearer  the  beak. 

The  specimen  from  Troy,  New  York,  figured  by  Walcott 
as  the  young  of  Stenotheca  rugosa,  — 3  mm.  long,  and  with 
overhanging  beak,  —  seems  distinct  from  that  species  and 
related  to  the  present  one.  The  curvature  of  the  beak  is  too 
great  for  a  shell  of  that  size,  to  correspond  to  that  of  the  earlier 
stages  of  growth  of  the  normal  form  of  S.  rugosa.^ 

iDr.  G.  F.  Matthew  has  recently  described  Platyceras  transversum  from  the 
Etcheminian  of  Smith  Sound,  Newfoundland,  which  he  considers  related  specitically 
to  the  shell  fi};ured  by  Walcott  as  the  young  of  Stenotheca  rugosa  from  Troy. 
(Trans,  roy.  soc.  Canada,  vol.  5,  sect.  4,  p.  101,  pi.  5,  figs.  4  a,  b,  1899.) 


631 

The  dimensions  of  the  specimen  from  the  Cohasset  bowlder 
are  :  length,  1.5  mm.  ;  greatest  width,  1.1  mm.  The  beak  over- 
hangs about  as  much  as  in  the  Troy  specimen.  It  is  not  im- 
probable that  this  specimen  represents  a  young  individual. 

Horizon  and  Locality.  In  the  bowlder  of  red  Lower  Cam- 
brian limestone  from  Sandy  Cove,  Cohasset,  1  specimen  (coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  11,965). 


Genus  Watsonella  gen.  nov. 

This  name  is  proposed  for  a  small  shell  which  appears  to 
represent  the  group  of  Heteropoda  in  Lower  Cambrian  strata 
of  this  region.  The  material  on  which  this  genus  is  based,  is 
in  the  form  of  moulds  in  a  limestone  matrix,  and  some  of 
the  features  are  obscure.  The  following  diagnostic  features 
may  be  provisionally  assigned  to  this  genus. 

Shell  small,  laterally  flattened  and  nearly,  if  not  quite,  bi- 
laterally symmetrical.  Beak  incurved.  Dorsal  region  flat- 
tened with  apparently  a  sinuosity  in  the  apertural  margin. 
The  flattening  of  the  dorsum  produces  a  distinct  angulated 
ridge  on  each  side,  which  is  crossed  by  the  lines  of  growth. 
Aperture  elongate,  ovate,  the  anterior  end  broadly  rounded  or 
truncate,  the  posterior  acute.  Type  Watsonella  crosbyi  sp. 
nov. 

This  shell,  as  represented  by  the  specimens  now  known,  has, 
when  partly  embedded  in  the  matrix,  veiy  much  the  appear- 
ance of  a  Haliotis-like  gastropod  shell.  This  is  emphasized 
by  the  fact  that  nearly  all  the  specimens  known  expose  the 
right  side,  the  angulation  between  this  side  and  the  dorsum 
resembling  strongly  the  umbonal  ridge  of  Haliotis,  but  with- 
out the  characteristic  openings  of  that  shell.  The  resemblance 
is  furthermore  brought  out  by  the  fact  that  in  some  of  the 
specimens  the  lines  of  growth  after  crossing  the  angulation 
apparently  turn    toward    the   umbo,   as   they  do   in    Haliotis. 


632 

For  a  long  time  the  shell  was  regarded  as  Haliotoid  in  form, 
until  the  discovery  of  another  specimen  showed  part  of  the  left 
side  exposed,  and  a  fortunate  fracture  showed  a  complete  cross- 
section.  An  attempt  at  sectioning  one  of  the  more  perfect 
specimens  proved  only  partly  successful,  as  the  left  side  was 
broken,  apparently  by  the  intrusion  of  hyolithid  shells. 


Watsonella  crosbyi.     sp.  nov. 

Plate  31,  figs.  9a-f.      . 

Shell  in  the  form  of  a  laterally  compressed  hood.  Beak 
incurved,  but  not  overhanging,  the  posterior  basal  portion  pro- 
jecting beyond  the  beak.  Lateral  surface  gently  arcuate, 
sometimes  quite  flat  in  the  middle  and  sloping  off  gently  toward 
either  end.  Margins  of  the  dorsum  strongly  angulated  in  the 
older  portion,  i.  e.,  the  more  recently  added  portions,  and  more 
uniformly  rounded  in  the  young  shell  or  the  earlier  formed  part 
of  an  adult.  In  some  of  the  specimens  a  median  carina  ap- 
pears on  the  centre  of  the  flattened  dorsum,  being  of  a  some- 
what similar  nature  to  the  keel  of  Bellerophon.  The  lines  of 
growth  are  gently  arcuate  forward,  marking  a  lateral  rounded 
projection  or  lip  on  each  lateral  face.  They  are  deflected  back- 
ward on  the  dorsum,  and  arch  over  the  carina,  producing  a 
pronounced  dorsal  sinus.  The  finer  lines  of  growth  frequently 
alternate  with  coarser  wrinkles.  Fine  radiating  striae  are 
indicated  on  some  specimens.  Aperture  broad  anteriorly, 
pointedly  acute    posteriorly. 

There  is  considerable  resemblance  between  this  shell  and  the 
recent  heteropod  Carinaria,  though,  from  the  direction  of  the 
lines  of  growth  on  our  shells,  there  appears  to  be  a  considerable 
sinuosity  in  tlie  dorsal  margin.  There  are  many  points  of  cor- 
respondence between  this  shell  and  Matthew's  Pelagiella 
atlanloides  (Trans,  roy.  soc.  Canada,  vol.  11,  sect.  4,  p.  94, 


633 


pi.  16,  fig.  8  a,  b,  1894;  Trans."  N.  Y.  acad.  sci.,  vol.  14, 
p.  131,  pi.  6,  figs.  6  a— c,  1895),  though  the  two  are  readily 
disthiguished.  The  strong  lateral  angles,  the  scarcely  inrolled 
beak,  and  the  surface  markings,  distinguish  the  present  species 
at  once. 

The  dimensions  of  the  specimens  illustrated  are  as  follows  : 


PL  31, 

Length  of 

Width  of 

Height  of 

Fig. 

aperture 

aperture 

shell 

9a. 

9b. 
9c. 

4.  mm. 

1.  mm. 

2.75  mm 

5.75  ram. 



4.  mm. 

9d. 

3.  mm. 

1.2  mm. 

— - 

Shaler  and  Foerste's  Fordilla  from  North  Attleboro  probably 
represents  a  fragmentary  impression  of  this  species. 

Ho7'izon  and  Locality.  —  In  the  red  limestone  bowlders 
of  Lower  Cambrian  age  from  Pleasant  Beach  (one  specimen) 
and  Sandy  Cove,  Cohasset,  (5  specimens)  (coll.  Boston  soc. 
nat.  hist.,  cat.  nos.,  11,951—11,954). 

The  o^eneric  name  is  o^iven  in  honor  of  Mr.  Thomas  A.  Wat- 
son  of  Weymouth,  the  specific  name  in  honor  of  Prof.  W.  O. 
Crosby.  In  thus  uniting  their  names  in  that  of  this  shell,  I 
wish  to  pay  a  well-earned  tribute  to  these  friends  who  have 
for  so  long  a  time  worked  hand  in  hand  on  the  solution  of  the 
intricate  problems  of  our  local  geology. 


Genus  Raphistoma  Hall. 


Raphistoma  attleborensis  Shaler  and  Foerste. 

PL  31,  figs.  10  a-c. 

1888.  Plexirotomaria  {Raphistoma)  attleborensis,  Shaler 
and  Foerste,  Bull.  mus.  comp.  zooL,  vol.  16,  p.  30, 
pi.  2,  fig.  11. 


634 

1890.  Pleurotomaria  {Raphistoma^  attlehorensis  "VValcott, 
10th  ann.  rep.  U.  S.  geol.  surv.,  p.  619,  pi.  74, 
figs.  12,  12  a. 
"  Shell  small,  flattened,  composed  of"  three  Avhorls.  The 
first  Avhorl  is  very  small ;  the  succeeding  ones  increase  rapidly 
in  size.  The  surface  in  general  slopes  at  a  low  angle  from  the 
apex  of  the  shell  to  the  sides.  In  the  last  whorl  of  the  cast, 
the  outside  margin  of  the  coil  thickens  a  little,  forming  an 
indistinct  border  alono-  the  maroin  of  the  shell,  which  becomes 
more  evident  as  it  approaches  the  orifice.  The  edge  of  the 
whorl  is  compressed  and  rather  narrowly  rounded.  The  surface 
of  the  shell  is  marked  by  fine,  transverse,  closely  set  striae, 
which  apparently  are  directed  backward  toward  the  earlier 
formed  parts  of  the  shell,  but  in  reality  indicate  various  stages 
of  o;rowth  of  the  shell.  The  internal  cast  does  not  show  these 
fine  stria?,  but  broader  and  more  widely  separated  elevations, 
having  the  same  direction  as  the  striae.  The  diameter  of  the 
shell  is  3.2  mm.  ;  the  height  is  a  little  less  than  1  mm.  Owing 
to  the  shape  of  the  shell  it  is  difficult  to  measure  its  height  ac- 
curately "  (Shaler  and  Foerste). 

The  type  specimen  came  from  station  1,  North  Attieboro, 
and  is  the  only  specimen  discovered  at  that  place.  Walcott 
gives  the  best  figure  of  it. 

A  specimen  from  Cohasset  (PI.,  31,  figs.  10  a,  b.)  has  a 
diameter  of  3.4  mm.  at  the  base,  it  being  the  largest  of  several 
specimens  found.  This  specimen  retains  a  portion  of  the 
aperture  entire,  showing  a  moderately  shallow,  broad  and 
rounded  notch  in  the  upper  portion  of  the  outer  lip.  This 
notch  has  all  the  appearance  of  being  original,  though  from  the 
fact  that  a  line  of  fracture  runs  through  the  rock  at  that  point, 
one  might  be  led  to  regard  it  as  of  accidental  origin.  The 
shell  is  broadly  and  deeply  umbilicated,  and  the  under  side  of 
the  whorl  is  flattened.  The  aperture  is  considerably  extended 
below  the  plane  of  the  body  whorl.  Another  specimen  from 
the  second  Cohasset  bowlder  (PI.  31,  fig.  10  c.)  shows  three 


635 

whorls,  all  of  which  are  very  depressed,  and  separated  by  deep 
sutures,  the  volutions  resting  loosely  upon  one  another.  Dis- 
tinct and  sti-ong  lines  of  growth  pass  outward  and  backward 
from  the  suture. 

Horizon  and  Locality. —  In  the  bowlders  of  reddish  brown 
Lower  Cambrian  limestone  from  Pleasant  Beach  and  Sandy 
Cove,  Cohasset ;  several  specimens  (coll.  Boston  soc.  nat. 
hist.,  cat.  nos.,  11,955,  11,957).  Also  at  station  1,  North 
Attleboro,  Mass. 


Genus  Steapaeollina  Billings. 

Steapaeollina  eemota  Billings. 
PI.  31,  figs.  11  a,  b. 

1872.    Straparollina  remota  Bill.,  Can.  nat.  and  geol.,  n.  s., 

vol.  6,  p.  471,  fig.  7. 
1874.    Btraparollina  remota  ^S^.^YdX.  foss.,  vol.    2,   pt.    1, 

p.  70,  fig.  38. 
1890.    Btra'parollina  remota  Walcott,  10th  ann.   rep.  U.  S. 

geol.  surv.,  p.  619,  pi.  74,  figs.  13,  13  a. 
1892.  Straparollina  remota  Y^ nicoii,  Proc.  biol.  soc.  Wash- 
ington, vol.  7,  p.  155. 
"  Shell  small  hemispherical,  spire  depressed  and  rounded  in 
outline,  height  2  to  3  lines  [4.5  to  6.75  mm.] ,  width  3  to  4  lines 
[6.75  to  9  mm.],  whorls  about  three;  suture  deep.  The 
whorls  are  nearly  uniformly  rounded,  more  narrowly  so  on  the 
upper  side  close  to  the  suture,  and  also  on  the  basal  side.  On 
a  side  view  the  minute  apical  whorl  is  scarcely  at  all  seen  ;  the 
next  below  it  is  elevated  about  half  its  own  diameter  above  the 
body  whorl.  In  a  specimen  4  lines  [9  mm.]  wide,  the  width 
of  the  aperture  is  about  IJ  lines  [3.4  mm.],  as  nearly  as  can 


636 

be  determined  from  an  individual  partly  buried  in  the  matrix. 
Surface  nearly  smooth  "  (Billings). 

Our  largest  specimen  is  not  oyer  4  mm.  wide,  the  majority 
being  very  much  smaller.  Lines  of  growth  are  indistinctly 
visible,  and  there  appear  to  be  occasional  heavier  concentric 
lines.  Revolving  striae  also  appear  to  be  present,  as  noted  by 
Walcott.  Mr.  Walcott  has  noticed  the  similarity  between  this 
species  and  Raj^histoma  attleborensis  S.  and  F.  from  North 
Attleboro.  The  two  are  associated  in  the  bowlders  of  red 
limestone  found  in  the  Boston  Basin,  but  it  is  not  likely  that 
they  will  prove  identical.  The  whorls  o^  SU'cq^arollina  reinota 
are  rounded,  with  usually  a  slight  angulation  on  the  lower, 
outer  margin  of  the  whorl.  The  base  of  the  spire  of  some  of 
our  specimens  is,  however,  as  flat  as  that  of  R.  attleborensis. 
The  whorls  of  this  latter  species  are  much  compressed,  and 
have  a  strongly  angulated  outer  margin,  and  a  comparatively 
flat  under  side.  In  some  specimens  of  jS.  remota  the  spire  is 
much  more  elevated  than  usual,  the  apical  angle  becoming  less 
than  a  right  angle. 

Horizon  and  Locality.  — In  the  red  limestone  bowlders  of 
Lower  Cambrian  age  from  Pleasant  Beach  and  Sandy  Cove, 
Cohasset,  and  from  Bass  Point,  Nahant.  Over  fifty  specimens 
were  obtained,  mostly  from  the  first  bowlder  (coll.  Boston  soc. 
nat.  hist.,  cat.  nos.,  11,958,    11,959). 


Genus  Stenotheca  Salter. 

Note  :  In  1890  Dr.  G.  F.  Matthew  referred  the  laterally 
compressed  species  of  Stenotheca  to  the  phyllopod  crustaceans, 
regai'ding  them  as  univalvular  folded  carapaces,  without,suture 
along  the  dorsal  margin.  (Trans,  roy.  soc.  Canada,  vol.  8, 
sect.  4,  pp.  132,  133.)  Stenotheca  ruc/osa  (Hall)  was,  how- 
ever, retained  among  the  Gastropoda  and  referred  to  the  genus 
Platyceras  (  ?).      Dr.  Matthew's  determination  is  borne  out  by 


637 

the  structure  of  some  of  the  specimens  from  this  region.  Never- 
theless, the  genus  is  for  the  present  retained  among  the 
Gastropoda,  until  more  is  known  of  these  organisms.  Their 
generic  distinction  from  Hall's  Metoptoma  ( ?)  rugosa  is 
unquestionable. 


Stenotheca  abrupta  Shaler  and  Foerste. 

Plate  31,  figs.  12  a-c. 

1888.    Stenotheca   rugosa  var.    abrupta   S.   and  F.,     Bull, 
mus.  com.  zooL,  vol.  16,  p.  29,  pi.  1,  figs.  9  a,  b. 

1890.    Stenotheca  rugosa  var  ahrupta   Walcott,   10th  ann. 

rep.  U.  S.  geol.  surv.,  p.  617,  pi.  74,  figs.  6,  6  a. 
1890.    Stenotheca  rugosa  Sears,  Bull.   Essex  inst.,  vol.  22, 
p.  32. 

"Shell  small,  decreasing  rapidly  in  size  towards  the  apex. 
The  apex  never  strongly  incurved  ;  usually  within  a  moderate 
distance  of  a  line  vertical  to  the  base  at  its  centre.  Shell  never 
more  than  slightly  curved.  Shell  corrugated  into  four  or  five 
rounded  ridges,  passing  transversely  around  the  shell,  very 
strong  below,  decreasing  rapidly  in  size  towards  the  apex. 
Greatest  diameter  of  the  aperture,  4  mm.  :  height  of  the 
shell,  the  same.  Longitudinal  stride  very  fine  and  closely 
set"  (Shaler  and  Foerste). 

Our  specimens  from  the  Nahant  bowlder  resemble  the  larger 
of  the  two  figures  given  by  Shaler  and  Foerste,  but  they  are 
much  smaller  than  the  size  indicated  for  that  specimen.  They 
are  also  more  cornucopia-shaped,  being  longer  and  more  slen- 
der. In  the  specimen  figured,  the  first  corrugation  is  partly 
broken  away.  The  diameter  at  the  base  is  about  2.5  mm., 
the  height  being  about  3  mm.  Nine  corrugations  are  dis- 
tinctly   recognizable,    decreasing    in    size    upward.     A    tenth 


638 

appears  to  have  been  present,  but  the  apex  is  imperfectly  pre- 
served. A  part  of  the  shell  of  this  specimen  adheres  to  the 
rock  matrix  of  the  counterpart  (Plate  31,  fig.  12  b).  It  is 
thick,  and  a  rounded  thickened  edge  marks  the  concave  ven- 
tral margin  of  the  shell,  which  is  not  broken  by  the  corruga- 
tions. This  thickened  edge  has  all  the  appearance  of  a  free 
margin,  indicating  that  the  shell  was  split  along  the  concave 
ventral  border.  This  feature  favors  Matthew's  view  of  the 
crustacean  character  of  these  remains.  Faint  striae  parallel 
to  the  thickened  margin  are  visible  on  the  interior  of  the 
shell,  and  these  are  also  visible  on  some  of  the  internal  moulds 
of  this  species. 

Horizon  and  Locality.  —  In  the  reddish  brown  limestone 
bowlder  of  Lower  Cambrian  age  from  Bass  Point,  Nahant,  4 
specimens  (coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11,962). 
Also  at  stations  2  and  3,  North  Attleboro,  Mass.  (S.  and  F.). 
A  section  of  the  white  limestone  at  Nahant  shows  an  outline 
apparently  of  a  specimen  of  this  species  (PI.  31,  fig.  12  c,  coll. 
Peabody  acad.  sci.,  Salem,  Mass.). 


Stenotheca  curvirostra  Shaler  and  Foerste. 

Plate  31,  fig.  13. 

1888.    Stenotheca  ctirvirostra  Shaler  and  Foerste,  Bull.  mus. 

comp.  zool.,  vol.  16,  p.  30,  pi.  1,  fig.  8. 
1890.    Stenotheca  curvirostra  Walcott,  10th  ann.  rep.  U.  S. 

geol.  surv.,  p.  618,  pi.  74,  fig.  10. 
"  Shell  small,  rather  elongate  ;  the  lower  part  gently  curved, 
the  curvature  more  marked,  especially  at  the  beak  ;  the  beak 
always  considerably  elevated  above  the  aperture  of  the  shell. 
The  transverse  ribs  are  narrow  and  sharp  ;  from  ten  to  eighteen 
are  found  on   a   single   shell ;    the  interspaces    are   broad  and 


639 

flat.  The  longitudinal  stride  are  fine  and  closely  set.  Diame- 
ter of  the  aperture  of  the  shell  in  the  largest  specimen  found,  4 
mm.  ;  height  of  the  shell  5  mm."  (Shaler  and  Foerste). 

Our  specimens  are  considerably  smaller  than  the  type  speci- 
men. The  figure  given  by  Messrs.  Shaler  and  Foerste  shows 
very  narrow,  band-like,  concentric  ridges,  separated  by  wide 
interspaces.  This  appearance  is  observed  in  some  of  our  spe- 
mens,  when  viewed  with  the  light  at  a  certain  angle.  The 
figure  given  by  Walcott  on  plate  74  of  the  10th  Annual  Report 
of  the  U.  S.  Geological  Survey  represents  better  the  normal 
characteristics  as  seen  in  our  specimens.  Occasionally  the  cor- 
rugations become  somewhat  broader  and  more  rounded  in 
our  specimens.  The  longitudinal  strife  are  faintly  visible  in 
the  majority  of  specimens,  and  quite  strongly  so  in  one.  As  in 
the  preceding  species,  the  corrugations  continue  over  the  dor- 
sum and  die  out  toward  the  ventral  side.  The  largest  speci- 
men found  measures  1.5  mm.  across  the  aperture,  with  a 
height  of  about  2.5  mm. 

Horizon  and  Locality.  — In  the  reddish  brown  limestone 

bowlder   of  Lower  Cambrian   age,  Sandy  Cove,  Cohasset,   4 

specimens    (coll.    Boston  soc.   nat.   hist.,   cat.  no.,    11,964). 

■Also  at  station  2,  North  Attleboro,  Mass.,  5  specimens  (Shaler 

and  Foerste) . 


Stenotheca  pauper  Billings. 

Plate  31,  fig.  14. 

1872.    Stenotheca  2Jaiiper^\\\mgB ,  Can.  nat.  andgeol.,  n.  s., 

vol.  6,  p.  479. 
1886.    Stenotheca  jjaujDe?'   Walcott,  Bull.  30,   U.    S.    geol. 

surv.,  p.  129. 
1888.    Stenotheca  rugosa  var.  pauper  Shaler  and   Foerste, 

Bull.  mus.  comp.  zool.,  vol.  16,  p.  29,  pi.  1,  fig.  7, 


640 

1890.  Stenotheca  rugosa  var  paii'pera  Walcott,  10th  ann. 
rep.  U.  S.  geol.  snrv.,  pi.  74,  fig.  7.  (Not  Steno- 
theca rugosa  y2iX pawpera  Waloott,  ibid.,  fig.  3). 
1899.  Parmoj)horella  (?)  paxipera  Matthew,  Bull.  nat. 
hist,  soc.  N.  B.,  vol.  18,  p.  190,  and  Trans,  roy.  soc. 
Canada,  vol.   5,  sect.  4,  p.  100. 

"  Shell  small,  conical,  with  the  apex  incurved,  laterally  com- 
pressed. Aperture  ovate,  elongated  in  the  plane  in  which  the 
curvature  of  the  apex  occurs.  Surface  with  four  or  five  small 
engirdling  convex  ridges.  Length  of  aperture  about  IJ lines  ; 
width  about  1  line  ;  height  of  shell  about  1  line"  (Billings). 

The  type  specimen  came  from  the  red  limestone  of  Brigus, 
Conception  Bay,  Newfoundland. 

Shaler  and  Foerste's  description  is  as  follows:  "Shell 
small,  decreasing  rapidly  in  size  towards  the  apex.  Apex 
strongly  incurved.  Shell  corrugated  into  from  five  to  eight 
rounded  ridges,  passing  transversely  around  the  shell.  These 
are  crossed  by  very  fine,  closely  set  striae,  passing  longitu- 
dinally along  the  shell.  In  the  casts  the  transverse  ridges  are 
less  distinct,  and  the  longitudinal  stride  are  not  seen  at  all. 
When  not  crushed,  the  apex  is  broad  oval  in  outline."  Bil- 
lings' description  was  unaccompanied  by  illustrations,  and 
hence  it  is  difficult  to  know,  without  resort  to  the  type,  whether 
his  form  was  specifically  identical  with  that  described  by  Shaler 
and  Foerste  from  North  Attleboro.  Walcott  mentions  having 
examined  the  type  specimen  in  the  collection  of  the  Geological 
Survey  of  Canada,  but  he  gives  no  description  beyond  speak- 
ing of  it  as  "  a  coarsely  ribbed  variety  of  S.  rugosa,  such  as 
occurs  both  at  Troy,  New  York,  and  Bic  Harbor,  Canada " 
(Bull.  30,  p.  129).  On  plate  74  of  the  10th  Annual  Keport 
of  the  U.  S.  Geological  Survey  he  gives,  in  figure  3,  a  view 
of  a  specimen  from  Manuel's  Brook,  Conception  Bay,  New- 
foundland, which  he  refers  to  8.  rugosa  var.  pawpera.  This 
is  clearly  a  distinct  species  from  the  North  Attleboro  form.  If 
it  corresponds  to  tlie  type  of  Billings,  the  Massachusetts  speci- 


641 

mens  must  be  referred  to  another  species.  Billings'  descrip- 
tion, however,  coi'responds  better  with  the  North  Attleboro 
specimens  than  with  that  figured  by  Walcott  from  New- 
foundland. 

This  species  is  certainly  not  congeneric  with  Pavmojjhorella 
acaclica,  and  hence  Matthew's  reference  cannot  be  accepted. 

Shaler  and  Foerste's  largest  specimen  had  an  apertural 
diameter  of  2.5  mm.  and  a  similar  height.  A  specimen  from 
the  second  Cohasset  bowlder  (PL  31,  fig,  14)  has  an  antero- 
posterior diameter  of  1.8  mm.,  and  a  height  of  1.5  mm.  The 
beak  is  somewhat  more  incurved  than  that  of  the  North  Attle- 
boro specimen  figured,  but  the  shell  otherwise  corresponds 
closely  to  that  form. 

Horizon  and  Locality.  — In  the  bowlder  of  reddish  brown 
Lower  Cambrian  limestone  from  Sandy  Cove,  Cohasset  (coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  11,963).  Also  at  North 
Attleboro,  Mass.  (Shaler  and  Foerste). 


Stenotheca  LEVIS  Walcott. 

PI.  31,  fig.  15. 

1890.    Stenotheca    rugosa,    var.    levis    Walcott,    10th    ann. 
rep.  U.  S.  geol.  surv.,  p.  617,  pi.  74,  figs.  5,  5  a. 

Shell  elongate,  rather  stout,  laterally  compressed,  higher 
than  long,  with  the  beak  overhanging  posterioi'ly,  but  not 
incurved.  Apex  rounded.  Surface  marked  by  about  three 
strong  corrugations,  which  are  most  pronounced  on  the  ventral 
side,  and  do  not  continue  over  the  dorsum. 

The  original  specimen  figured  by  Walcott  is  from  Concep- 
tion Bay,  Newfoundland.  The  corrugations  are  narrow  and 
far  apart.  Our  shell  is  much  smaller,  being  1.6  mm.  high,  by 
1    mm.    across    the     base.      The    corrugations    are    stronger. 

OCCAS.  PAPERS  B.  S.  N.  H.     IV.     41. 


642 

Aside    from    these    variations,   our  shell  agrees  well  with  the 
Newfoundland  specimen. 

Horizon  and  Locality.  — In  the  reddish  brown  Lower 
Cambrian  limestone  bowlder  from  Pleasant  Beach,  Cohasset, 
one  specimen  (coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11,961). 


Hyolithidae. 

Genus  Hyolithes  Eichwald. 
Hyolithes  shaleri  Walcott. 

Plate  32,  figs.  1  a,  b.     Plate  35,  figs.  4,  4  a-c. 

1884.    Hyolithes  shaleri  Walcott,  Bull.  10,  U.  S.  geol.  surv., 
p.  44,  pi.  7,  figs.  4,  4  a-c. 

"  Form  an  elongate  triangular  pyramid,  slightly  arching 
towards  the  dorsal  side  and  expanding  regularly  from  the 
apex  towards  the  aperture.  Transverse  section  midway  of  the 
length,  semielliptical,  with  a  width  twice  as  great  as  the  height ; 
the  lateral  angles  acute.  Ventral  [dorsal  face]  gently  convex 
transversely,  curving  slightly  longitudinally.  Dorsal  [ventral] 
face  strongly  convex,  and  showing  a  slight  tendency  to  become 
angular  at  the  centre,  a  little  concave  longitudinally.  From 
the  direction  of  the  surface  lines  the  aperture  appears  to  have 
been  oblique.  Operculum  unknown.  Surface  marked  by  lines 
of  growth  that  on  the  dorsal  [ventral]  side  are  nearly  trans- 
verse, and  on  the  ventral  [dorsal]  side  arched  forward  ;  traces 
of  fine  longitudinal  lines  are  shown  in  the  matrix  of  the  ventral 
[dorsal]  side  (Walcott)." 

Tlie  dimensions  of  the  type  specimen  are  as  follows  :  length, 
about  90  mm.  ;  breadth  of  aperture,  23  mm.  ;  height  of  aper- 
ture, about  14  mm. 


643 

Only  a  few  specimens  of  this  species  are  known.  Some- 
times all  that  is  shown  is  a  section  presenting  two  convergent 
lines,  which,  if  the  section  is  right,  are  curved.  The  species 
is  readily  recognized  by  its  great  size  and  peculiarly  flattened 
character.  A  strong,  rounded,  anterior  lip  occurs  on  the  flat 
face. 

Horizon  and  Locality. —  In  the  Middle  Cambrian  argil- 
lites  at  Hayward  Creek,  Braintree,  Mass.  Type  specimen 
(Plate  35,  fig.  4)  in  the  collection  of  the  Museum  of  Compar- 
ative Zoology.  Two  fragmentai-y  specimens  (PL  32,  figs. 
1  a,  b)  in  the  collection  of:  Mr.  W.  W.  Dodge.  Other,  less 
perfect  specimens  in  the  collections  of  the  Boston  Society  of 
Natural  History  and  the  National  Museum  at  Washington. 


Hyolithes  princeps  Billings. 

Plate  32,  figs.  2  a-h. 

1872.    Hyolithes  jJ^inceps  BiWmgs,  Can.  nat.  andgeol.,  n.  s., 

vol.  6,  p.  216,  figs.  4  a,  b  (p.  213). 
1886.    Hyolithes  princej^s  Walcott,    Bull.   30,   U.   S.   geol. 

surv.,  p.  135,  pi.  13,  figs.  5,  5  a,  b. 

1889.  Hyolithes      incequilateralis    Foerste     (pars),     Proc. 

Boston  soc.  nat.  hist.,  vol.  24,  p.  2(32. 

1890.  Hyolithes  princeps    Walcott,    10th  ann.    rep.   U.  S. 

geol.  surv.,  p.  621,  pi.  76,  figs.  1,  1  a— e. 
1890.    Hyolithes  i^rinceps  Sears,   Bull.  Essex  inst.,  vol.  22, 
p.  32. 
(Not   Hyolithes  princeps    Shaler    and    Foerste,   Bull, 
mus.    comp.    zool.,    1888,    vol.   16,    p.   33,    pi.    2, 
fig.  25). 

Shell  elongate,  regularly  tapering,  often  attaining  the  length 
of  several  inches.      Apical  angle  13°  to  15°.      Dorsal  side  flat 


644 

or  gently  convex,  produced  anteriorly  into  a'  rounded  lip  whose 
greatest  length  is  equal  to  about  one  fourth  of  the  transverse 
diameter  of  the  tube  at  the  mouth. 

Lateral  margins  abruptly  rounded,  sometimes  angular. 
Ventral  face  varying  from  uniformly  arcuate  to  asymmetrical, 
one  side  being  more  abruptly  curved  than  the  other,  thus 
bringing  the  median  doi'sal  line  to  one  side  of  the  median 
ventral  line,  instead  of  just  below  it.  Surface  marked  by  fine 
engirdling  striae,  which  arch  forwai'd  on  the  flattened  dorsal 
face  in  conformity  with  the  curvature  of  the  anterior  margin 
of  the  lip. 

Most  of  our  specimens  are  small,  representing  either  young 
individuals  or  a  small  variety.  Some  of  our  specimens,  how- 
ever, equal  the  largest  Canadian  specimens.  The  following 
were  the  measurements  of  the  apical  angle  obtained  from  seven 
specimens  :  13°-,  13%  13.5°,  14°-,  15°-,  15°,  15°+.  Walcott 
gives  the  apical  angle  for  the  species  as  15°,  but  his  figures 
show  a  variation  from  15°  to  19°+.  The  inequilateral  form  is 
marked  in  some  of  the  specimens  ;  and  concomitantly,  the  lateral 
angle  of  the  dorsum  corresponding  to  the  more  steeply  curving 
side,  is  more  rounded,  while  the  opposite  one  becomes  angular 
and  strongly  acute.  Foerste  based  his  species  H.  incequilater- 
alis  on  this  feature,  his  types  being  subsequently  referred  by 
Walcott  to  H.  communis  var.  emmojisi  Ford  (^Orthotheca 
emmo7isi  (Ford)).  It  may  perhaps  seem  advisable  in  the 
future  to  restore  Foerste's  name  at  least  to  varietal  rank, 
recognizing  the  relation  of  the  variety  of  II.  princejys. 

There  appears  to  be  some  considerable  variation  in  the  dorso- 
ventral  diameter  of  the  tube.  Much  of  this  variation  in  the 
sections  as  well  as  variation  in  the  lateral  angles,  is  due  to  the 
direction  in  which  the  section  is  cut,  as  will  be  seen  from  the 
following  considerations  :  — 

1.  Sections  made  parallel  to  transverse  and  dorso-ventral 
axis,  and  normal  to  longitudinal  axis,  alone  give  true  cross- 
section. 


645 

2.  Sections  made  parallel  to  transverse  axis  and  oblique  to 
the  other  two  give  : 

a.  In  a  tube,  normally  of  circular  section  ( Orthotheca  cy- 
lindrica),  an  oval  or  ellipse  in  which  the  smaller  diameter  is 
constant  and  corresponds  to  the  transverse  axis  ;  the  longer  axis 
of  the  oval  will  be  determined  by  the  obliquity  of  the  section. 

b.  In  a  tube  with  normally  an  oval  section,  with  the  trans- 
verse axis  longer  than  the  dorso-ventral  (^FI.  imjjar),  an  oval 
approaching  a  circle,  which  is  reached  when  the  section  is  at 
such  an  angle  that  the  two  diameters  are  equal.  Passing  this, 
an  oval  of  increasing  size  is  formed  in  which  the  constant  trans- 
verse axis  becomes  the  shorter  diameter. 

c.  In  a  laterally  symmetrical  form,  but  with  flattened  dor- 
sum {H.  princeps)  the  vertical  diameter  of  the  arch  will  be 
increased,  the  horizontal  (transverse)  remaining  the  same,  the 
section  at  the  same  time  remaining  equilateral.  The  vertical 
diameter  can,  however,  never  be  decreased  below  the  true 
height. 

3.  Sections  cut  parallel  to  the  dorso-ventral  axis  and  oblique 
to  the  other  two  in  all  cases  increase  the  transverse  diameter. 
A  normally  circular  section  is  changed  to  an  oval,  a  normally 
oval  to  an  ellipse,  and  a  normally  equilateral,  but  dorso-ven- 
trally  unequal  section,  will  become  inequilateral  (from  the 
tapering  of  the  tube). 

4.  Sections  cut  obliquely  in  either  circular  or  oval  tubes, 
will  produce  oval  or  elliptical  sections,  and  in  equilateral  tubes, 
which  are  dorso-ventrally  unequal,  irregular  sections. 

5.  Finally  sections  cut  in  any  direction  in  inequilateral 
tubes,   will  always   be  irregular. 

It  is  important  to  bear  in  mind  these  facts  when  interpreting 
the  sections  seen  on  the  rock  surfaces,  for  in  most  cases  it  is 
not  readily  possible  to  determine  the  direction  of  the  section. 

There  is,  however,  every  probability  that  some  of  the  more 
convex  sections  represent  specimens  with  a  greater  dorso-ventral 
diameter.      This  is  especially  true  of  the  larger  sections,  which 


646 

represent  more  correctly  the  adult  characteristics  of  the  species. 
Some  of  the  larger  cross-sections  with  strongly  rounded  dorso- 
lateral angles  probably  represent  the  next  species. 

Horizon  and  Locality. — In  the  white  metamorphosed 
Lower  Cambrian  limestone  of  East  Point,  Nahant  (coll.  Pea- 
body  acad.  sci.,  Salem,  Mass.,  cat.  nos.,  698,  695,  708,  709, 
694,  700). 


Hyolithes  excellens  Billino;s. 

Plate  32,  figs.  3  a— c. 

1877.    Hyolithes    excellens    Billings.      Can.   nat.   and   geol., 
n.  s.,  vol.  6,  p.  471,  fig.  8. 

Shell  elongate,  tapering  rapidly  and  uniformly  to  a  point. 
Dorsal  surface  flat  or  very  gently  convex,  the  sides  curving 
downward  so  as  to  brino^  the  o;reatest  width  of  the  shell  about 
one  third  of  the  dorso-ventral  diameter  below  the  flat  dorsum. 
Ventral  curve  a  uniform  or  slightly  distorted  arch.  Apical 
angle  between  27°  and  28°  in  our  specimens.  Where  the 
transverse  diameter  is  11  mm.,  the  dorso-ventral  diameter  is 
7  mm.  Our  largest  section  (probably  not  a  normal  one) 
referable  to  this  species  (Plate  32,  fig.  3  c)  has  a  transverse 
diameter  of  19  mm.,  the  dorso-ventral  being  indeterminable. 
Anterior  lip  and  surface  striae  not  preserved. 

Billings'  type  specimen  tapers  only  at  an  angle  of  22^,  and 
the  diameters  of  the  sections  are  as  17  to  12.  It  may  be  con- 
sidered that  these  differences  of  section  and  angle  are  sufficient 
to  warrant  placing  our  specimens  in  another  species.  In 
apical  angle,  our  specimens  correspond  well  with  H.  cliscors 
Barrande  (Syst.  sil.  Boh.,  vol.  3,  pis.  13  and  16),  but  in  cross- 
section  and  surface  characters,  they  differ  widely  from  that 
species. 


647 

Horizon  and  Locality.  —  In  the  white  metamorphic 
limestone  of  East  Point,  Nahant  (coll.  Peabody  acad.  sci., 
Salem,  Mass.,  cat.  nos.,  708  a,  708  b). 


Hyolithes  quadricostatus  Shaler  and  Foerste. 

1888.    Hyolithes  quadricostatus  S.  andF.,  Bull.  mus.  comp. 

zooL,  vol.  16,  p.  31,  pi.  2,  fig.  15. 
1890.    Hyolithes    quadricostatus    Walcott,    10th    ann.    rep. 

U.  S.  geol.  surv.,  p.  621,  pi.  78,  figs.  1,  1  a,  b. 

This  species  was  obtained  by  Shaler  and  Foerste  in  the 
Lower  Cambrian  limestone  at  station  2,  North  Attleboro, 
Mass. 


Hyolithes  americanus  Billino-s. 

Plate  32,  figs.  4  a,  b. 

1847.    Theca    (?)    triangidaris    Hall,    Pal.   N.   Y.,    vol.    1, 
p.  313,  pi.  87,  figs.  1  a-d. 

1871.  Theca  tr ianfjidarns  Yord.,  Amer.  journ.  sci.,  3d  ser., 

vol.  2,  p.  33. 

1872.  Hyolithes  afnericanus  Billings,  Can.   nat.   and  geol., 

2d  ser.,  vol.  6,  p.  215,  figs.  2  a,  b  (p.  213)  ;    Amer. 

journ.  sci.,  3d  ser.,  vol.  3,  p.  353,  figs.  2  a,  b. 
1886.    Hyolithes  americanus  Walcott,  Bull.   30,  U.  S.  geol. 

surv.,  p.  132,  pi.  13,  figs.  6,  6  a— f. 
1888.    Hyolithes  a')nericanus  Shaler  and  Foerste,  Bull.  mus. 

comp.  zool.,  vol.  16,  p.  32,  pi.  2,  fig.  16. 
1890.    Hyolithes  atnericanus  Walcott,  10th  ann.   rep.   U.  S. 

geol.  surv.,  p.  620,  pi.  75,  figs.  2,  2  a-f. 

"  Length  from  twelve  to  eighteen  lines,  tapering  at  the  rate 
of  about  four  lines  to  the  inch.      Section  triano^ular,  the  three 


648 

sides  flat,  slightly  convex  or  slightly  concave,  the  dorsal  [ven- 
tral] and  lateral  edges  either  quite  sharp  or  acutely  rounded. 
Lower  [upper]  lip  rounded,  projecting  about  two  lines  in  full- 
grown  individuals.  Surface  finely  striated,  the  striae  curving 
forwards  on  the  ventral  [dorsal]  sides,  and  passing  upwards 
on  the  sides  at  nearly  a  right  angle,  curve  slightly  backwards 
on  the  dorsum  [ventrum] .  In  a  specimen  eighteen  lines  in 
length,  the  width  of  the  aperture  is  about  six  lines  and  the 
depth  about  four,  the  proportion  being  slightly  variable  "  (Bil- 
lings). 

Shaler  and  Foerste  described  a  specimen  of  this  species  from 
station  2,  North  Attleboro.  The  apical  angle  of  that  specimen 
is  20°,  which  is  low  for  the  species,  the  angle  being  about 
24°  in  the  specimens  figured  by  Walcott,  while  an  unusually 
broad  specimen  figured  by  him  (10th  ann.  rep.,  pi.  75,  fig. 
21),  and  referred  to  this  species,  has  an  apical  angle  of  36°  +  . 
In  a  specimen  figured  by  Walcott  (ibid.,  p.  620)  the  apical 
angle  is,  according  to  the  figure,  a  trifle  less  than  21°.  This 
specimen  shows  a  central  longitudinal  ridge  on  the  dorsal 
surface,  a  feature  indicating  close  relation  to  H.  hillingsi  from 
Nevada.  The  North  Attleboro  specimen  has  a  length  of  11 
mm.,  and  does  not  show  the  surface  markings.  The  ventral 
angle  is  slightly  elevated,  forming  an  "  incipient  wing." 

Among  the  specimens  collected  by  Mr.  Sears  at  Nahant  is 
one  which  is  distinctly  referable  to  this  species,  though  showing 
some  marked  diflerences  from  the  normal  form.  The  most 
important  of  these  is  the  smaller  apical  angle,  which  in  our 
specimen  is  not  over  19°,  and  may  be  a  trifle  less.  The  ventral 
angle  is  obtusely  rounded,  the  lateral  faces  are  gently  convex, 
and  diverge  at  an  angle  of  approximately  76°.  The  section  is 
less  sharply  triangular  than  that  of  normal  specimens  of  this 
species,  but  it  is  much  too  angular,  and  the  sides  are  too  flat 
to  allow  its  being  referred  to  H.  princejis,  with  which  it  occurs. 
There  is  no  evidence  of  mechanical  deformation.  The  section 
is  much  like  that  of  II.   searsi,  differing,  liowever,  in  the  ab- 


G49 

sence  of  the  dorso-lateml  keels,  and  in  tlie  gently  convex  char- 
acter of  the  faces,  which  in  that  species  are  flat.  The  dorsum 
of  the  Nahant  specimen  is  very  gently  convex,  the  convexity 
being  similar  to  that  of  the  sides.  No  lines  of  growth  are 
visible.     The  leng'th  of  the  fragment  is  about  22  mm. 

Horizon  and  Locality.  —  In  the  white  limestone  of  East 
Point,  Nahant,  one  specimen  (coll.  Peabody  acad.  sci.,  cat. 
no.,  708).  Also  at  station  2,  North  Attleboro  (Shaler  and 
Foerste) . 


Hyolithes  billingsi  Walcott  (  ?) . 

1861.    Salterella  ohtusa  Billings,  geol.   of  Vermont,  vol.   2, 

p.  955.     Idem,  1865,  Pal.  foss.,  vol.  1,  p.  18. 
1874.    Hyolithes  primordialis  (?)    White,  Geogr.  and  geol. 

expl.    and   surv.    west   of  100th   mer.,   prelim,   rep. 

invert,   foss.,  p.  6,   and  vol.  4,  pt.  1,  p.  37,   pi.  1, 

figs.  5  a— e. 
1886.    Hyolithes    billingsi  Walcott,  Bull.   30,   U.   S.   geol. 

surv.,  p.  134,  pi.  13,  figs.  1,  1  a— d. 
1888.    Hyolithes  billingsi  (  ?)  Shaler  and  Foerste,  Bull.  mus. 

comp.  zool.,  vol.  16,  p.  34,  pi.  2,  fig.  20. 
1890.    Hyolithes   billiyigsi   Walcott,    10th    ann.    rep.   U.   S. 

geol.  surv.,  p.  620,  pi.  75,  figs.  1,  1  a-e. 

A  number  of  small  specimens  from  station  1,  North  Attle- 
boro, are  referred  to  this  species  by  Shaler  and  Foerste. 


Hyolithes  seaesi  sp.  nov. 

PL  32,  figs.  5  a,  b. 
Known  at  present  only  from  cross-sections,  which  are  in  the 


660 

form  of  obtuse  isosceles  triano-les,  with  the  base  o-reater  than 
the  height,  and  the  baso-lateral  margins  produced  into  auricles, 
which  mark  the  cross-sections  of  dorso-lateral  keels.  Apex  of 
triangle  rounded. 

As  inferred  from  the  cross-sections,  and  a  partial  cast,  the 
form  of  the  shell  appears  to  be  elongate  and  gently  tapering, 
with  flat  sides,  and  abruptly  rounded  obtuse  ventrum,  and  flat 
but  depressed  dorsum,  bounded  on  both  sides  by  projecting, 
acute,  lateral  keels.  The  cross-sections  resemble  very  closely 
those  of  modern  species  of  Cleodora,  in  the  flat  sides,  laterally 
produced  margins  and  rounded  ventrum.  A  slight  central 
bulge  is  perceptible  in  the  dorsal  side  of  some  sections,  appar- 
ently corresponding  to  the  ridge  in  the  centre  of  the  dorsal  face 
of  Cleodora.  The  Cambrian  species  has,  however,  apparently 
not  the  pyramidal  form  of  Cleodora,  but  seems  to  taper  more 
gently. 

It  may  be  necessary  to  separate  this  species  generically  from 
Hyolithes. 

Named  in  honor  of  Mr.  John  H.  Sears,  the  indefatigable 
student  of  the  geology  of  Essex  County,  and  the  first  dis- 
coverer of  fossils  at  Nahant. 

Horizon  and  Locality.  —  In  the  white  crystalline  lime- 
stone of  Nahant.  The  first  specimen  of  this  species  was  ob- 
tained by  Mr.  Sears  from  Nahant  Head.  This  is  the  largest 
section  known,  having  a  transverse  diameter  of  9  mm.  and  a 
dorso-ventral  diameter  of  5.8  mm.  (pi.  32,  fig.  5  a).  Prof. 
Crosby  and  Mr.  Watson  subsequently  found  a  specimen  of 
weathered  limestone  containing  a  group  of  sections  (PI.  32, 
fig.  5  b)  at  an  outcrop  on  the  northern  side  of  the  Nahant 
headland.  Other  specimens  were  subsequently  found  by  Mr. 
Sears. 

Types  in  the  collection  of  the  Peabody  Academy  of  Sciences, 
cat.  no.,  710,  and  in  the  collection  of  the  Boston  Society  of 
Natural  History,  cat.  no.,  11,967. 


651 


Hyolithes  communis  Billings. 


PI.  32,  fig.  6. 

1872.    Hyolithes    coniinunis  Billings,    Can.    nat.    and   geol., 

n.  s.,  vol.  6,  p.  214,  figs.  1  a,  b.  (p.  213). 
1886.    Hyolithes  communis  Walcott,   Bull.  30,   IT.  S.  geol. 

surv.,  p.  136,  pi.  14,  figs  3,  3  a-e. 
1890.    Hyolithes  comm^unis  Walcott,    10th  ann.  rep.  U.  S. 

geol.  surv.,  p.  620,  pi.  77,  figs.  3,  3  a-g. 
1892.    Hyolithes  communis  Walcott,  Proc.  biol.  soc.  Wash., 

vol.  7,  p.  155  (partim?). 

"This  species  attains  a  length  of  about  eighteen  lines,  al- 
though the  majority  of  the  specimens  are  from  ten  to  fifteen 
lines  in  length.  The  ventral  [dorsal]  side  is  flat  or  only 
slightly  convex  for  about  two  thirds  the  width,  and  then 
rounded  up  to  the  sides.  The  latter  are  uniformly  convex. 
The  dorsum  [ventrum] ,  although  depressed  convex,  is  never 
distinctly  flattened,  as  is  the  ventral  [dorsal]  side.  The  lower 
lip  projects  forward  for  a  distance  equal  to  about  one  fourth  or 
one  third  the  depth  of  the  shell.  In  a  specimen  whose  width 
is  three  lines,  the  depth  is  two  lines  and  a  half"  (Billings). 

Walcott  states  that  specimens  from  Troy,  N.  Y.,  show  im- 
perforate septa  near  the  apex. 

The  identification  of  this  species  in  our  rocks  is  unsatisfactory. 
Several  cross-sections  in  the  white  limestone  at  Nahant  lead  to 
the  belief  that  some  of  the  slender  hyolithids  of  that  horizon 
belong  to  this  species.  The  apical  angle  of  this  species  is  given 
by  Walcott  as  13°  (Bull.  30,  p.  139),  but  the  specimens  from 
our  rocks  which  appear  to  be  referable  to  this  species  show  a 
smaller  apical  angle. 

Horizon  and  Locality.  —  In  the  white  limestone  of  East 
Point,  Nahant  (coll.  Peabody  acad.  sci.,  cat.  no.,  708  c). 


652 

Hyolithes  impar  Ford. 

PL  32,  figs.  7  a-c. 

1872.    Hyolithes  im'paT^  Ford,  Amer.  journ.   sci.,  3d  ser., 

vol.  3,  p.  419,  figs.  1,  2. 
1886.    Hyolithes  impar  Walcott,  Bull.  30,  U.  S.  geol.  surv., 

p.  139,  pi.  14,  figs.  1,  1  a-e. 
1890.    Hyolithes  hni^ar  Walcott,  10th  ann.  rep.  U.   S.  geol. 

surv.,  p.  621,  pi.  77,  figs.  1,  1   a— f. 
1890.    Hyolithes  impar  Sears,  Bull.  Essex  inst.,  vol.  22,  p. 

32. 

Shells  elongate,  tapering  to  an  acute  point.  Usual  length 
about  1^  inches.  Cross-section  generally  broadly  and  regularly 
oval,  though  some  specimens  described  by  Ford  show  a  dorsal 
flattening.  Apical  angle  about  10°  in  typical  specimens,  but 
in  one  of  the  Nahant  specimens  it  is  only  8°  (PI.  32,  fig.  7  a) 
and  in  others  it  is  still  less.  The  surface  ornamentation  is 
described  by  Ford  as  consisting  of  "fine  engirdling  lines,  which 
upon  the  ventral  [dorsal]  side  curve  gently  forward,  thence 
more  sharply  backward  upon  the  sides  until  they  reach  a  point 
at  about  the  middle  of  the  depth,  where  they  are  again  deflected, 
and  flow  across  the  dorsum  [ventrum]  in  uninterrupted  slightly 
forward-bending  curves.  There  are  also  prominent  subimbri- 
cating  lines  of  growth,  which  give  to  some  of  the  specimens  an 
exceedingly  rugose  aspect."  In  none  of  our  specimens  are  these 
surface  characters  shown. 

This  species  may  be  recognized  by  its  oval  cross-section,  and 
its  apical  angle  of  10°  or  less.  Walcott  has  shown  (Bull.  30, 
p.  139)  that  H.  impar  is  connected  by  intermediate  forms  with 
H.  communis.  The  relation  in  which  these  two  species  stand 
to  each  other  must  be  determined  by  the  study  of  the  young  of 
both.      It  may  then  be  found  that  the  one  has  in  its  adult  stage, 


653 

features  characteristic  of  the  earlier  stages  of  the  other,  and 
that  the  intermediate  types  represent  later  and  later  acquisitions 
of  the  more  specialized  characteristics. 

Horizon  and  Locality.  —  In  the  white  metamorphic  lime- 
stone at  East  Point,  Nahant  (coll.  Peabody  acad.  sci.,  cat. 
nos.,  698  a,  700,  702). 


HyOLITHES  (?)   HAYWAEDENSIS  Sp.  nOV. 

Plate  32,  fig.  11. 

Shell  thick,  consisting  of  at  least  two  layers,  the  outer  one 
thinner  than  the  inner  one.  Form,  a  slightly  bent,  tapering 
tube,  with  a  subcircular  cross -section.  Surface  markings  not 
preserved,  though  the  inner  layer  appears  to  have  had  a  reticu- 
lated surface.  This,  however,  may  be  a  feature  of  the  preser- 
vation. The  length  of  the  shell  was  30  ram.  or  more,  the 
lower  portion  being  broken  away.  The  diameter  near  the  upper 
end  is  5  mm.  Thickness  of  the  shell  about  .8  mm.,  of  which 
.5  mm.  or  more  is  made  up  by  the  inner  layer. 

This  shell  resembles  Hyolithes  daniayius ,  but  its  cross- 
section  is  more  nearly  circular  than  in  that  species.  It  may, 
however,  prove  closely  related  to,  though  probably  not  identi- 
cal with,  that  species. 

The  reference  to  Hyolithes  is  provisional. 

Horizon  and  Locality.  —  In  the  Middle  Cambrian  argil- 
lites  of  Hayward's  Quarry,  Braintree,  Mass.,  associated  with 
Paradoxides  harlani-,  etc.  (coll.  Boston  soc.  nat.  hist.,  cat. 
no.,  11,974. 

Genus  Orthotheca  Nov^k 

(Sitzungsber.  d.  kon.  bohm.    Gesellschaft  d.  Wissenschaften, 
1886,  p.  673.) 


654 

Oethotheca  cylindrica  sp.  nov. 
Plate  32,  figs.  8  a-f. 

1888.  Hyolithellus  micans  Shaler  and  Foerste  (partim), 
Bull.mus.  comp.  zool.,  vol.  16,  p.  34. 

1892.  Hyolithes  commu7iis  —  slender  var.,  Walcott,  Proc. 
biol.  soc.  Wash.,  vol.  7,  p.  155. 

Shell  small,  slender,  almost  cylindrical,  circular  or  subcir- 
cular  in  cross-section,  occasionally  somewhat  compressed. 
Apical  angle  low.  The  older  portion  of  the  tube  is  septate, 
the  septum  uniformly  convex  backward.  Surface  marked  by 
encircling  striae,  which  pass  around  the  shell  at  right  angles 
to  the  longitudinal  axis.      No  lip  is  present. 

This  species  justifies  its  reference  to  Orthotheca  by  the  uni- 
formly engirdling  striae,  which  have  not  been  observed  to 
bend  forward  in  any  of  the  specimens  examined.  The  striae 
are  moderately  fine,  with  frequently  coarser  annulations.  This 
character  of  the  striae  and  the  low  apical  angle  separate  it  from 
H.  communis,  with  which  it  was  formerly  identified.  The 
apical  angle  in  a  specimen  from  one  of  the  Cohasset  bowlders 
is  5°  (PI.  32,  fig.  8  c).  A  specimen  from  the  white  limestone 
of  Nahant  shows  a  tapering  of  1  mm.  in  30,  which  is  equiva- 
lent to  an  apical  angle  of  a  little  less  than  two  degrees.  The 
nearly  circular  cross-section  is  another  distinctive  feature. 

Horizon  and  Locality.  —  In  the  reddish  brown  Lower 
Cambrian  limestone  bowlders  from  Pleasant  Beach  and  Sandy 
Cove,  Cohasset,  and  from  the  Barrier  Beach  at  Bass  Point, 
Nahant  (coll.  Boston  soc.  nat.  hist.,  cat.  nos.,  11,963  and 
11,959).  Also  in  the  red  slates  at  Mill  Cove  and  at  station 
no.  1,  North  Attleboro  (coll.  H.  T.  Burr).  This  species 
seems  to  be  not  uncommon  in  the  white  limestone  at  Nahant, 
but  the  specimens  from  that  rock  seldom  show  the  surface 
striae.     The  identification,   therefore,  depends  on  the  slender 


655 

form,  low  angle,  and  circular  cross-section.  In  thin  sections 
of  this  rock,  circular  cross-sections  are  not  uncommon,  and 
these  in  general  may  be  regarded  as  cross-sections  of  this 
species,  though  some  may  represent  Hyolithellus  micans. 
Some  of  these  cross-sections  may  also  represent  the  lower  ends 
of  other  species,  which  either  are  circular,  or,  as  in  the  case  of 
oval  species,  may  appear  circular  from  the  direction  in  which 
the  section  is  cut  (coll.  Peabody  acad.  sci.,  cat.  no.,  693,  etc.). 


Orthotheca  emmonsi  (Ford). 
Plate  32,  figs.  9  a— d. 

1871.    Salterella Ford,  Amer.  journ.   sci.,  3d  ser.,  vol. 

2,  p.  83. 
1873.    Hyolithes  emmonsi  Ford,   Amer.  journ.  sci.,  3d  ser., 

vol.  5,  p.  214,  figs.  3  a— c. 
1886.    Hyolithes  communis  var.  emmonsi  Walcott,  Bull.  30, 

U.  S.  geol.  surv.,  p.  137,  pi.  14,  figs.  4,  4  a,  b. 

1888.  Hyolithes     communis    var.      emmonsi     Shaler     and 

Foerste,  Bull.  mus.  comp.  zool.,  vol.  16,  p.  32, 
pi.  2,  fig.  17. 

1889.  Hyolithes    incequilateralis    Foerste    (partim),    Proc. 

Boston  soc.  nat.  hist.,  vol.  24,  p.  262. 

1890.  Hyolithes    communis    var.    emmonsi    Walcott,    10th 

ann.  rep.  U.  S.  geol.  surv.,  p.  621,  pi.  77,  figs.  4, 
4  a,  b. 

1890.  Hyolithes  communis  var.  emmonsii  Sears,  Bull.  Essex 
inst.,  vol.  22,  p.  32. 

1893.  Orthotheca  emmonsi  Matthew,  Trans,  roy.  soc.  Can- 
ada, vol.  11,  sect.  4,  p.  95. 

Shell  elongate  and  slender.  Apical  portion  frequently  de- 
ciduous above  the  transverse  diaphragm,  the  shells  terminating 
in  a  blunt  end.      Apical  angle  about  8°.     Dorsal  (  ?)  side  flat- 


656 

tened,  having,  in  the  most  typical  specimens,  a  wide  shallow 
depression  along  the  middle,  which  runs  the  whole  length  of 
the  shell.  In  our  specimens  the  depression  is  seldom  well 
marked.  The  sides  of  typical  specimens  are  gently  rounded, 
and  meet  to  form  a  tolerably  prominent,  though  often  scarcely 
perceptible,  ventral  (  ?)  ridge  in  the  forward  part  of  the  shell, 
while  the  portion  near  the  apex  is  almost  semicircular  in 
section. 

Surface  ornamented  by  fine  concentric  lines  of  growth,  which 
run  directly  around  the  shell,  or  at  inght  angles  to  its  longitu- 
dinal axis. 

In  a  specimen  from  Nahant  (PI.  32,  fig.  9  a)  the  apical 
angle  is  about  5°,  and  the  dorsum  (?)  is  flat,  with  a  slight 
central  concavity  in  the  lower  portion.  No  lines  of  growth  are 
shown.  The  fragment  has  a  length  of  30  mm.,  but  the  origi- 
nal length  must  have  been  nearly  twice  that.  In  another 
specimen  from  Nahant  (PI.  32,  fig.  9  d)  the  angle  of  divergence 
is  a  trifle  over  6°,  the  dorsum  (?)  flat,  and  the  ventrum  (?) 
an  unsymmetric  curve. 

In  1889  Foerste  described  some  specimens  of  Hyolithes  from 
Nahant  under  the  name  H.  incBquilateralis.  He  also  re- 
ferred the  specimens  previously  described  by  Shaler  and 
Foerste  as  H.  jjriyicejjs  from  North  Attleboro  to  this  species. 
Walcott  in  1890,  after  a  critical  examination  of  Foerste's  type 
specimens,  as  well  as  other  specimens  collected  by  Mr.  Sears, 
came  to  the  conclusion  that  the  Nahant  specimens  were  identical 
with  a.  coTiimunis  var.  emtnonsi  (  =  Orthotheca  emmonsi) 
(10th  ann.  rep.  U.  S.  geol.  surv.,  p.  544). 

Dr.  Foerste  called  attention  to  the  fact  that  the  cross-sections 
have  lateral  slopes  of  unequal  curvature,  thus  lacking  the  bilat- 
eral symmetry  usually  found  in  this  genus.  This  feature  is 
commonly  shown  in  Nahant  specimens,  and  is  equally  well 
marked  in  H.  prince2)s  from  that  locality.  Similar  inequilat- 
eral development  is  found  in  //.  communis,  examples  of 
which  are  figured  by  Walcott. 


657 

A  specimen  of  0.  emmonsi  was  found  by  Shaler  and  Foerste 
at  station  2,  North  Attleboro.  The  length  of  the  specimen  is 
estimated  at  55  or  60  mm,,  and  the  apical  angle  is  11°.  The 
dorsum  (?)  is  flat  and  shows  a  depression,  Avhich  becomes  more 
distinct  at  a  distance  from  the  apical  extremity.  The  ven- 
trum  (  ?)  is  evenly  rounded,  without  the  inequilateral  develop- 
ment of  most  of  the  Nahant  specimens. 

Horizon  and  Locality.  —  In  the  white  limestone  at  Nahant 
(coll.  Peabody  acad.  sci.,  cat.  nos.,  703,  706,  697),  and  doubt- 
fully in  the  Lower  Cambrian  limestone  bowlder  from  Pleasant 
Beach,  Cohasset  (coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11,969). 
Also  at  station  2,  North  Attleboro,  1  specimen. 

OrTHOTHECA   (?)   FOERSTEI  sp.   nov. 

PI.  32,  fig.  10. 

1888.  Ilyolithes  princeps   Shaler  and   Foerste,   Bull.    mus. 

comp.  zool.,  vol.  16,  p.  33,  pi.  2,  fig.  25. 

1889.  Hyolithes    inrnquilateralis    Foerste    (partim),    Proc. 

Boston  soc.  nat.  hist.,  vol.  24,  p.  262. 

"  Shell  large,  straight,  gradually  tapering,  very  thin. 

"The  shells  vary  greatly  in  size,  reaching  at  maturity  a 
diameter  of  9  or  even  11  mm.  and  a  length  of  100  mm.  or 
more.  The  dorsal  side  is  flattened  or  moderately  convex.  The 
ventral  side  is  decidedly  convex ;  usually  the  convexity  is  more 
marked  on  one  side  of  the  ventral  surface  than  on  the  other, 
the  latter  side  being  often  almost  flat.  The  median  line  is 
rounded,  yet  usually  distinct  enough  to  be  recognized.  The 
apical  angle  is  very  moderate.     In  some  specimens  it  is  as  low 

as   6° The  surface  is  marked   by  fine  transverse  striae" 

(Shaler  and  Foerste). 

This  species  differs  from  Ilyolithes  princeps  in  its  more 
rounded    lateral    faces    and    low    apical    angle.      The    cross- 

OCCAS.  PAPERS  B.  S.  N.  H.  IV.  42. 


658 

section  is  sometimes  almost  circular,  while  in  H.  lyrincei^s  it  is 
more  nearly  semicircular. 

Foerste's  name  H.  incBquilateralis  might  be  applied  to  this 
species,  were  it  not  for  the  fact  that  it  was  originally  coined 
for  the  Nahant  specimens,  which  Walcott  has  shown  to  belong 
to  Orthotheca  emmonsi. 

The  specific  name  is  proposed  in  honor  of  Dr.  August  F. 
Foerste. 

Horizon  and  Locality.  —  This  species  is  common  in  the 
Lower  Cambrian  limestone  at  station  1,  North  Attleboro,  Mass. 
(Shaler  and  Foerste).  It  also  occurs  in  the  Lower  Cambrian 
limestone  bowlders  from  Cohasset  (coll.  Boston  soc.  nat.  liist., 
cat.  no.,  11,973). 


Genus  Hyolithellus  Billins^s. 


HroLiTHELLUS  MiCANS  Billings. 
Plate  32,  fig.  12. 

1872.    Hyolithes  micans  Billings,  Can.  nat.  and  geol.,  n.  s., 

vol.  6,  p.  215,  figs.  3  a,  b  (p.  213). 
1872.    HyolithelliLs  micans  Billings,  Can.  nat.   and  geol.,  n. 

8.,  vol.  6,  p.  240. 
1886.    Hyolithellus  micans  Walcott,  Bull.  30,  U.   S.  geol. 

surv.,  p.  142,  pi.  14,  figs.  2,  2  a-e. 
1888.    Hyolithellus  micans  Shaler  and  Foerste,  Bull.   mus. 

comp.  zooL,  vol.  16,  p.  34,  pi.  2,  fig.  23    (partim). 
1890.    Hyolithellus  micans  Walcott,   10th  ann.   rep.   U.   S. 

geol.  surv.,  p.  624,  pi.  79,  figs.  1,  1  a-e. 
1899.    Hyolithellus  micans    Matthew,   Bull.    nat.   hist.    soc. 

N.   B.,   vol.    18,  p.    192,   pi.    2;    Trans,    roy.    soc. 

Canada,  vol.  5,  sect.  4,  p.  109,  pi.  6,  figs.  1  a-d. 


659 

Shell  long,  very  slender,  cylindrical,  tapering  very  gently 
to  an  acute  point,  cross-section  circular  or  broadly  ovate. 
Apical  angle  very  low.  Billings  states  that  the  rate  of  taper- 
ing amounts  to  scarcely  half  a  line  in  the  length  of  eighteen 
lines,  where  the  width  of  the  tube  is  from  one  to  two  lines, 
giving  thus  an  apical  angle  of  about  one  and  one  half  degrees. 
Walcott  states  that  the  "  shell  for  the  first  10  mm.  or  15  mm., 
is  often  curved  and  almost  twisted  in  some  examples."  Sur- 
face commonly  smooth,  but  in  well-preserved  specimens  show- 
ing fine  concentric  lines  of  growth,  which  "  in  some  examples 
are  quite  strong  and  regular  in  arrangement  "  (Walcott). 

Horizon  and  Localities.  —  In  the  reddish  Lower  Cambrian 
limestone  bowlders  from  Cohasset  and  Nahant  (coll.  Boston 
soc.  nat.  hist.,  cat.  no.,  11,966).  Some  of  the  specimens  identi- 
fied as  this  species  may  represent  the  young  of  Orthotheca 
cylindrical  from  which  they  can  be  distinguished  only  by  their 
smaller  apical  angle.  What  appears  like  the  inner  mould  of 
an  operculum  is  associated  with  a  specimen  from  the  Nahant 
bowlder.  The  species  also  occurs  at  North  Attleboro,  as 
identified  by  Shaler  and  Foerste. 


Genus  Ueotheca  Matthew. 

Urotheca  pervetus  Matthew. 

1899.     t//'o^/iecaper^;e/!^(5  Matthew,  Bull.  nat.  hist.  soc.  N.  B., 
vol.  18,  p.  192,  pi.  1,  fig.  8. 

1899.  Urotheca  pervetus    Matthew,  Trans,    roy.   soc.    Can- 

ada, vol.  5,  sect.  4,  p.  106,  pi.  5,  fig.  8. 

1900.  Urotheca  pervetus  Burr,  Amer.  geol.,  vol.  25,  p.  48. 

"  Proximal  end  unknown  ;  distal  end  exhibits  a  slender, 
gently  curved,  chitinous  tube.  All  the  examples  are  pressed 
flat  in  the  shale  and  so  the  form  of  the  orifice  is  not  seen. 


660 

^^  Sculpture. — No  transverse  stri^  were  observed,  but 
faint  longitudinal  strife  are  present,  these  may  be  due  to 
compression." 

"  Size.  —  Length  of  the  part  preserved,  35  mm.  ;  w^idth  at 
the  small  end,  1^  mm.  ;  at  the  aperture,  3j  mm.  Hate  of 
tapering  of  this  part  of  the  tube,  1  to  17  "  (Matthew). 

A  single  curved  individual  was  obtained  by  Mr.  Burr  from 
the  Pearl  Street  slates.  It  shows  faint  annulations  and  very 
faint  longitudinal  striations  near  the  aperture.  It  is  flattened 
in  the  slate.  Length  of  fragment,  14.5  mm.  ;  apparent  diam- 
eter at  aperture,    2.2    mm. 

Horizon  and  Locality.  —  In  the  Lower  Cambrian  slates 
on  Pearl  Street,  North  Weymouth,  Mass.,  associated  with 
Olenellus,  etc.  The  species  was  originally  described  by  Mat- 
thew from  the  Etcheminian  of  Smith  Sound.  Our  specimen 
agrees  closely  with  specimens  from  the  type  locality. 


Genus  Salterella  Billings. 

Salterella  curvatus  Shaler  and  Foerste. 

1888.    Salterella  ctirvatus    Shaler  and  Foerste,  Bull.  mus. 

comp.  zooL,  vol.  16,  p.  34,  pi.  2,  fig.  22. 
1890.    Salterella   curvatus  Walcott,    10th  ann.    rep.   U.   S. 

geol.  surv.,  p.  625,  pi.  79,  figs.  3,  3  a. 

This  species  was  obtained  by  Shaler  and  Foerste  from  the 
red  Lower  Cambrian  limestone  of  station  1,  North  Attleboro, 

Mass. 


661 


OSTEACODA. 

Genus  Leperditia  Rouault. 

Lepei'ditia,  cf.  L.  solitaria  Barrande. 

Compare    Lejjerditia    solitaria    Barrande,    Syst.    Sil.    de   la 

Boheme,  vol.  1,  suppL,  pi.  23. 
Leperditia,  cf.   L.  solitaria  Barrande,   Burr,  Amer.    geol., 

vol.  25,  p.  47. 

"  Five  individuals  have  been  found  which  appear  to  be  os- 
tracods  of  the  Leperditia  type.  The  smallest  of  these  is  8.5 
mm.  long,  and  5  mm.  in  height.  Its  hinge  line  is  nearly 
straight,  and  curves  abruptly  at  the  ends.  The  valve  is  only 
moderately  convex.  The  two  ends  are  apparently  symmetri- 
cal. Three  of  the  other  individuals  resemble  this  one  in  size 
and  shape.  The  fourth  is  considerably  larger,  having  a  length 
of  11,5  mm.  It  differs  from  the  others  in  form,  also.  The 
hinge  line  is  less  straig-ht  and  the  valves  more  convex.  It 
shows  distinct  lines  of  growth.  It  bears  a  ridge  comparable 
to  the  umbonal  ridge  of  a  lamellibranch.  This  specimen  is 
only  provisionally  referred  to  Leperditia"   (Burr). 

The  material  here  described  is  of  a  very  doubtful  character. 


Malacostraca. 

Genus  Aristozoje  Barrande. 

Aristozoe  (?)  Shaler  and  Foerste.     Bull.  mus.  comp.  zooL, 
vol.  16,  p.  35,  pi.  2,  fig.  18. 

A  valve   referred    doubtfully    to    this    genus    was    obtained 
by  Shaler   and   Foerste  at  station,  no.   2,  North   Attleboro. 


662 
Teilobita. 

Genus  Olenellus  Hall. 

Olenellus  (Holmia)  beoggeri  Walcott. 
PI.  33,  figs.  1  a-j.i 

1888.    Olenellus  broggeri  Walcott,  Nature,  vol.  38,  p.  551. 
1890.     Olenellus  {Holmia)  broggeri  Y^fulcott,  10th  ann.  rep. 

U.  S.  geol.  surv.,  p.  638,  pi.  91,  fig.  1,  pi.  92,  figs. 

1,    1  a— h. 
1900.     Olenellus  (Holmia)  broggeri  Burr,  Amer.  geol.,  vol. 

25,  p.  43. 

Among  the  specimens  obtained  from  the  slates  of  North 
Weymouth,  are  a  number  of  fragments  of  large  individuals  of 
this  species,  and  a  nearly  perfect  head  of  a  young  one.  A 
fragment  of  the  head  (pi.  33,  fig.  1  c),  slightly  compressed 
laterally,  shows  the  glabella  very  gently  widening  forward  to 
a  point  just  beyond  the  anterior  terminations  of  the  eye-lobe, 
after  which  it  narrows  rapidly  to  an  acutely  rounded  anterior 
end.  The  difference  between  the  glabella  of  this  specimen  and 
that  of  the  type,  is  easily  seen  to  be  due  to  lateral  compression 
of  the  former.  As  a  result  of  this  compression  the  centre  of 
the  glabella  is  rather  abruptly  elevated  into  a  low,  rounded 
ridge.  Three  shallow  glabellar  furrows  are  observable.  The 
anterior  one  is  opposite  the  anterior  end  of  the  eye-lobe  and 
extends  very  gently  forward  and  inward  to  a  little  over  a  third 
of  the  width  of  the  glabella,  the   furrows  on  opposite  sides  not 

1  Since  the  specimens  on  this  plate  were  drawn  and  described,  Mr.  Burr  has  con- 
tinued his  exploration  of  the  ledge  on  Pearl  Street,  North  AVeyuiouth,  for  the  U.  S. 
Geological  Survey,  and  has  found  numerous  specimens  of  Jlohnia  brscrgeri  together 
with  other  specimens  of  the  species  here  described,  many  of  which  are  much  more 
perfect  than  those  noted  in  this  paper. 


663 

uniting.  The  second  furrow  extends  inward  with  a  slight 
backward  curvature,  and  the  third  slopes  gently  backward. 
The  occipital  furrow  is  not  shown  on  this  specimen. 

Eye-lobe  crescentiform,  narrow  and  elongate.  It  begins 
anteriorly  close  to  the  glabella,  and  opposite  the  anterior  gla- 
bellar furrow,  curves  outward  and  backward,  and  again  very 
slightly  inward,  to  a  point  opposite  where  the  occipital  furrow 
should  reach  the  margin  of  the  glabella,  and  some  distance  from 
it.  The  marginal  rim  appears  rather  narrow,  and  the  shallow 
groove  just  within  it  rather  wide,  but  the  original  form  in  both 
cases  has  been  altered  by  compression.  A  narrow,  rather  sharp 
ridge  limits  the  groove  posteriorly. 

A  comparison  of  this  specimen  with  the  type  in  the  collection 
of  the  U.  S.  jSTational  Museum,  shows  no  differences  which  can- 
not readily  be  accounted  for  by  compression.  The  glabella  is 
less  spreading  forward,  and  the  curvature  of  the  eye-lobe  is  less, 
in  our  specimen.  Length  of  head,  approximately  50  mm.  ; 
width,  about  80  mm.  Length  of  glabella  approximately  38  mm. 
A  stout  spine  found  in  the  North  Weymouth  slates  apparently 
represents  the  occipital  spine  of  this  species.  Lateral  cheek 
represented  by  several  fragments.  The  largest  of  these  (PI. 
33,  fig.  1  e)  has  the  characteristic  strong  "  inter-ocular"  spine 
just  within  the  genal  spine.  The  width  of  the  genal  spine  at 
this  point  is  14  mm.,  the  corresponding  width  of  the  spine  on 
the  cheek  figured  by  Walcott  on  plate  42,  fig.  1  b,  being  11 
mm. 

Surface  marked  by  fine,  raised,  inosculating  striae,  which 
Walcott   states   to   be  characteristic  of  the   genus    Olenellus. 

The  hypostoma  is  represented  by  several  specimens.  A 
comparison  of  our  best  specimen  with  that  figured  by  Walcott 
on  plate  42,  fig.  1  f,  shows  a  somewhat  more  ovate  anterior 
lobe,  and  a  prominent,  rounded,  posterior  fold,  separated  from 
the  anterior  lobe  by  a  rounded  groove  which  disappears  toward 
the  lateral  wings.  The  posterior  rim  is  narrow  and  not  very 
prominent.      Length,  16  ram.      Width,  including  the  anterior 


664 

wings,  about  20  mm.  A  number  of  pleura  that  occur  in  the 
North  Weymouth  slates  show  the  characteristics  of  this  species 
in  their  form  and  in  the  profound  and  narrow  groove  which 
marks  the  centre,  and  which  dies  out  part  way  toward  the 
extremity.  They  also  show  the  characteristic  surface  markings. 
Several  specimens  apparently  representing  the  detached  doub- 
lure occur  associated  with  the  other  fragments.  One  of  these 
is   figured  herewith.     No  hypostoma  has  been  found  in  place. 

A  nearly  perfect,  though  distorted,  small  head  shows  a  nar- 
row frontal  rim,  within  which  is  a  slightly  narrower  well- 
marked  groove,  bordered  posteriorly  by  a  thin  raised  ridge. 
The  glabella  has  the  characteristics  of  the  adult,  with  the 
three  pairs  of  glabellar  furrows  deep  and  strongly  marked, 
though  extending  rather  less  than  a  third  across  the  orlabella 
from  each  side.  The  occipital  furrow  is  extended  across  the 
glabella  by  a  faint  depression  (which  may,  however,  be  acci- 
dental). The  occipital  ring  is  marked  by  a  projecting  spine 
base.  The  eye-lobes  are  crushed,  but  appear  to  be  slightly 
less  crescentic  than  those  of  the  adult,  with  their  posterior  end 
nearer  to  the  glabella.  This  may,  however,  be  due  to  com- 
pression. Inter-ocular  and  genal  spines  well  marked,  a  very 
faint  line  extending  from  the  former  to  the  posterior  end  of 
the  eye-lobe.  Length  of  the  head,  13.5  mm.  ;  of  glabella, 
10.5  mm.  Width  of  head  between  postero-lateral  spines, 
21  mm.  This  head  is  very  similar  to  the  smaller  one  figured 
by  Walcott,  except  that  the  glabella  is  more  developed,  partak- 
ing more  of  the  character  of  the  adult.  Several  other  more 
or  less  fragmentary  small  heads  are  referable  to  this  species. 

Horizon  and  Locality.  —  In  the  dark  purplish  slates  of 
Lower  Cambrian  age.  Pearl  Street,  North  Weymouth.  The 
specimens  figured  were  obtained,  with  others,  by  Mr.  H.  T. 
Burr,  with  the  exception  of  one,  —  the  young  head  figured, 
which  was  found  by  Mr.  Henry  Ash  of  North  Weymouth. 


665 


Olenellus    sp. 


PL  34,  figs.  1  a,  b/ 

1900.     Olenellus     sp.,  Burr,  Amer.  geol.,  vol.  24,  p.  45. 

Among  the  material  collected  by  Mr.  H.  T.  Burr  at  North 
Weymouth  are  two  very  perfect  heads  of  a  species  of  Olenellus 
which  appears  to  be  undescribed.  A  third  head,  vertically 
compressed,  was  collected  by  Mr.  P.  S.  Smith  at  the  same 
locality.  The  following  descriptions  apply  to  the  two  heads 
figured  herewith,  the  third  —  that  collected  by  Mr.  Smith  — 
showing,  however,  the  same  characters,  though  much  more 
distorted. 

The  larger  head  (PL  34,  fig.  1  a)  (as  compressed  upon  the 
slate)  describes  almost  a  semicircle  in  outline,  with  the  centre 
at  the  base  of  the  g-labella.  Width  of  head,  47  mm.  Lenoth 
at  the  centre,  24  mm.  The  postero-lateral  angles  project 
about  4  mm.  behind  the  base  of  the  occipital  ring. 

The  glabella  has  the  form  of  an  acute  triangle,  the  anterior 
end  forming  the  bluntly  rounded  apex  of  the  triangle.  The 
length  of  the  glabella  is  20  mm.,  and  the  width  of  the  occipital 
ring,  12.5  mm.  There  are  three  pairs  of  glabellar  furrows  in 
front  of  the  occipital  furrow.  The  latter  is  strongly  marked 
for  about  a  third  of  the  distance  inward,  from  the  margin  of 
the  glabella  on  each  side,  and  is  continued  across  by  a  faint 
depression.  It  appears  more  like  a  pair  of  glabellar  furrows, 
which  are  deep  and  rather  wide  at  the  margin,  becoming  shal- 
lower as  they  pass  inward  and  gently  backward.  Posterior 
pair  of  glabellar  furrows  similar  and  parallel  to  occipital  fur- 
row, and,  like  it,  continued  across  by  a  very  faint  depression. 

1  Although  additional  specimens  of  this  species  have  since  been  found  by  Mr. 
Burr,  none  more  perfect  than  those  here  iigured  have  been  obtained,  nor  has  any 
other  portion  than  the  cephalon  been  found. 


666 

The  two  anterior  pairs  of  glabellar  furrows  are  of  the  same 
type  as  the  posterior  one,  and  parallel  to  it,  but  are  not  con- 
tinued across  the  glabella.  The  furrows  are  progressively 
shorter  toward  the  anterior  end  of  the  glabella.  In  each  case 
they  occupy  about  one  third  of  the  width  of  the  glabella  on 
each  side.  The  anterior  lobe  forms  a  little  more  than  one 
third  of  the  length  of  the  glabella,  and  is  subovoid  in  outline, 
the  anterior  end  being  the  narrower.  On  each  side  of  the 
posterior  glabellar  lobe  is  a  well-marked,  rounded  tubercle  or 
monticule,  separated  from  the  lobe  by  a  depression.  This 
destroys  the  triangular  appearance  of  the  glabella,  producing 
an  abrupt  dilation  just  in  front  of  the  occipital  furrow. 

Eye  lobes  crescentic,  relatively  narrow.  The  anterior  end 
begins  at  a  point  opposite  the  anterior  glabellar  furrow,  and  a 
short  distance  from  the  glabella.  It  then  curves  outward,  at 
first  gently  and  then  abruptly,  the  outermost  margin  of  the 
lobe  reaching  a  distance  nearly  half  way  between  the  glabella 
and  the  lateral  margin  of  the  head.  The  posterior  third  of  the 
lobe  curves  inward  to  a  point  opposite  the  occipital  furrow, 
and  slightly  more  distant  from  the  glabella  than  the  point  of 
commencement  of  the  anterior  end.  A  line,  that  of  the  inter- 
ocular  spine,  passes  outward  and  backward  from  the  posterior 
end  of  the  eye  lobe,  cutting  the  posterior  margin  of  the  cepha- 
lon  about  half-way  between  the  margin  and  the  glabella.  A 
faint  marginal  fold  is  indicated,  limited  by  a  very  faint  depres- 
sion within  the  margin,  and  a  faint  furrow  is  visible  parallel 
and  close  to  the  posterior  margin. 

The  smaller  head  (pi.  34,  fig.  1  b)  is  nearly  12  mm.  in 
length,  and  has  a  more  prominent  glabella,  Avhich  is  somewhat 
less  tapering.  The  head  appears  to  be  slightly  compressed 
laterally.  The  occipital  furrow  is  well  marked,  but  the  others 
are  fainter,  especially  the  anterior  one,  which  is  almost  invisible. 
The  tubercle,  shown  on  the  left  side,  opposite  the  posterior 
glabellar  furrow,  is  strong  and  distinct.  The  left  eye-lobe 
alone  is  shown  ;   it  is  strong,   and  from  its  base  continues  a 


667 


well-marked  line,  that  of  the  inter-ocular  spine.  The  marginal 
fold  is  very  faintly  outlined  ;  the  posterior  fold  is  rather  strong, 
though  narrow. 

The  most  pronounced  chai*acteristics  of  this  species  are  the 
triangular  glabella  with  its  deep  furrows,  the  strong  lateral 
tubercles,  and  the  very  faint  or  obsolete  marginal  fold. 

No  specific  name  is  at  present  proposed  for  this  species 
which  is  undoubtedly  new,  since  the  material  is  insufficient  to 
establish  a  complete  specific  diagnosis.  It  probably  belongs 
to  a  new  subgenus  of  Olenellus. 

Horizon  and  Locality.  —  In  the  dark  purplish  Lower  Cam- 
brian slates  of  Pearl  Street,  North  Weymouth,  Mass. 

Olenellus  (Mesonacis)  asaphoides  (Emmons)  (?). 

PI.  34,  figs.  2,  a,  b. 

(For  description  and  synonymy  see  Walcott,  Bull.  30,  U.  S. 
geol.  surv.,  1886,  p.  168;  also  10th  ann.  rep.  U.  S.  geol. 
surv.,  p.  637,  pis.  16,  88,  89,  90). 

1900.     Olenellus  {ISIesonacis)   asaphoides  (Emmons)   Burr, 
Amer.  geol.,  vol.  25,  p.  45. 

Among  the  material  collected  by  Mr.  Burr  and  the  writer 
at  North  Weymouth  are  the  central  portion  of  the  head  of  a 
young  individual,  several  lateral  cheeks,  and  a  small  nearly  per- 
fect head,  which  appear  to  be  referable  to  this  species.  Although 
the  material  is  fragmentary,  a  careful  comparison  with  the 
types  in  the  collection  of  the  United  States  National  Museum 
leads  to  the  belief  that  this  species  is  represented. 

The  glabella  in  our  largest  specimen  narrows  toward  the 
centre,  and  then  becomes  wider  again  forward.  The  occipital 
furrow  is  not  continued  across,  but  consists  of  a  short  furrow 
on  each  side,  passing  obliquely  backward  in  about  the  same 
degree  as  is  seen  in  the  adult.     The  occipital  ring  bears  in  the 


668 

centre  a  rather  pronounced  tubercle.  This  is  unlike  the  type 
specimens  of  the  species,  but  in  a  specimen  of  about  the  same 
age  from  Troy,  N.  Y.,  in  the  collection  of  the  U.  S.  National 
Museum,  cat.  no.,  15,413,  a  similar  tubercle  occurs  on  the  oc- 
cipital ring.  Three  pairs  of  glabellar  furrows  are  shown.  The 
posterior  two  are  similar,  to  and  parallel  to  the  occipital  pair; 
the  anterior  pair  passes  straight  inward.  None  of  the  furrows 
reach  more  than  about  a  third  of  the  way  across  the  glabella. 
Anterior  lobe  not  shown.  The  eye-lobe  is  narrow  and  cres- 
centic,  beginning  opposite  the  anterior  glabellar  furrow,  and  a 
short  distance  from  it,  and  curving  around  to  a  point  opposite 
the  anterior  end  of  the  occipital  ring,  and  somewhat  more  dis- 
tant from  the  glabella  than  is  the  anterior  end  of  the  eye-lobe. 
The  length  of  the  head  is  estimated  at  about  20  mm.  ;  the 
width  of  the  occipital  ring,  about  14  mm.  In  the  character 
of  the  glabellar  furrows  and  of  the  eye-lobe,  this  specimen 
resembles  more  the  adult  stage.  The  eye-lobe  begins,  how- 
ever, somewhat  further  back.  The  presence  of  the  occipital 
tubercle  apparently  indicates  an  immature  stage. 

Mr.  Burr  has  suggested  that  this  specimen  may  be  a  distorted 
young  of  Hohnia  hroggeri.  There  is  nothing  in  the  specimen 
to  furnish  absolute  pi'oof  for  or  against  this  view,  but  my 
opinion  is  that  the  characters  point  to  its  being  Mesonacis  asa- 
'phoides.  The  young  individual  obtained  from  these  slates 
agrees  very  closely  with  specimens  of  the  species  of  the  same 
size  from  Troy  and  Washington  County,  N.  Y.  The  head  is 
slightly  crushed  and  perhaps  somewhat  drawn  out  laterally. 
It  is  semicircular,  with  a  strong  marginal  and  posterior  fold 
and  well-developed,  long  and  narrow,  genal  spines.  The  gla- 
bella narrows  slightly  in  front  of  the  occipital  ring,  continues 
with  parallel  sides  for  a  short  distance,  and  then  widens  out  to 
a  suboval  anterior  end  which  comprises  the  two  anterior  lobes. 
The  occipital  furrow  is  extended  across,  at  first  gently  sloping 
backward,  then  straight  across.  Occipital  ring  with  a  small 
median  tubercle.     Posterior  pair  of  glabellar  furrows  parallel 


669 

to  occipital  furrows,  extending  only  part  way  across.  The 
other  two  furrows  are  faint ;  the  anterior  one  is  just  in  front 
of  the  broadest  part  of  the  glabella.  Its  character  is  not  well 
shown,  but  it  appears  to  slope  forward  slightly.  All  these 
characters  are  shown  on  a  specimen  of  0.  asaphoides  of  the 
same  size  from  Washington  County,  N.  Y.,  (Nat.  Mus.,  cat. 
no.,  17,480),  except  that  the  anterior  two  pairs  of  furrows 
appear  to  extend  all  the  way  across  the  glabella.  This  is  prob- 
ably due  to  compression.  The  eye-lobes  are  somewhat  more 
crescentic  in  our  specimen  than  in  that  from  New  York,  being 
also  somewhat  further  removed  from  the  glabella.  They  begin 
anteriorly  about  opposite  the  anterior  furrow,  and  close  to  the 
glabella,  and  curve  around  to  a  point  opposite  the  antero-lateral 
margin  of  the  occipital  ring  and  some  distance  from  it.  From 
its  posterior  end  a  raised  line,  that  of  the  inter-ocular  spine, 
passes  outward  and  backward,  cutting  the  posterior  margin  a 
little  more  than  one  third  the  distance  from  the  lateral  mare:in 
to  the  glabella.  Surface  showing  a  pitted  character  some- 
what like  that  figured  by  Walcott  in  pi.  88,  fig.  1  e,  of  his 
Olenellus  Fauna.  Length  of  head,  7.5  mm.  Width,  15  mm. 
Length  of  glabella,  6  mm.  Width  of  occipital  ring,  3.5  mm. 
Ho7nzon  and  Locality.  — In  the  purplish  slates  of  Lower 
Cambrian  age  on  Pearl  Street,  North  Weymouth,  Mass. 


Olenellus  walcotti  (Shaler  and  Foerste). 

1888.    Paradoxides  walcotti  Shaler  and  Foerste,  Bull.  mus. 

comp.  zool.,  vol.  16,  p.  36,  pi.  2,  fig.  12. 
1890.     Olenellus    walcotti   Walcott,    10th   ann.    rep.   U.    S. 

geol.  surv.,  p.  636,  pi.  88,  fig.  2. 

(Compare  0.  asaphoides  (Emmons). 

A  single  specimen  of  this  species  was  found  at  station  2, 
North  Attleboro,  Mass.,  by  Shaler  and  Foerste.  Its  close 
similarity  to  and  possible  specific  identity  with  0.  asaphoides 
Emmons,  was  pointed  out  by  Walcott. 


670 


Genus  Metadoxides  Bornemann. 

Metadoxides  MAGNiFicus  Matthew  (?). 
PL  34,  figs.  4-6. 

1898.  Metadoxides  magnificus  Matt.,  Bull.  nat.  hist.  soc. 

N.  B.,  no.  17,  p.  137,  pi.  3. 

1899.  Metadoxides  magnijiciis  Matt.,  Trans,  roy.  soc.  Can- 

ada, vol.  5,  sect.  4,  p.  83,  pi.  7,  figs.  1  a-e. 

1900.  Metadoxides  magnificus  (?)  Burr,  Amer.   geol.,  vol. 

24,  p.  46. 

Several  fragments  in  the  collections  from  North  Weymouth 
indicate  the  presence  of  a  large  trilobite  with  long  genal  spines. 
They  have  been  provisionally  identified  with  Matthew's  species, 
the  material  being  too  fragmentary  to  allow  precise  determi- 
nation. The  surface  of  the  spines  shows  the  raised  inosculating 
lines  characteristic  of  Olenellus. 

Horizon  and  Locality.  —  In  the  purplish  Lower  Cambrian 
slates  on  Pearl  Street,  North  Weymouth. 


Genus  MiCRODisous  Emmons. 

MiCRODiSCUS  BELLI-MARGINATUS  Shaler  and  Foerste. 

1888.    Microdiscus  belU-marg inatus  Shaler  and  Foerste,  Bull. 

mus.  comp.  zool.,  vol.  16,  p.  35,  pi.  2,  figs.  19  a,  b. 
1890.    Microdiscus  hellimarg inatus  Walcott,  10th  ann.  rep. 

U.  S.  geol.  surv.,  p.  630,  pi.  81,  figs.  2,  2  a,  b. 

This  s[)ecies  was  found  by  Shaler  and  Foerste  in  the  red 
Lower  Cambrian  limestone  at  stations  2  and  3,  North  Attle- 
boro,  Mass.     It  is  fairly  abundant. 


671 


MiCRODISCUS    LOBATUS   (Hall). 

1847.    Agnostus  lobatus  Hall,  Pal.  N.  Y.,  vol.  1,  p.  258, 

pi.  67,  figs.  5  a-f. 
1873.    Mic7'odiscus  lobatics  Ford,  Amer.  journ.  sci.,  3d  ser., 

vol.  6,  p.  135,  foot-note. 
1886.    Microdiscus  lobatus  Walcott,  Bull.  30,  U.  S.  geol. 

siirv.,  p.  156,  pi.  16,  figs.  1,  1  a,  b. 
1888.    Microdiscus  lobatus  Shaler  and  Foerste,  Bull.   mus. 

comp.  zooL,  vol.  16,  p.  36,  pi.  2,  fig.  13. 
1890.    Microdiscus  lobatus  Walcott,    10th  ann.   rep.   U.  S. 

geol.  surv.,  p.  632,  pi.  81,  figs.  4,  4  a,  b. 
A  single  specimen  of  this  species  was  found  by  Shaler  and 
Foerste  at  station  2,  North  Attleboro,  Mass. 


Microdiscus  sp. 
PI.  33,  fig.  3. 

(Compare   Microdiscus  helena    Walcott,    10th    ann.    rep. 
U.  S.  geol.  surv.,  p.  632,  pi.  81,  figs.  1,  la). 
1900.     Microdiscus,  cf.  M.  helena  Burr,  Amer.   geol.,  vol. 

25,  p.  47. 

A  small  pygidium,  showing  some  of  the  characteristics  of  il/. 
helena  from  Conception  Bay,  Newfoundland,  was  obtained  by 
Mr.  Burr  at  North  Weymouth.  The  axial  lobe  is  delimited  by 
rather  deep  furrows,  and  tapers  uniformly  to  near  the  posterior 
margin,  before  reaching  which  it  becomes  abruptly  rounded. 
Transverse  furrows  occur  on  the  axial  lobe,  but  their  precise 
characters  are  not  determinable.  Length,  2  mm.  Greatest 
width,  2-|-  mm. 

Horizon  and  Locality.  —  In  the  dark  purplish  Lower  Cam- 
brian slates  of  Pearl  Street,  North  Weymouth,  one  specimen. 


672 


Genus  Strenuella  Matthew. 

Strenuella  strenua  (Billings). 
PI.  34,  figs.  7  a-c,  8. 

1874.    A.graulos  sti^enuus  Billings,  Geol.   surv.  Canada,  pal. 

foss.,  vol.  2,  pt.  1,  pp.  71,  72. 
1886.    A.graulos  {St7'enueIIa)  sty^emnis  ^latthew.  Trans,  roj. 

soc.  Canada,  vol.  4,  sect.  4,  p.  154. 
1888.     Ptychoj}aria   niucronatus  Shaler  and  Foerste,  Bull. 

mus.  comp.  zooL,  vol.  16,  pp.    37,  38,  pi.  2.,  figs. 

21  a-d. 
1890.     Agraulos  strenuusy^dXcott,  10th  ann.  rep.  U.  S.  geol. 

surv.,  pp.  653,  654,  pi.  97,  figs.  1,  1  a— c. 

1899.  Strenuella  strenua  Matthew,   Trans,   roy.  soc.    Can- 

ada, vol.  5,  sect.  4,  p.  76. 

1900.  Agraidus  (Strenuella)  strenuus  var.  nasutus  (Wal- 

cott).  Burr,  Amer.  geol.,  vol.  25,  p.  46. 

"Head  (without  the  movable  cheeks)  irregularly  quadran- 
gular, broadly  rounded  in  front.  Glabella  rather  strongly 
convex,  conical,  variable  in  its  proportional  length  and  width, 
either  smooth  or  with  several  obscure  impressions  on  each  side 
representing  the  glabellar  furrows  ;  neck  segment  with  a  strong 
triangular  projection  backwards,  neck  furrows  all  across  but 
usually  obscurely  impressed.  In  some  specimens  the  front  of 
the  head  has  a  thick,  convex  marginal  rim  separated  from  the 
front  of  the  glabella  by  a  narrow  groove.  In  others  this  rim 
is  scarcely  at  all  developed.  The  eyes,  shown  by  the  form  of 
the  lobe,  appear  to  have  been  semi-annular  and  about  one 
third  the  length  of  the  head.  The  surface  appears  to  be 
smooth  "  (Billings) . 

This  species  is  common  at  North  Attleboro,  but  it  has  not 
vet  been  found  in  either  the  white  limestone  of  Nahant  or  the 


673 


red  limestone  bowlders  from  Coliasset  and  Nahant.  A  speci- 
men has,  however,  been  obtained  from  a  pebble  of  dark  red- 
dish brown  calcareous  slate  found  at  Nahant.  The  specimen 
(Plate  34,  fig-.  8)  is  mutilated,  destitute  of  free  cheeks,  and 
with  the  glabella  broken  away.  The  mai^ginal  rim  is  strongly 
developed,  and  is  separated  from  the  remainder  of  the  head 
by  a  deep  rounded  furrow.  The  dorsal  and  occipital  furrows 
are  strong,  the  latter  extending  onto  the  fixed  cheeks.  The 
ocular  ridges,  connecting  the  anterior  ends  of  the  palpebral 
lobes,  appear  to  have  been  strong.  The  palpebral  lobes  are 
elevated,  slightly  curved,  and  separated  from  the  cheeks  by 
strong  rounded  furrows.  Spine  stout,  moderately  long,  taper- 
ing rapidly  to  an  acute  point.  The  centre  of  the  fixed  cheek 
is  strongly  marked,  elevated,  and  flattened  on  top,  and  has  the 
form  of  a  spherical  triangle. 

This  species  is  represented  in  the  collection  from  North 
Weymouth  by  a  considerable  number  of  specimens.  The 
largest  head  obtained  (pi.  34,  fig.  7  b)  shows  the  glabella  nar- 
rowing toward  the  front,  which  is  somewhat  abruptly  rounded. 
The  occipital  furrow  extends  across  and  in  front  of  it.  Two 
glabellar  furrows  show  faintly.  They  extend  obliquely  back- 
ward, for  about  a  third  of  the  distance  from  each  side.  These 
furrows  are  not  always  well  shown  on  the  other  specimens. 
The  anterior  fold  is  usually  well  marked  in  the  North  Wey- 
mouth specimens,  and  defined  by  a  rounded  furrow,  which  in 
some  specimens  is  rather  broad.  The  fixed  cheeks  are  usually 
strongly  elevated,  flat  on  top,  and  margined  by  pronounced 
furrows.      The  ocular  ridges  are  usually  well  marked. 

The  specimens  from  North  Weymouth  agree  closely  with 
that  found  in  the  pebble  of  purple  slate  at  Nahant.  They  are 
perhaps  referable  to  the  variety  nasutus  of  Walcott. 

A  specimen  from  North  Weymouth  shows  a  part  of  the 
thorax,  but  the  preservation  is  not  perfect  enough  to  permit 
description.  Associated  with  the  cephalon  in  the  Nahant  peb- 
ble were  found  several  cross-sections  of  the  thorax  of  probably 

OCCAS.  PAPERS  B.  S.  X.  H.    IV.  43. 


674 


the  same  individual.     They  show  a  strongly  convex  axis,  deep 
axial  furrows,  and  arched  pleura. 

Horizon  and  Locality. —  Lower  Cambrian.  In  a  pebble 
of  dark  reddish  brown,  faulted,  calcareous,  shale,  found  by  Mr. 
Watson  at  Nahant,  west  of  the  point  known  as  John's  Peril, 
one  specimen  (coll.  Boston  soc.  nat.  hist.,  cat.  nos.,  11,968,. 
11,969) .  In  the  purplish  slates  of  Pearl  St.,  North  Weymouth, 
about  37  specimens.^  The  species  is  abundant  at  stations  2  and 
3,  North  Attleboro,  Mass.  (Shaler  and  Foerste). 


Genus  Agraulos  Corda. 

Agraulos  quadrangularis  (Whitfield). 
Plate  34,  figs.  9,  10  ;    Plate  35,  fig.  1. 

1862.    Elli'psoce'phalus  ?     Ordway,  Proc.  Boston  soc. 

nat.  hist.,  vol.  8,  p.  5. 
1884,    A.rionellus  quadrangularis  Whitfield,    Bull.    Amer. 

mus.  nat.  hist.,  vol.  1,  p.  147,  pi.  14,  fig.  8. 
1884.    Agi'aulos  quadrangularis  Walcott,  Bull.    10,  U.   S. 

geol.  surv.,  p.  48,  pi.  7,  fig.  1. 

Cephalic  shield  semioval  to  semicircular  in  outline,  with  the 
postero-lateral  angles  produced.  The  original  outline  is  often 
distorted  by  compression.  Glabella  convex,  longer  than  wide, 
gently  tapering  forward,  its  anterior  end  uniformly  rounded 
to  correspond  to  the  curvature  of  the  anterior  border  of  the 
cephalic  shield.  Transverse  contour  a  uniform  arch  ;  longi- 
tudinal contour  of  uniform  elevation  in  the  posterior  two 
thirds,  descending  in  the  anterior  third,  at  first  gently,  and 
then  more  abruptly,  to  the  frontal  limb.  Transverse  furrows 
extending    obliquely  backward,   usually  obsolete,   but    one  or 

1  Recent  discoveries  at  North  Weymouth  have  greatly  added  to  this  number. 


675 

two  pairs  distinctly  shown  in  some  specimens.  Dorsal  and 
occipital  furrows  strongly  marked.  Occipital  ring  moderate, 
and  bearing  a  long,  stout,  cylindrical  spine.  Cephalic  limb 
broad,  rounded,  and  without  marginal  rim.  Fixed  cheeks  of 
about  the  same  width  anteriorly  as  the  cephalic  limb,  with 
which  they  are  uniformly  continuous.  They  become  broader 
posteriorly,  and  terminate  in  acute  postero-lateral  points. 
Occipital  furrow  extended  out  onto  the  fixed  cheek,  and  dis- 
tinctly marked.  Palpebral  lobes  small,  semi-lunar  in  outline, 
rather  strongly  elevated.  Free  cheeks  with  the  spine  extend- 
ing to  about  the  sixth  or  seventh  thoracic  segment  in  the  only 
specimen  observed.  The  facial  sutures  begin  on  the  posterior 
margin,  distant  from  the  glabella  by  about  its  width,  pass  for- 
ward and  inward  by  very  gentle  sigmoid  curves,  then  around 
the  palpebral  lobes  with  very  short  convex  curves,  and  then 
forward  in  nearly  straight,  slightly  converging  lines  to  the 
anterior  mai'gin.  Thorax  with  sixteen  ( ?)  segments,  the 
axis  convex  and  gently  tapering  posteriorly.  Pleura  ending 
in  short  backward-curving  points,  and  strongly  grooved,  the 
grooves  parallel,  well  rounded  and  broad,  and  gradually  dying 
out  toward  the  extremity.  Pygidium  with  the  axis  elevated, 
convex,  and  terminating  in  a  rounded  end. 

When  the  specimens  are  longitudinally  compressed,  the 
glabella  becomes  sub-quadrangular  in  outline.  This  is  the 
condition  of  the  type  specimen.  The  proportions  of  length  to 
width  of  the  cephalic  shield  are  also  varied  in  this  manner.  It 
is  not  unlikely,  however,  that  some  of  this  difference  in  propor- 
tional length  and  width  is  original,  as  it  is  shown  by  several 
specimens  which  otherwise  differ  little  from  the  typical  forms. 
We  may  perhaps  assume  that  a  broad  and  a  narrow  form 
existed,  the  former  representing  the  female,  and  the  latter  the 
male. 

The  spines  are  shown  on  two  specimens,  one  a  broad  form, 
probably  somewhat  compressed  (Plate  34,  fig.  10),  and  one  a 
narrow  form,  apparently  with  normal  proportions  (Plate  34, 


676 

fig.  9).  A  third,  in  the  collection  of  Mr.  W.  W.  Dodge, 
shows  the  impression  of  the  spine.  The  following  restoration 
is  made  from  available  material,  and  is  believed  to  be  correct, 
at  least  in  its  main  features. 


Fig.  49.  Agraulos  quadrangular  is  (Whitf.).  A  restoration  made  from 
specimens  in  the  collection  of  the  Boston  Society  of  Natural  History,  the 
Student  Palaeontological  collection  of  Harvard  University,  and  tlie  collection 
of  Mr.  W.  W.  Dodge.  —  Cephalon  from  specimen  no.  13,  natural  size  (left 
palpebral  lobe  restored).  Free  cheeks  and  pygidium  from  specimen,  no.  7. 
Thorax  from  specimen,  no.  18  (right  side  restored).  Spine  from  specimen, 
no.  4.  (Parts  taken  from  smaller  specimens  are  enlarged  to  correspond  to 
the  cephalon.) 

Doubtful  features  of  the  restoi-ation.  —  Width  of  free 
cheeks  and  their  relation  to  the  cephalon.  Distance  of  genal 
spine  from  thorax.  Character  of  distal  end  of  posterior  pleura. 
Outline  and  grooving  of  pygidial  limb.  Number  of  segments 
in  thorax  ;  —  the  number  may  be  only  fifteen,  instead  of  six- 
teen as  here  represented. 

The  following  table  (p.  678)  shows  the  measurements  of  all 
the  specimens  of  this  species  which  have  come  un,der  my 
observation.  The  characters  and  present  whereabouts  of  these 
specimens  are  as  follows. 


677 

1.  Type  specimen.  Coll.  Amer.  mus.  nat.  hist.,  New 
York.  Cephalon,  with  free  cheeks,  occipital  ring,  and  part 
of  the  postero-lateral  angles  of  the  fixed  cheek,  wanting. 
The  width  of  the  cephalon  at  the  middle  of  the  glabella  is 
9  mm.  The  dimensions  of  this  specimen  were  kindly  fur- 
nished by  Dr.  E.  O.  Hovey  of  the  American  Museum.  The 
specimen  is  illustrated  in  the  Bulletin  of  the  American  Mu- 
seum, vol.  1,  pi.  14,  fig.  8,  twice  natural  size. 

2.  Specimen  figured  by  Walcott.  (See  plate  35,  fig.  1). 
Cephalon,  exclusive  of  free  cheeks  and  spine.  Coll.  mus. 
comp.  zool. 

3.  Small  specimen.  Cephalon,  exclusive  of  free  cheeks; 
part  of  spine  preserved.  Coll.  Boston  soc.  nat.  hist.,  cat. 
no.,  23. 

4.  Cephalon  of  broad  type.  Probably  compressed  longi- 
tudinally ;  with  mould.  Free  cheeks,  palpebral  lobes,  and 
occipital  ring,  wanting ;  strong  spine  shown  (Plate  34,  fig. 
IQ).      Coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11,975. 

5.  Cephalon,  exclusive  of  free  cheeks,  palpebral  lobes  on 
occipital  ring ;  slightly  compressed  laterally.  Coll.  Boston 
soc.  nat.  hist.,  cat.  no.,  11,976. 

6.  Cephalon,  exclusive  of  free  cheeks ;  normal  uncom- 
pressed form  showing  palpebral  lobes,  spine,  and  two  oblique 
transverse  glabellar  furrows  (PI.  34,  fig.  9).  Stud.  pal. 
coll.  Harv.  univ.,  cat.  no.,  333. 

7.  Nearly  entire,  imperfect  and  crushed  specimen,  with 
mould.  Shows  cephalon  with  free  cheeks,  thorax  and  pygi- 
dium  ;  no  spine.      Stud.  pal.  coll.  Harv.  univ.,  cat.  no.,  332. 

8.  Mould  showing  cephalon,  exclusive  of  free  cheeks,  and 
part  of  thorax.      Stud.  pal.  coll.  Harv.  univ.,  cat.  no.,  19. 

9.  Broad  specimen  of  cephalon,  exclusive  of  free  cheeks; 
shows  occipital  ring,  no  spine.  Stud.  pal.  col,  Harv.  univ., 
cat.  no.,  21. 

10.  Normal  form ;  specimen  showing  part  of  cephalon, 
exclusive   of  free  cheek ;  a   strong  glabellar   furrow,  occipitnl 


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679 

ring  and   part  of  thorax  are   shown.      Stud.   pal.   coll.  Harv. 
univ.,  cat.  no.,  331. 

11.  Small  specimens  of  cephalon,  exclusive  of  free  cheeks. 
Stud.  pal.  coll.  Harv.  univ.,  cat.  no.,  334. 

12.  Small  specimen  showing  part  of  cephalon.  Stud.  pal. 
coll.  Harv.  univ.,  cat.  no.,  159. 

13.  Large  cephalon,  exclusive  of  free  cheeks. 

14.  Cephalon,  exclusive  of  free  cheeks,  left  side  broken. 

15.  Small  specimen,  cephalon,  exclusive  of  free  cheeks. 

16.  Similar  to  preceding,  but  slightly  compressed,  anterior 
ends  of  cephalon  and  glabella  appearing  truncate,  and  more 
quadrangular  than  in  the  preceding  specimens. 

17.  Cephalon,  exclusive  of  free  cheeks,  base  and  sides 
broken,  — retains  impression  of  spine. 

18.  Cephalon  and  thorax  to  14th  segment,  the  cephalon 
without  the  free  cheeks  ;  pygidium  wanting. 

Nos.  13  to  18,  and  three  other  small  and  less  perfect  speci- 
mens, are  from  the  collection  of  Mr.  W.  W.  Dodge  of  Cam- 
bridge. 

19.  Cephalon  and  portion  of  thorax  preserved.  U.  S.  nat. 
mus.,  cat.  no.,  14,601. 


Genus  Ptychoparia  Corda. 

Ptychoparia  rogersi  Walcott. 

Plate  35,  fig.  2. 

1884.  Ptychoidaria  rogersi  Walcott,  Bull.  10,  U.  S.  geol. 
surv.,  p.  47,  pi.  7,  fig.  2. 

' '  This  species  is  known  only  by  the  central  portions  of  the 
head,  and  two  specimens  showing  portions  of  the  thorax. 

"  Glabella  cylindro-conical,  rounding  rather  abruptly  in 
front,  posterior  pair  of  glabellar  furrows  very  faintly  shown  in 


680 

one  specimen  ;  dorsal  furrow  strongly  defined ;  occipital  fui*- 
row  rounded,  well  marked  and  extending  out  across  the  fixed 
cheeks  ;  occipital  ring  rather  narrow  rising  at  the  centre  and 
extending  backwards  in  a  short,  strong  spine  ;  fixed  cheeks  of 
medium  width,  moderately  convex,  and  sloping  forward  to 
unite  with  the  frontal  limb  :  ocular  ridges  shown  only  on  one 
specimen  ;  starting  a  little  back  of  the  anterior  end  of  the 
glabella,  they  extend  obliquely  backward  to  the  small  palpebral 
lobe  :  frontal  limb  rather  narrow  ;  it  curves  downward  for  a 
short  distance  in  front  of  the  glabella  and  then  up  to  the 
frontal  rim.  The  facial  sutures  cut  the  anterior  margin  so  as 
to  leave  a  narrow  frontal  limb,  and  then  extend  obliquely  out- 
ward and  backward  to  the  palpebral  lobe  ;  back  of  this  they 
extend  obliquely  outward  to  the  posterior  margin  of  the   head. 

"Thorax  formed  of  well-marked,  strongly  trilobed,  narrow 
segments  ;  the  axial  lobe  about  one  third  of  the  entire  width 
anteriorly,  and  tapering  rather  rapidly  backward ;  pleural 
grooves  very  narrow.  Number  of  segments  in  the  thorax 
unknown. 

"  The  condition  of  the  preservation  of  the  surface  renders  its 
character  uncertain.      It  is  apparently  roughened  or  granulose. 

"  Owing  to  the  lateral  compression  of  the  specimen  illus- 
trated, the  form  of  the  glabella  is  too  elongate.  In  hopes  of 
getting  better  specimens  the  further  illustration  of  the  species 
is  deferred. 

"  The  specific  name  is  given  in  honor  of  Mr.  Yi.  B.  Rogers, 
the  distinguished  geologist,  who  took  so  strong  an  interest  in 
the  discovery  of  the  Braintree  Paradoxides  beds  in  1856." 

No  new  specimens  of  this  species  have  been  found,  so  far  as 
I  am  aware,  since  the  type  specimens  were  described  in  1884. 

Horizon  and  Loculit[i.  —  In  the  Middle  Cambrian  argil- 
lites  of  Hayward's  Quarry,  Braintree,  Mass.  (Coll.  mus. 
comp.  zool.,  Cambridge,  Mass.). 


681 


Ptychoparia  (?)     ATTLEBORENSis  Shaler  and  Foerste. 

1888.    PtychoiJciria  attleborensis  Shaler  and  Foerste.      Bull. 

mus.  comp.  zool. ,  vol.  16,  p.  39,  pi.  2,  fig.  14. 
1890.    Ptychoparia    (?)    attleborensis  Walcott,   10th    ann. 

rep.  U.  S.  geol.  surv.,  p.  649,  pi.  95,  fig.  2. 
1899.    Strenuella   ( ?)    attleborensis    Matthew,    Trans,    roy. 

soc.    Canada,   vol.    5,  sect.    4,    p.    77,   pi.    4,   figs. 

4  a-c. 

This  species  was  described  from  North  Attleboro,  where 
twenty  specimens  were  obtained  at  station  2,  by  Shaler  and 
Foerste. 


Genus  Paradoxides  Brono-t. 

Paradoxides  harlani  Green. 
Plate  35,  fig.  3  ;  Plates  36,  37,  38,  39. 

1834.    Paradoxides  harlani  Green,  Amer.  journ.   sci.,  vol. 

25,  p.  3S6. 
1856.    Paradoxides    harlani  W.   B.    Rogers,    Proc.    Boston 

soc.  nat.  hist.,  vol.  6,  pp.  27-29,  40-44. 
1856.    Paradoxides  harlani  Stodder,  Proc.  Boston  soc.  nat. 

hist.,  vol.  6,  p.  369. 
1856.    Paradoxides  harlani  W.   B.    Rogers,   Amer.  journ. 

sci.,  2d  ser.,  vol.  22,  p.  296. 

1858.  Paradoxides  spinosus  H.  D.   Rogers,  Geol.   Penna., 

vol.  2,  p.  816,  fig.  590. 

1859.  Paradoxides   harlani  C.    T.   Jackson,    Proc.    Boston 

soc.  nat.  hist.,  vol.  7,  pp.  54,  75. 
1859.    Paradoxides  harlani  W.   B.  Rogers,   Proc.   Boston 
soc.  nat.  hist.,  vol.  7,  p.  86. 


682 

1860.    Paradoxides  spinosus  Barrande,  Bull,  de  la  soc.  geol. 

de  France,  t.  17,  p.  551. 
1860.    Paradoxides  sjjinosus   Barrande,   Proc.   Boston   soc. 

nat,  hist.,  vol.  7,  p.  369. 

1860.  Paradoxides  harlani  Ordway,  Proc.  Boston  soc.  nat. 

hist.,  vol.  7,  p.  427. 

1861.  Paradoxides  harlani  Ordway,  Proc.  Boston  soc.  nat. 

hist.,  vol.  8,  pp.  1-5,  fig.  2. 
1861.    Paradoxides  harlani  C.  T.  Jackson,   Proc.    Boston 

soc.  nat.  hist.,  vol.  8,  p.  58. 
1863.    Paradoxides  harlani  Dana,  Man.  of  geol.,  p.   189, 

fig.  245. 
1884.    Paradoxides  harlani  Walcott,  Bull.   10,  TJ.  S.  geol. 

surv.,  p.  45,  pi.  7,  fig.  3,  pi.  8,  figs.  1,  1  a-e,  pi.  9, 

fig.  1. 

Head-shield  crescentic,  varying  from  semicircular  to  trans- 
versely semioval  in  outline,  with  the  posterior  lateral  angles 
produced  into  long  tapering  spines.  Glabella  much  elevated 
above  the  surface  of  the  cheeks  ;  rounded  in  front,  with  the 
greatest  width  in  the  anterior  third,  from  which  point  it  grad- 
ually narrows'  toward  the  neck  furrow.  Three  transverse  fur- 
rows divide  the  glabella  into  four  parts,  of  which  the  anterior 
one  is  the  largest,  while  the  remaining  three  resemble  the  seg- 
ments of  the  axis  of  the  thorax,  of  which  they  appear  to  be  the 
forward  continuation.  The  anterior  furrow  is  usually  more 
concave  forward  than  the  other  two,  though  this  is  not  always 
the  case  ;  and  it  is,  as  a  rule,  much  less  distinct.  In  fact  in 
many  specimens  it  can  only  be  distinguished  with  difficulty. 
The  last,  or  occipital,  ring  of  the  glabella  frequently  bears  a 
median  tubercle.  In  a  young  individual  of  the  broad  form 
(no.  2)  the  anterior  pair  of  furrows  is  faint  and  trending  back- 
ward. The  two  succeeding  furrows  extend  all  the  way  across 
the  glal)clla,  the  second  being  concave  forward,  the  third 
straight.     The  occipital  and  dorsal  furrows  are  strong,  and  the 


683 

occipital  ring  bears  a  prominent  tubercle.  In  a  young  individ- 
ual of  the  narrow  type  the  two  anterior  pairs  of  furrows  are 
faint,  the  third  is  stronger  and  extends  nearly,  but  not  quite,  to 
the  centre  from  each  side.  The  cephalic  limb  is  well  developed 
in  adult  individuals,  and  margined  by  a  broad  and  flat  marginal 
rim.  In  the  young  the  limb  is  very  short,  and  more  or  less 
rounded  ;  but  with  increasing  size  the  space  between  the  glab- 
ella and  the  marginal  rim  widens,  and  the  rim  becomes  broader 
and  flatter.  Palpebral  lobe  crescent-shaped,  long  and  narrow, 
the  length  in  some  specimens  exceeding  three  times  the  width 
at  the  centre. 

The  facial  suture  comprises  a  gentle  sigmoid,  and  several 
simple,  curves.  It  begins  on  the  posterior  margin  of  the  cheek 
at  a  point  about  midway  between  the  glabella  and  the  outer 
margin  of  the  cephalon,  passes  inward  and  forward  by  a  gentle 
sigmoid  curve  half-way  to  the  margin  of  the  glabella,  then, 
suddenly  bending  out  again  at  an  angle  less  than  ninety  de- 
grees, it  describes  an  unsymmetrical  curve  around  the  outer 
margin  of  the  palpebral  lobe,  after  which  it  forms  a  sharply 
rounded  re-entrant,  and  bends  outward  again  in  a  nearly  per- 
fect crescent,  the  chord  of  which  is  considerably  longer  than 
the  line  joining  the  ends  of  the  curve  around  the  palpebral 
lobe.  The  suture  finally  reaches  the  anterior  margin  at  a  point 
nearer  the  median  line  than  that  at  which  it  commenced. 
On  each  side  of  the  glabella  is  a  deep  furrow,  —  the  continu- 
ation of  the  occipital  furrow,  —  running  across  the  cheek 
parallel  to  its  posterior  margin.  The  free  cheeks  are  margined 
by  a  rim  which  is  often  very  broad  and  appears  to  have  been 
hollow  and  continued  backward  in  a  long  tapering  spine.  The 
length  of  the  spine  varies  greatly  in  different  individuals.  It 
appears  to  be  longest  in  the  narrow  forms,  and  shortest  in  the 
broad  forms.  In  an  unusually  large  specimen  in  the  collection 
of  the  U.  S.  National  Museum,  cat.  no.,  14,601,  in  which  the 
width  of  the  cheek,  measured  from  the  posterior  end  of  the  eye- 
lobe  to  the  lateral  margin,  along  a  line  normal  to  the  longi- 


684 

tudinai  axis  of  the  head,  is  45  4-  mm.,  the  spine  has  a  length 
of  nearly  140  mm. 

The  hypostoma  is  usually  detached  and  commonly  found 
entirely  isolated.  It  is  suboval  in  outline,  and  resembles  in  a 
general  way  the  anterior  lobe  of  a  very  broad  glabella.  It 
was  originally  attached  by  the  round  anterior  end  to  the  doub- 
lure or  infolded  portion  of  the  frontal  limb,  and  a  very  perfect 
specimen  thus  attached  is  figured  by  Walcott  (see  Plate  38, 
fig.  1  e).  The  surface  of  the  hypostoma  is  covered  with  con- 
centric corrugations,  which  character,  together  with  the  form,, 
serves  to  distinguish  it. 

Thorax  composed  of  eighteen  segments  in  the  adult,  which 
decrease  in  length  from  the  head  to  the  pygidium.  The  axis 
occupies  less  than  one  third  the  width  of  the  thorax,  is  rounded 
and  elevated  along  the  centre,  and  is  bounded  on  either  side  by 
a  well-defined  axial  furrow.  It  gradually  tapers  toward  the 
pygidium.  Some  of  the  segments  of  the  axis  bear  tubercles 
in  young  individuals.  The  pleura  are  flattened,  and  each  end& 
in  a  spinous  prolongation.  These  spines  or  fimbriae  progres- 
sively increase  in  length  from  before  backward  to  the  fourth  or 
fifth  segment  from  the  end,  when  they  again  decrease  in  length,- 
They  are  increasingly  deflected  backward,  especially  in  the 
posterior  segments.  A  deep  farrow  passes  obliquely  across 
each  pleuron,  from  the  inner  anterior  margin  outward  and 
backward. 

Pygidium  small,  consisting  of  a  single  or  sometimes  double^ 
narrow,  anterior  ring  and  a  broad,  more  or  less  circular,  pos- 
terior portion,  in  the  centre  of  which  is  an  elevated  but  gener- 
ally ill-defined  continuation  of  the  axis,  with  an  obtusely  rounded, 
or  more  or  less  angular  termination.  Sometimes  this  axial 
portion  is  very  strongly  defined. 

Green's  original  description  of  this  species  is  as  follows  :  — 

"  The  contour  of  the  buckler  in  this  species,  cannot  be  satis- 
factorily determined  from  our  present  specimen  :  the  anterior 
and  posterior  parts   of  it  are  well  defined,  but  the  cheeks  on 


685 

each  side  are  either  mutihited  or  obscured.  T1\\q  front  is  very 
much  elevated  above  the  surface  of  the  cheeks.  It  rises  a  little 
before  the  anterior  edge  of  the  buckler,  is  rounded  in  front, 
and  gradually  tapers  towards  the  middle  lobe  of  the  abdomen, 
with  which  it  forms  a  i*egular  continuation.  On  its  posterior 
surface  there  are  thi-ee  transverse  furrows  ;  the  upper  one 
crosses  it  a  little  obliquely,  and  there  is  on  each  side  above,  a 
considerable  protuberance.  The  cheeks  were  no  doubt  in  the 
form  of  spherical  triangles,  but  whether  the  outer  angles  termi- 
nated in  acute  prolongations,  cannot  from  our  specimen  be 
determined.  The  organs  of  vision  appear  to  be  entirely 
wanting.  There  are  two  shallow  depressions  on  each  side  of 
the  cheeks,  commencing  near  the  protuberances  on  the  front, 
and  running-  towards  the  lateral  edo'es  of  the  buckler.  The 
posterior  border  of  the  buckler  where  it  joins  the  lobes  of  the 
abdomen,  is  marked  by  a  transverse  groove,  nearly  continuous 
with  the  lower  transverse  furrow  on  tlie  front ;  this  groove  at 
its  commencement,  appears  to  bifurcate  outwards. 

"  The  abdomen  and  tail  cannot  be  distinguished  from  each 
other.  There  are  seventeen  distinct  articulations  in  both.  The 
middle  lobe  is  very  convex,  and  is  separated  from  the  lateral 
ones,  by  a  deep  channel ;  it  gradually  tapers  to  an  obtuse  tip. 
In  our  specimen  there  is  a  small  part  of  the  tail  of  another 
trilobite  deposited  in  this  place,  which  at  first  sight  appears  to 
be  a  dislocated  fragment  of  our  animal . 

' '  The  lateral  lobes  are  flattened  ;  the  costal  arches  are  very 
distinct  near  their  insertion,  and  for  about  half  their  length, 
but  towards  their  free  extremities  they  are  a  good  deal  obliter- 
ated. There  appears  to  have  been  a  delicate  membranaceous 
'prolongation  for  a  considerable  distance  beyond  the  solid  por- 
tion of  each  rib.  This  organization  is  very  apparent  on  the 
costal  arches  of  the  tail.  There  is  a  deep  groove  running 
obliquely  over  the  upper  surface  of  eacii  rib.  Length  of  the 
fossil  about  9  inches  ;  breadth,  about  4  inches." 

This  specimen  (Plate  36)    is  now  in  the  collection  of  the 


686 

Boston  Society  of  Natural  History  (cat.  no,,  6264).  It  is 
on  a  rock  fragment  twelve  inches  long,  by  a  little  over  three 
inches  v^^ide.  The  greater  part  of  the  left  side  is  cut  off  by  a 
joint  face,  and  the  cheeks  are  wanting.  The  left  facial  suture 
can  be  traced  part  way  across  the  cephalon,  and  the  left  palpe- 
bral lobe  is  visible  in  outline,  though  the  greater  part  is  broken 
away.  The  right  free,  and  a  portion  of  the  right  fixed, 
cheek,  with  the  palpebral  lobe,  are  broken  away,  though  the 
suture  can  be  traced  part  way.  The  fimbriae  of  the  anterior 
two  or  three  pleura  of  the  right  side,  as  well  as  those  of 
several  of  the  more  posterior  segments,  ax"e  mutilated.  The 
pygidium,  which  Green  regarded  as  a  part  of  another  trilobite, 
is  moved  out  of  place.  The  two  protuberances  on  the  glabella, 
mentioned  by  Green ,  are  not  a  part  of  the  fossil  but  adhesions 
of  the  rock  matrix.  The  two  anterior  glabellar  furrows  are 
also  filled  with  the  rock  matrix,  and  hence  barely  visible.  The 
axis  of  the  thorax  is  perfect  down  to  about  the  eleventh  seg- 
ment. The  last  two  segments  are  considerably  mutilated,  and 
were  evidently  counted  as  one  by  Green,  who  states  that  the 
fossil  has  seventeen  thoracic  segments.  The  number  is  dis- 
tinctly eighteen.  The  last  two  segments  are  each  shorter  than 
the  antepenultimate  segment,  but  together  they  are  longer, 
being  6.5  mm.  in  combined  length,  while  the  segment  preceding 
them  has  a  length  of  only  5  mm.  The  axis  is  very  convex. 
The  pleura  are  strongly  grooved,  the  grooves  running  from 
before  backward  and  outward,  being  deeply  rounded.  Some  of 
the  anterior  pleura  are  sheathed  over  those  lying  posterior  to 
them,  thus  partly  obliterating  the  dividing  line  between  them. 
There  is  a  slight  forward  displacement  of  all  the  pleura  on 
both  sides,  owing  no  doubt  to  a  down-crushing  of  the  axis,  the 
lateral  furrows  at  the  same  time  being  deepened.  This  dis- 
placement brings  the  grooves  on  the  pleura  opposite  the  sutures 
between  the  axial  rings.  It  has  also  affected  the  relative 
position  of  the  transverse  groove  of  the  fixed  cheeks.  The 
fimbriae  of  the  last  six  segments  are  very  long,  and  progres- 
sively decrease  in  length  toward  the  pygidium. 


687 

The  pygidium  is  of  the  transverse  type,  with  two  narrow 
rings  and  a  short  triangular  protuberance  behind  the  rings. 
The  dimensions  of  the  specimen  are  given  in  the  table  (p.  690 
no.  16). 

A  small  specimen  from  the  collection  of  the  Bridgewater 
Normal  School  shows  some  features  not  usually  seen  (Plate 
37).  The  laterally  displaced  hypostoma  is  seen  from  the 
inside,  attached  to  the  doublure.  From  this  specimen  it 
appears,  that  the  facial  suture,  after  reaching  the  frontal  mar- 
gin, passes  over,  and  bends  inward  at  a  slight  angle,  on  the 
doublure,  and  then  gently  curves  outward  again  to  the  poste- 
rior margin  of  the  doublure.  On  the  free  cheek  is  indicated  the 
hollow  character  of  the  marginal  rim,  a  portion  of  the  under 
side  of  which  is  shown  by  the  breaking  away  of  the  upper  por- 
tion. This  under  portion  shows  a  sutural  margin  correspond- 
ing to  that  of  the  doublure  of  the  frontal  limb,  but  not  to  that 
on  the  fixed  cheek.  The  spines  appear  also  to  have  been  hol- 
low. The  number  of  thoracic  segments  in  this  specimen  is 
seventeen  (see  table,  p.  690,  no.  8). 

A  young,  very  perfect  specimen  (except  for  the  lateral 
cheeks)  in  the  collection  of  the  U.  S.  National  Museum,  cat. 
no.,  14,601,  shows  seventeen  segments  in  the  thorax,  thus 
corresponding  in  age  to  the  specimen  from  the  Bridgewater 
Normal  School  (see  specimen,  no.  7  in  table,  p.  690).  It  has 
a  pygidium  of  the  circular  type  —  being  as  long  as  broad,  and 
having  one  narrow  but  strongly  defined  anterior  axial  ring. 

Two  distinct  forms  of  this  species  may  be  recognized,  a  nar- 
row, and  a  broad  one.  Following  Barrande's  suggestion,  the 
narrow  form  may  be  regarded  as  the  male  and  the  broad  one  as 
the  female. 

Among  the  American  relatives  of  P.  harlani,  the  most 
nearly  related  to  the  broad  form  is  P.  regina  Matthew,  from 
the  St.  John  Group  of  New  Brunswick.  In  fact  I  have  been 
unable  to  find  very  much  difference  between  that  species  and 
some  of  our  very  broad  forms  with  short  cephalic  shield  and 


688 

broad  marginal  rim.  In  a  specimen  in  the  collection  of  the 
Boston  Society  of  Natural  History  (no.  19)  which  most  nearly 
resembles  the  New  Brunswick  species,  the  genal  spines  are 
defective,  but  they  probably  did  not  greatly  exceed  in  length 
those  of  jP.  regina.  In  another  specimen  (no.  26)  the  dimen- 
sions are  almost  identical  with  P.  regina.  Mr.  Walcott 
regards  P.  hennetti  Salter,  from  the  Cambrian  of  St.  Mary's 
Bay,  Newfoundland,  as  a  closely  related  form.  He  speaks  of 
two  specimens  from  that  locality  which  closely  resembles  P. 
harlani.  The  original  figure  and  description  by  Mr.  Salter 
(Quart,  journ.  geol.  soc,  vol.  15,  p.  552)  show  some  impor- 
tant differences,  e.  g.,  in  the  small  size  of  the  free  cheeks, 
which  brings  the  broad  marginal  rim  nearer  to  the  glabella 
than  in  any  of  our  specimens,  in  the  very  short  genal  spines 
which  are  close  to  the  thorax,  and  in  the  smaller  extension  of 
the  pleura,  especially  those  near  the  pygidium,  though  these, 
in  the  specimen  figured,  have  been  restored. 

The  nearest  relative  of  the  narrow  form,  is  the  Bohemian 
P.  spinostis  Boeck.  Barrande  considered  the  two  identical, 
making  his  comparison  with  a  cast  of  the  type  specimen  of 
P.  harlani  and  photographs  of  others,  sent  him  by  Professor 
Rogers.  In  a  communication  published  in  the  Proceedings  of 
the  Boston  Society  of  Natural  History,  vol.  8,  1861,  Mr.  Al- 
bert Ordway  discussed  the  comparison  made  by  Barrande, 
and  showed  some  important  differences  between  the  two  species, 
establishing  the  distinctness  of  each.  His  comparisons  were 
evidently  made  with  specimens  of  the  broad  form  of  P.  harlani, 
an  outline  of  the  head  of  which  he  figures.  The  resemblance 
would  have  been  much  greater  had  he  figured  a  narrow  form 
oi  P.  harlani.  In  this,  however,  the  spines  are  longer  than 
in  the  Bohemian  species,  and  slightly  curved  outward  rather 
than  inward  near  the  extremity.  The  outline  figure  of  the 
head  given  by  Mr.  Ordway  as  that  of  P.  harlani  resembles 
P.  regina  very  closely. 

The  following   list  comprises  the  specimens  of  Paradoxides 


689 

harlani  which  I  have  been  able  to  examine.  Numbers  1  to 
27  have  been  measured,  and  the  results  are  given  in  the  ap- 
pended table  of  actual  measurements.  They  have  been  ar- 
ranged progressively  from  the  smaller  to  the  larger,  taking  the 
length  of  the  cephalon  —  the  most  constant  measurement  ob- 
tainable—  as  a  basis.  In  the  table  of  proportional  measure- 
ments the  length  of  the  cephalon  is  taken  as  100,  and  the 
others  calculated  on  that  basis.  While  this  will  readily  allow 
comparison  between  the  specimens,  it  is  apt  to  lead  to  misap- 
prehension, unless  the  fact  is  borne  in  mind  that  proportions 
are  apt  to  vary  with  the  age  of  the  individual.  For  this  rea- 
son, only  individuals  of  nearly  the  same  size  should  be  com- 
pared for  purposes  of  showing  variation.  The  measurements 
of  Paradoxides  regina  Matthew,  taken  from  a  cast  of  the 
type  specimen,  are  given  to  show  the  close  resemblance  be- 
tween this  species  and  the  broader  forms  of  P.  harlani. 
Except  in  the  greater  width  of  the  glabella,  the  type  of  P. 
regina  scarcely  differs  in  actual  measurements  from  specimen 
number  26  of  P.  harlani  from  Braintree. 

1.  Cephalon  of  a  young  individual  of  narrow  form,  exclu- 
sive of  free  cheek.     Coll.  W.  W.  Dodgfe. 

2.  Cephalon  of  a  young  individual  of  broad  form,  the  occi- 
pital ring  showing  a  strong  tubercle.  Free  cheeks  wanting. 
Coll.  W.  W.  Dodge. 

3.  Cephalon  of  young  individual,  exclusive  of  free  cheeks 
(Plate  38,  fig.  1).      Coll.  mus.  comp.  zool. 

4.  Cephalon  of  young  individual,  exclusive  of  free  cheeks 
(Plate  38,  fig.  1  a).      Coll.  mus.  comp.  zool. 

5.  An  immature  individual,  with  10  segments  shown  in  the 
thorax,  the  lower  part  of  which,  with  the  pygidium,  is  want- 
ing. Free  cheeks  also  wanting.  Coll.  mus.  comp.  zool. 
(With  counterpart.) 

6.  Young  individual.  Stud.  pal.  coll.  Harv.  univ.,  cat. 
no.,  328. 

7.  Young,  nearly  pei'fect  specimen,  except  for  lateral  cheeks  ; 
17  segments  in  thorax;  U.  S.  nat.  mus.,  cat.  no.,  14,601. 
(With  counterpart.) 

OCCAS.  PAPERS   B.   S.  N.  H.   IV.  44. 


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692 

8.  Nearly  complete  small  specimen,  showing  genal  spines, 
hypostoma,  etc.,  and  17  segments  in  the  thorax.  Coll.  Bridge- 
water  normal  school  (Plate  37). 

9.  Immature  individual,  with  15  thoracic  segments,  and 
with  cephalon  and  pygidium  displaced.  Free  cheeks  wanting. 
Coll.  mus.  comp.  zool. 

10.  Central  portion  of  a  large  cephalon.  Coll.  mus.  comp. 
zool. 

11.  Cephalon,  exclusive  of  free  cheeks.  Coll.  mus.  comp. 
zool. 

12.  Imperfect  cephalon,  exclusive  of  free  cheeks.  Coll. 
mus.  comp.  zool. 

13.  Nearly  perfect  specimen,  except  free  cheeks.  Shows 
tubercle  on  occipital  ring.  Coll.  Boston  soc.  nat.  hist.,  cat. 
no.,  2753. 

14.  Nearly  perfect  specimen,  parts  restored.  Coll.  Boston 
soc.  nat.  hist.,  cat.  no.,  2754  (Plate  39).  Counterpart,  cat. 
no.,  6265. 

15.  Specimen,  with  free  cheeks  and  great  part  of  thorax 
wanting.      Coll.  mus.  comp.  zool. 

16.  The  type  specimen.  Coll.  Boston  soc.  nat.  hist.,  cat. 
no.,  6264  (Plate  36). 

17.  Individual  showing  part  of  head  and  anterior  thoracic 
segments.      Stud.  pal.  coll.  Harv.  univ.,  cat.  no.,  324. 

18.  Large  specimen  ;  anterior  portion  of  cephalon,  one  side 
of  cephalon  and  thorax,  and  posterior  part  of  thorax,  bi'oken 
away.      Coll.  mus.  comp.  zool.,  exhibition. 

19.  Head  shield  with  anterior  thoracic  segments  and  short 
spines.     Coll.  Boston  soc.  nat.  hist.,  cat.  no.,  12040. 

20.  Showing  cephalon  and  part  of  thorax.  Coll.  Boston 
soc.  nat.  hist.,  cat.  no.,  4573. 

21.  Fragmentar}'  specimen,  showing  17  thoracic  segments 
and  a  fragment  of  the  18th,  also  very  long,  slender,  palpebral 
lobes.  One  side  of  thorax,  free  cheeks,  and  part  of  head, 
broken  away.      Stud.  pal.  coll.  Harv.  univ.,  cat.  no.,  325. 


693 

22.  Imperfect  specimen  showing  part  of  head  and  part  of 
thorax  (with  counterpart).  Coll.  Boston  soc.  nat.  hist.,  cat. 
nos.,  2  and  3. 

23.  Showing  free  cheek  and  spine.  Part  of  head  and 
thorax  broken  away.  Stud.  pal.  coll.  Harv.  univ.,  cat.  no., 
556. 

24.  Large  specimen  with  counterpart,  the  two  showing 
nearly  all  the  features.  Coll.  Boston  soc.  nat.  hist.,  cat.  no., 
2756. 

25.  Large  specimen  with  imperfect  glabella  and  thorax. 
Cheek  figured  by  Walcott.  (See  Plate  35,  fig.  3).  Coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  1. 

26.  Large  specimen,  showing  head  with  part  of  the  free 
cheeks,  and  a  part  of  the  thorax.  Stud.  pal.  coll.  Harv.  univ., 
cat.  no.,  330. 

27.  Showing  fragment  of  cephalon  and  part  of  thorax. 
Free  cheeks  and  part  of  fixed  cheeks  wanting.  Coll.  Mass. 
inst.  technology. 

Also  other  fragmentary  specimens  in  that  collection. 

28.  Paradoxides  regina  Matthew.  From  a  cast  of  the 
type  specimen.     Stud.  pal.  coll.  Harv.  univ. 

Horizon  and  Locality.  —  In  the  siliceous  argillites  of  Mid- 
dle Cambrian  age  at  Hay  ward  Creek,  near  South  Quincy, 
township  of  Braintree,  Massachusetts,  numerous  specimens. 
Also  in  similar  rock  on  the  land  of  Mr.  T.  A.  Watson  in 
Weymouth,  Mass.,  and  in  shaly  beds  at  East  Braintree. 

Thanks  are  due  to  the  following  gentlemen  for  the  use  of 
specimens  of  fossils  :  To  Prof.  Alpheus  Hyatt  for  access  to  the 
specimens  in  the  collections  of  the  Boston  Society  of  Natural 
History,  and  the  Museum  of  Comparative  Zoology.  To  Prof. 
R.  T.  Jackson  for  access  to  the  specimens  in  the  Student 
Palaeontological  Collection  of  Harvard  University.  To  Mr. 
Charles  D.  Walcott  and  Mr.  Chas.  Schuchert  for  access  to  the 
collections  of  the  U.  S.  Geological   Survey  and  of  the  U.  S. 


694 

National  Museum.  To  Mr.  H.  T.  Burr  for  the  use  of  his 
collection  of  North  Weymouth  fossils.  To  Mr.  Boyden  of 
the  Bridgewater  Normal  School  for  the  loan  of  the  specimen 
of  Paradoxides  illustrated  on  Plate  37.  To  Mr.  John  H. 
Sears  I  am  greatly  indebted  for  the  loan  of  the  extensive  col- 
lection of  Nahant  fossils  which  he  has  made  for  the  Peabody 
Academy  of  Sciences.  Mr.  T.  A.  Watson  has  supplied  much 
new  material,  and  has  also  aided  greatly  in  the  exploration  of 
the  ledges  on  Pearl  Street,  North  Weymouth.  To  Mr.  W. 
W.  Dodge  I  am  much  indebted  for  the  use  of  his  collection  of 
Braintree  fossils.  Other  acknowledgments  are  due  Mr.  J. 
B.  Woodworth,  Prof.  W.  H.  Niles,  Dr.  E.  O.  Hovey,  Mr. 
W.  E.  Hobbs,  and  Mr.  Robert  Burke. 

To  Miss  Elvira  Wood  and  Miss  L.  R,.  Martin  thanks  are 
due  for  the  care  and  labor  bestowed  on  the  illustrations  of  the 
fossils.  The  U.  S.  Geological  Survey  granted  the  privilege  of 
making  electrotype  copies  from  their  original  plates  of  Brain- 
tree  fossils. 


ERRATA. 

Page    299,  fourth  line  from  bottom,  for  PL  16  read  PL  20. 

"  300,  first  and  second  lines  from  top,  for  PL  17  read 
PL    21 ;  for  PL  18  read  PL  22. 

"     300,  fifth  line  from  bottom,  for  PL  15  read  PL  19. 

"  301,  eleventh  line  from  bottom,  for  PL  19  read 
PL  26. 

"     340,  ninth  line  from  the  top,  for  PL  14  read  PL  16. 

"  610,  in  the  table,  before  "31.  Aristozoe?"  insert: 
''''  Leperditia^  cf.  L.  solitaria  (?)";  and  oppo- 
site it,  in  column  10,  insert ;  "  r." 


Occas.  Papers,  Vol.  IV. 


PLATE   31. 

(Figures  3,  4,  6  a-c,  9  a-f,  10  c,  11  b,  and  12  a  were  drawn  by  Miss  Elvira 
Wood,  Instructor  in  Palaeontology  in  the  Massachusetts  Institute  of  Tech- 
nology ;  the  remainder  were  drawn  by  Miss  L.  R.  Martin. ) 

Eigs.  1  a-d.    Acrothele  gamagei  (Hobbs.)  P.  615. 

1  a.    Pedicle  valve  showing  a  large  foramen  just  posterior  to  the  beak. 
The  area  is  not  well  shown.     X  2. 

1  b.     Pedicle  valve  showing  a  ratlier  abrupt  flattening  of  the  posterior 
portion.'   There  is  a  suggestion  of  a  faint  median  pseudo-deltidium. 
No  foramen  is  shown.      X  2. 

1  c.     Pedicle  valve  with  the  sides  merging  posteriorly  into  the  flattened 
area.     No  foramen  is  shown.     X  2. 

1  d.    Exterior  of  a  brachial  valve  showing  the  hinge-line  and  the  con- 
cavity on  either  side  of  tlie  beak. 
Paradoxides  beds,  Hayward's  Quarry.     Coll.  W.  W.  Dodge. 

Eig.  2.     Iphidea  bella  Billings  (?).  P.  617. 

Ventral  valve,  showing  area  and  pseudo-deltidium.     X  1^. 
Nahant  limestone,  Nahant,  Mass.      Coll.  Peabody  acad.  sci.,  cat.  no.,  705. 

Eig  3.     Obollelacrassa  (Jisll)  {?).  P.   619. 

Surface  and  lateral  views.     X  2. 
Nahant  limestone,  Nahant,  Mass.     Coll.  Boston  soc.  nat.  hist.,  cat.  no., 
11,970. 

Eig.  4.    Lingulella  rogersi  Walcott.  P.  624. 

Valve   from    the    original    specimen  described    by   Professor   Rogers. 
(From  a  gutta  percha  cast.)     X  2. 
Quartzite  pebble,  from  conglomerate,  Fall  River,  Mass.     Coll.  Boston  soc. 
nat.  hist.,  cat.  no.,  2750. 

Eig.  5.     Parmophorella  acadica  (Hartt).  P.  625. 

A  crushed  and  broken  shell. 
Paradoxides  beds,  Hayward  Creek.     Stud.  pal.  coll.  Harv.  Univ.,  cat.  no., 
503. 

Eigs.  6  ar-c.     Scenella  (?)  sp.  P.  627. 

Summit,  lateral  and  posterior  view  of  a  specimen  from  which  the  shell 
has  been  removed.      X  8. 
Red  Lower  Cambrian  limestone  bowlder.  Pleasant  Beach,  Cohasset.    Coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  11,971. 


PLATE  ol  (continued). 

Figs.  7  a,  b.     Platyceras  prlmaevum  Billings.  P.  028. 

7  a.     The  shell  from  the  apical  end  of  the  spire.     The  centre  is  broken 

away.     X  12. 
7  b.     Dorsal  view  of  the  same,  showing  the  obliquity  of  the  body  whorl, 
its  angulation  along  the  dorsum,  and   the  rapid  widening  toward 
tlae  aperture.     X  12. 
Red  Lower  Cambrian  limestone  bowlder,  Sandy  Cove,   Cohasset.     Coll. 
Boston  soc.  nat.  hist.,  cat.  no.,  11,960. 

Figs.  8  a,  b.     Platyceras  cleflectum  Grabau.  P.  630. 

Top  and  side  view  of  the  type.     X  16. 
Lower  Cambrian  limestone  bowlder,  Sandy  Cove,  Cohasset.     Coll.  Boston 
soc.  nat.  hist.,  cat.  no.,  11,965. 

Figs.  9  a-f.     Watsonella  crosbyi  Grabau.  P.  632. 

9  a.     Right  side  of  a  specimen  with  the  angulation  somewhat  rounded 
and  the  lines  of  growth  arching  far  up  on  the  dorsum.     X  5. 
Pleasant  Beach,  Cohasset. 

9  b.     Right  side  of  another  individual  of  normal  form.     X  5. 
Sandy  Cove,  Cohasset. 
9  c.     Right  side  of  a  larger  individual  with  broken  beak,   and  faint 
radiating  striae.     X  5. 
Sandy  Cove,  Cohasset. 
9  d.     Left  side  of  a  fragment.     X  5. 

9  e.     Apertural  section  of  same,  shown  on  a.  broken  surface  below  the 
centre.     X  5. 

9  f.    View  of  dorsum  near  the  aperture,  showing  keel.     The  specimen 

is  compressed  near  the   aperture,  hence  the   dorsum  is  narrower 

below.     X  10. 
Red  Lower  Cambrian  limestone  bowlders  from  Cohasset. 
Coll.  Boston  soc.  nat.  hist.,  cat.  nos.,  11,952,  11,951,  11,953,  11,954. 

Fig.  10  a-c.     Eaphistoma  attleborensis  Shaler  and  Foerste.  P.  633. 

10  a.     Base  of   a  specimen  showing  dee,p   and  wide  umbilicus,  and  a 

notch  in  the  outer  lip.     X  5. 
10  b.     Side  view  of  same,  showing  fiat  base,  compressed  whorls,  and  pro- 
jecting aperture.     X  5. 
Pleasant  Beach,  Cohasset. 

10  c.     Lateral  view  of  a  specimen  with  the  whorls  less  compressed  than 

in  the  preceding,  and  rather  loosely  superimposed.     X  7. 
Sandy  Cove,  Cohasset.     Red  Lower  Cambrian  limestone  bowlders  from 
Cohasset. 
Coll.  Boston  soc.  nat.  hist.,  cat.  nos.,  11,955  and  11,957. 

Figs.  11  a,  b.     Straparollina  remota  Billings.  P.  635. 

11  a.    Top  view  of  a  nearly  complete  specimen  with  the  tip  broken. 

X  4. 


PLATE  31  (continued). 

11  b.    Profile  of  same,  showing  flat  base  and  rounded  upper  surface  of 

the  whorls.     X  4. 
Ked  Lower  Cambrian    limestone    bowlder,    Pleasant    Beach,    Cohasset. 
Coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11,958. 

Figs.  12  a-c.     Stenotheca  abrupta  Shaler  and  Foerste.  P.  637. 

12  a.     Longitudinal  section  of  a  specimen  probably  belonging  to  this 

species.     X  8. 
White  Nahant  limestone,  Nahant,  Mass.     Coll.  Peabody  acad.  sci. 

12  b.     Mould  of  the  interior,  showing  ten  corrugations.     The  basal  corru- 
gation is  broken  away,  and  the  apex  is  imperfect.     X  8. 
12  c.     Interior  of  the  shell  of  the  same  specimen,  adhering  to  the  rock 
matrix.     The  fragment  shows  the  thickened  margin  on  the  concave 
ventral  side  ;  also  faint  vertical  striae.      X  8. 
Red  Lower  Cambrian  limestone   bowlder,  Bass  Point,   Nahant.     Coll.. 
Boston  soc.  nat.  hist.,  cat.  no.,  11,962. 

Fig.  13.     Stenotheca  curvirostra  Shaler  and  Foerste.  P.  638. 

Lateral  view  of  a  specimen  witli  strong  corrugations  over  the  dorsum, 
fading  ventrally.     X  12. 
Red  Lower  Cambrian  limestone  bowlder,  Sandy  Cove,  Cohasset.     CoU.- 
Boston  soc.  nat.  hist,  cat.  no.,  11,964. 

Fig.  14.     Stenotheca  pauper  Billings.  P.  639. 

A  small  specimen  witli  coarse  corrugations  across  tlie  dorsum,  fading 
ventrally.     X  12.     The  shell  is  broken  along  the  dorsum. 
Red  Lower  Cambrian  limestone  bowlder,  Sandy  Cove,  Cohasset.      Coll. 
Boston  soc.  nat.  hist,  cat  no.,  11,963. 

Fig.  15.     Stenotheca  levis  Walcott.  P.  641. 

A  specimen  with  strong  rounded  corrugations  near  tlie  venter  only. 
X  12. 
Red  Lower  Cambrian  limestone  bowlder,  Pleasant  Beach,  Cohasset.     ColL 
Boston  soc.  nat.  hist,  cat.  no.,  11,961. 


Occas. Papers, Boston  Soc.  Nai.Hisi..Vol.IV. 


Plate  31 


'■fill 


Cambrian  Fossils  of  the  Boston  Basin. 


HcHotpc  Prirrtinfl  Ci-j,Bonior., 


Occas.  Papers,  Vol.  IV. 


PLATE  32. 

(Figures  1  a,  b,  7  c,  8  e,  f  and  9  c,  were  drawn  by  Miss  L.  R.  Martin.     The 
remainder  were  drawn  by  Miss  Elvira  Wood.) 

Figs.  1  a,  b.     Hyoliihes  shaleri  Walcott.  P.  642. 

1  a.     View  of  ventral  side  of  a  fragment,  with  cross-section.     Nat.  size- 

1  b.     Dorsal  face   of  a  fragment  embedded  in  rock,  showing  lip.     Nat. 

size. 
Paradoxides  beds,  Hayward  Creek.     Coll.  W.  W.  Dodge. 

Figs.  2  a-h.     HyoUthes  pr biceps  Billings.  P.  643. 

2  a.     Dorsal  side  of  a  young  individual  showing  forward-curving  striae 

and  lip.     X  1^- 
2  b.     Fragment  of  a  specimen  showing  dorsal  side  and  cross-section.  X  If- 
2  c.     Cross-section  referred  to  this  species,  obliquely  cut.     Enlarged.   . 
2  d,  e.     Sections  of  large  specimens  referred  to  this  species.     X  — . 

2  f-h.     Various    cross-sections,    the  variations    in    outline   being  due 

chiefly  to  obliquity  of  cut.     Enlarged. 
White  limestone,  Nahant.     Coll.  Peabody   acad.  sci.,  cat.  nos.  698,  695, 
708,  709,  694,  700,  694  a. 

Figs.  3  a-c.     HyoUthes  excellens  Billings.  P.  646. 

3  a     Dorsal  side  of  specimen  described.    Nat.  size. 
3  b.     Cross-section  of  same. 

3  c.     Cross-section  of  a  specimen  of  this  species  nat.  size.     White  lime- 

stone, Nahant.    Coll.  Peabody  acad.  sci.,  cat.  nos.,  708  a,  708  b. 

Figs.  4  a,  b.     HyoUthes  americanus  Billings.  P.  647. 

4  a.    Dorsal  view  of  the  specimen  described  ;  the  upper  part  is  broken 

away.     X  1^. 

4  b.     Cross-section  of  same. 

White  limestone  at  Nahant.     Coll.  Peabody  acad.  sci.,  cat.,  no.,  708. 

Figs.  5  a,  b.    HyoUthes  searsi  Grabau.  P.  649. 

5  a.     Cross-section  of  the  largest  specimen  known.     Nat.  size. 

White  limestone.  East  Point,  Nahant.     Coll.  Peabody  acad.  sci.,  cat.  no. 

710. 
5  b.    A  group  of  cross-sectioixs.    Nat.  size. 
White  limestone,  north  side  of  Nahant.     Coll.  Boston  soc.  nat.  hist.,  cat. 
no.,  11,967. 


PLATE  32  (continued). 

Fig  6.     Hyolithes  communis  Billings.  P.  651. 

Cross-section,  showing  normal  form. 
White  limestone,  Nahant.     Coll.  Peabociy  acad.  sci.,  cat.  no.,  708  c. 

Figs.  7  a-c.     Hyolithes  impar  Ford.  P.  652. 

7  a.     A  normal  shell  with  oval  cross-section.     X  1^- 
7  b.    Fragment  (restored),  witli  cross-section,     x  2. 

7  c.     A  cross-section.     Enlarged. 

White  limestone,  Nahant.    Coll.  Peabody  acad.  sci.,  cat.   nos.,  700,  698  a, 
702. 

Figs.  8  a-g.     Orthotheca  cylindrica  Grabau.  P.  654. 

8  a,     A  fragment  of  a  characteristic  small  specimen,  showmg  regular 

concentric  striae  and  a  constriction  at  one  end.  x  3. 
Red  limestone  bowlder,  Cohasset.     Coll.  Boston  soc.  nat.  hist.,  cat.  no., 
11,963. 

8  b.     Fragment  of  a  larger  specimen.  X  1  ^ 
White  limestone,  Nahant.     Coll.  Peabody  acad.  sci.,  cat.  no.,  693  a. 

8  c.     Cross  and  longitudinal  sections,  the  latter  showing  a  septate  apical 

portion.     Nat.  size. 
8  d.     Cross-section  of  two  invaginated  specimens.     Enlarged. 
8  e.     Fragment  with  septate  apical  portion.  X  3J. 
8  f.    Two   invaginated  specimens,   the  inner  one  showang  concentric 

striae.  X  3^.    Associated  with  the  preceding. 
8  c-f.     From  red  limestone  bowlders  from   Cohasset  and  Bass  Point. 
Coll.  Boston  soc.  nat.  hist.,  cat.  nos.,  11,963,  11,959,  etc. 

8  g.     A  small  curved  specimen  which  may  be  of  this  species.  X  1^. 
White  limestone,  Nahant.     Coll.  Peabody  acad.  sci.,  cat.  no.,  693. 

Figs.  9  a-d.     Orthotheca  emmonsi  (Ford).  P.  655. 

9  a.    Dorsal  view  of  a  specimen  showing  faint  concavity.     Nat.  size. 
9  b.     A  characteristic  ci'oss-section. 

9  c.     Cross-sections  of  invaginated  shells. 
9  d.     Ventral  view  of  a  specimen  with  cross-section. 
White  limestone  Nahant.     Coll.  Peabody  acad.  sci.,  cat.  nos.,  703,  697, 
703  a,  703  b. 

Fig.  10.     Orthotheca  (?)  foerstei  Grabau.  P.  657. 

Cross-section  of  a  group  of  invaginated  shells  referred  to  this  species. 
Enlarged. 
Red  limestone,  Pleasant  Beach,  Cohasset.     Coll.   Boston  soc.   nat.  hist., 
cat.  no.,  11,973. 

Fig.  11.     Hyolithes  (?)  haywardensis  Grabau.  P.  653. 

Type  specimen,  showing  several  layers.  X  1\. 
Paradoxides  beds,  Hayward  Creek.     Coll.  Boston  soc.  nat.  hist.,  cat.  no., 
11,974. 


PLATE   32  (Continued). 

Fig.  12.    Hyolithellus  micans  Billings.  P.  658. 

A  fragment  referred  to  this  species.  X  3. 
Ked  limestone  bowlder,  Nahant.      Coll.  Boston  soc.  nat.  hist.,  cat.  no. 
11,966. 

Fig.  13.     Longitudinal  section  of  a  hyolithid  (?),  showing  two  septa.  X  10. 
White  limestone,  Jeffrey's  Ledge.     Coll.  Peabody  acad.  sci.,  cat.  no.,  714. 

Fig.  14.    Longitudinal  section  of  an  undetermined  shell.  X  10. 

White  limestone,  Jeffrey's  Ledge.     Coll.  Peabody  acad.  sci.,  cat.  no.,  712. 

Fig.  15.     Longitudinal  section  of  a  Salterella  (?).  X  4.     White  limestone, 
Jeffrey's  Ledge.     Coll.  Peabody  acad.  sci.,  cat.  no.,  712  a. 


Occas. Papers, Boston  Soc.Nat.Hist„Vol.Iv: 


Plate  32. 


Cambrian  Fossils  of  the  Boston  Basin. 


Occas.  Papers,  Vol.  IV. 


PLATE  33. 

(All  the  figures  were  drawn  by  Miss  Elvira  Wood,  Instructor  in  Palaeon- 
tology in  the  Massachusetts  Institute  of  Technology.) 
Figs.  1  a-j.      Olenellus  {Holinia)  broggeri  Walcott.  P.  662. 

la.     A  young  head,  crushed  and  broken,  but  showmg  the  characteristic 
features.     X  2. 

1  b.     Fragment  of  cephalon. 

1  c.     A  large  fragmentary  c  phalon,  much  crushed. 

1  d.     Central  portion  of  ce^    alon,  laterally  compressed.    (From  a  gutta- 
percha cast.) 

1  e.     Part  of  lateral  cheek,  showing  interocular  spine. 

1  f .     Hypostoma  without  ihe  lateral  wings. 

1  g.     Ends  of  pleura,  referred  to  this  species. 

1  h.     A  pleuron,  showing  the  characteristic  groove. 

1  i.     Left  cheek  of  a  yorng  individual,  showing  interocular  s^Dine. 

1  j.     Doublure,  referred  to  this  species. 
(Figs.  1  b  to  1  j  are  natural  size.) 
Fig.  2.     Urotheca  pervetus  Matthew.  P.  6-59. 

A  fragment.     Enlarged. 

Fig.  3.     Microdisms   sp.  P.  671. 

A  pygidium.     X  4. 
Lower  Cambrian  Slates,  Pearl  St.,  North  Weymouth.     Coll.  H.  T.  Burr,  etc. 


Occas. Papers,  Boston  Soc.  Nat,Hist.,Vo] 


IV. 


Plate  33, 


'-if 


Id 


,1'  i 


!!•/ 


jh 


ig 


Carnbrian  Fossils  of  the  Boston  Basin. 


Hcliotype  Rindjis  Co.BosUm. 


Occas.  Papers,  Vol.  IV. 


PLATE  34. 

(Figs.  8-10  were  drawn  by  Miss  L.  K.  Martin,  the  others  by  Miss  Elvira 
Wood,  Instructor  in  Palaeontology  in  the  Massachusetts  Institute  of  Tech- 
nology.) 

1  a,'b.     Olenellus    sp.  P.  66-5. 

la.     A  nearly  perfect  cephalon.     Nat.  size. 

lb.     A  fragment  of  the  head  of  a  young  individual.     Nat.  size. 

2  a,  b.     Olenellus  {3Iesonacis)  asaphoides  (Emmons).  P.  667. 

2  a.     A  nearly  perfect  cephalon  of  a  young  individual.     X  2. 

2  b.     Fragment  of  the  cephalon  of  an  older  individual.     Nat.  size. 

3  a,  b.     Obolella  atlantica  Walcott.  P.  620. 
Two  valves  of  different  sizes,  .showing  external  characteristics.     X  3. 

4-6.     Metadoxides  7nagnijicus  M^tthevf  (?).  P.  670. 

Fragments  of  genal  spines,  referred  to  this  species  with  reservation.  Nat. 
size. 

7  a-c.     Strenuella  strenua  (Billings).  P.  672. 

7  a.     A  small  head  exclusive  of  free  cheeks.     X  2. 

7  b.     A  larger  head.     Nat.  size. 

7  c.     Fragment  of  a  cephalon.     X  2. 
All  the  above  are  from  the  purplish  Lower  Cambrian  slates  of  North  Wey- 
mouth.    Collected  by  Mr.  Burr,  with  exception  of  specimen,  fig.  2  a,  which 
was  collected  by  the  writer. 

Fig.  8.     Strenuella  strenua  (Billings) .  P.  672. 

A  fragmentary  cephalon.  X  Ij- 
Eed  slate  pebble,  Lower   Cambrian,  from   Nahant.     Coll.    Boston   soc. 
nat.  hist.,  cat.  no.,  11,968. 

Fig.  9.     Agraulos  quadrangularis  (Whitfield).  P.  674. 

Cephalon,   showing  spine   and    palpebral    lobes,    also  slight  glabellar 
grooves.     X  1^. 
Paradoxides  beds  Hayward  Creek. 
Stud.  pal.  coll.  Harv.  univ.,  cat.  no.,  333. 

Fig.  10.    Agraulos  quadrangularis  (Whitfield).  P.  674. 

A  large  and  broad  cephalon,  showing  spine.     X  1^. 
Paradoxides  beds,  Hayward   Creek.     Collected  by  Dr.   D.    F.   Lincoln. 
Coll.  Boston  soc.  nat.  hist.,  cat.  no.,  11.975. 


Occas. Papers,  Boston  Soc.  Nai.Hisi., Vol  .IV. 


Plate 


Cambrian  Fossils  of  the  Boston  Basin. 


Heliotype  hintiij  Co.B«3lon. 


Occas.  Papers,  Vol.  IV. 


PLATE    35. 

(Keproduced  from  plate  7,  Bull.  10,  U.  S.  geol.  surv.,  by  courtesy  of  the 
Director.) 

I'ig.  1.    Agraulos  quadrangular  is  (Whitfield).  P.  674. 

Head,  exclusive  of  the  free  cheeks.     Natural  size. 
Paradoxides  beds,  Hayward  Creek.     Coll.  mus.  comp.  zool. 

Fig.  2.    Ptychoparia  rogersi  Walcott.  P.  679. 

The  type  specimen.    Natural  size.     Paradoxides  beds,  Hayward  Creek. 
Coll.  mus.  comp.  zool. 

Fig.  3.     Paradoxides  harlani  Green.  P.  681. 

Large  free  cheek  flattened  by  compression. 
Paradoxides  beds,  Hayward  Creek.     Coll.  Boston  soc.  nat.  hist.,  cat.  no.,  1. 

Fig.  4,   4  a-c.     Hyolithes  shaleri  "Walcott.  P.  642. 

4,  4  a,  b.     Dorsal,  ventral  and  lateral  views  of  the  type  specimen. 
4  c.     Cross-section  of  same. 
Paradoxides  beds,  Hayward  Creek.     Coll.  mus.  comp.  zool. 


Occas.  Papers,  Boston  Soc.  Nat.  Ilisl.,  Vol.  IV. 


Half  35. 


CAMBRIAN  FOSSILS  OF  THE  BOSTON   BASIN 
(Paradoxides  Beds.) 


Occas.  Papers,  Vol.  IV. 


PLATE  36. 

Paradoxides  harlani  Green.  P.  681. 

The  type  specimen.     Two   thirds  natural   size. 
Presumably  from  the  Paradoxides  beds  of  Hay  ward  Creek.     Coll.  Bos- 
ton soc.  nat.  hist,  cat.  no.,  6264. 
Drawn  by  Miss  L.  E.  Martin. 


^ 


Occas.  Papers,  Vol.  IV. 


PLATE  37. 

Paradoxides  harlani  Green.  P.  681. 

A  small  nearly  complete  specimen  of  the  broad  type.     Natural  size. 
Paradoxides  beds,    Hayward  Creek.     Coll.   Bridgewater  Normal  School. 
Drawn  by  Miss  L.  R.  Martm. 


Occas. Papers, Boston  Soc.  Nai.Hisl.,Vol  .IV. 


lUr     '•/ 


Paradoxides  l^arlani  Green  Natural  size 


Heliolvpe  Co,Boston. 


Occas.  Papers,  Vol.  IT. 


PLATE  38. 

(Eeproduced  from  Plate  8  of  Bull.  10,  IT.  S.  geol.  siirv.,  by  courtesy  of 
the  Dii'ector.)  ■ 

Figs.  1  to  1  e.     Paradozides  harlani  Green.  P.  681. 

1,  1  a.     Two  medium-sized  heads,  showing  variations  in  the  frontal 

border.     Natural  size. 
1  b,  c.    Broad  and  elongate  forms  of  the  pygidium. 
1  d.     View  of  the  four  posterior  thoracic  segments  and  pygidium ;  the 

latter  is  crowded  up  beneath  the  segments. 
1  e.     Large  hypostoma  attached  to  the  frontal  doublure  of  the  head. 
Paradoxides  beds,   Hayward  Creek,  Braintree,  Mass.     Coll.  mus.  comp. 
zool. 


Occas.  Papers,  Boston  Soc.  Nat.  Hist,  Vol.  IV. 
1 


Hlalc  j«. 


lii 


le 


.7 


Id 


PARADOXIDES  HARLAN!   GREEN. 


Occas.  Papers,  Yol.  IV. 


PLATE  39. 

(Eeproduced  from  plate  9,  Bull.  10,  U.  S.  geol.  surv.,  by  coartesy  of  the 
Director.) 

Fig.  1.    Paradoxides  harlani  Green.  P.  681. 

A  large  individual,  preserving  the  body  and  parts  of  the  head.    The 

light-colored  portions  are  restored ;  and  the  pygidiuni,  vrhich  in  the 

specimen  is  pushed  a  little  out  of  position,  is  replaced.     Nat.  size. 

Paradoxides  beds,  Hayward  Creek,  Braintree,    Mass.     Coll.  Boston  soc. 

nat.  hist.,  cat.  no.,  2754. 


Occas.  Papers,  Boston  Soc,  Nat.  Hist.,  Vol.  IV. 


Plate  39. 


PARADOXIDES   HARLANl  GREEN- 


Table  of  tiie  Ilaritasket  ;^itrstn. 

rfi»an5.t©,  box^cterliig  tm&  iw%dorlvlng  the  b@«^M©d  a^cks. 

;'lj*st  COKglor^er^t© .  teasel.   r-nni-Xnti;  on  gi^Atiite   ..,,..,  E5*S0  fe©t» 
lioekj?  Ifecfeir  GrajTsitte  IPlatemj.,  Cliff  >M«tosts,,  ©-■  s 

First  Melaphyr,  ct»^;>act  and  .!aspei*:y  ««..«.•..'••.  2&-40  feet* 

rOcky  ileck,  last  ami  l/©st  Forpfi3n''lt©  llille  ar^  CllfT  l-latoau, 

i@cond  €<mgii:mormt&it  Jaspery  »  «  ♦  «  •■  #  •««.••...     25-50  feet, 
i-^ook'i*  itock»  >:as3t  sM  i;tost  Fot^hr^rrSt©  iftlls,  Oreat  Uili,  etc* 

i-'Oi*^srlfee  «.•..-..•••«..«••»«.•..*..     fjO-lOO  feot. 
B«st  5«i?i  V^est  .l?oii>h5^1t©  liiils,  Blaefe  |i:oeiE,  citc« 

f55lrd  Gor.glorK3rat<  .,,.♦,..,,•.  »    •     60-10©  foet, 

CemglOKKEfBte  ..^xi  -^^iiU  »^i*»t*5au,  Fol;30&>*a  IsIjj*-'.^,  ,<  v^uirid  ili,ll 
sBd  Oi»e©n  villi* 

•>o©ond  M©laph;ji»,  gs»©<m  snct  stEi^'gdaiotdal   .*.«•»»...  SiO-50  feet. 
.C^jacent  fill  I »  ^laph:?y  Platejm,  am  south  Slis^i'©. 

i^'oartii  Congi<^3erate   .•••».•.».••.. l,^~,u-5':'  fcet» 

Cr-i3scent  fUli,  J.|ai'»h  XsIrM,  «»tc* 

TMM  Meleplip**  ©E*e<3n  erid  araygtieiaidal  •.,,.«.,.•     S&-4S  fe@t» 

»J5d  Marsh  Island* 

Flftli  CongloiT^rat© ,,.»«,.»,•«,     25-30  feot. 

itofUcy  Keels  Msd  l*ari|.x  Beuoii  ttock. 

Pourtiv  M©l«p3i;Tr  itrscl  TMtt,  ^^en,  cosifmut  or  brecciRtrnt   •   «  ouu— fe4;u  re<;t. 
Atlantic  ttoi'i  GeTitrs  Hills »   Cim  Hock,  Little  Biaek  fmckp  etc. 

Sixtli  CojiniCKserute « 10-15  feet* 

iiocky  Keck* 

T"ifth  MQlftph;.'^,  g^&mi  nvA  *M'v;ga«loid!fil   • .*     id-iiu  \f:'ot* 

Hock 7  Itecit.  .  „„^._„„,,,  ..  ■-'.■„^,..,-^ 

600-960 


Table  of  the  Hlngham  Strata. 


Granitic  rocks  (diorlte,  granite,  and  feltiite), 

1.  Conglomerate  (basal).   Thickness  uncertain. 

2.  Melaphyr  ,  , ,  120-240  feet 

3.  Pine  conglomerate  and  sandstone,  alternating  .  .   120-200   " 

4.  Gray  slate  ,  . 40-  60   " 

5.  Gonglomerate,  sandstone,  and  slate,  alternating  .  100-170   " 

6.  Gray  and  red  slate  ,,..,.,....,...   90-130   " 

7.  Conglomerate  .„  ...o.  o  .........  ,   30-  50   " 

8.  Red  slate  ....................   20-  40   " 

9.  Conglomerate ,....o 40-  50 

10.  Red  slate  ...............   20-  30 

11.  Conglomerate  ,,...........,,..,    75-100 

12.  Red  slate 50-  75 

13.  Sandstone  and  conglomerate,  alternating  .....  200-300 

905-1445 

14.  Gray  slate 500-1- 


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Geology  of  the  Boston  Basin 


A  Special 

(iEou)oif.Ai,,\NuT(ii'o<iiui'iii(Ui.  Map 
NAHTASKET  BIS7g]gT. 


(-KOl.OI.Y  (IK  tin;  IKtSTOX  MaSIX.  MYAr.O.f.HOSISV.  Cl- .VKHAI.  MAI',  SIIKKT  I. 


1  ^■^ 


■1      C3     ^#=^     mt. 


Occu.  Papeis  Boson  Sot,  Nil 


TOPOGRAPHICAL  MAP  OF 

BLUE  HILLS  RESERVATION 


PubllBhed  wilh  permission  ol'  t!ie  Meiropolitaji  Park  C«m) 


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BOSTON  COLLEGE 


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MAY  20  1 

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